Module 7: Organic Chemistry

Table of Contents

This is one of the hardest modules, and both Chemistry in Focus and Pearson Chemistry 12 overlook significant syllabus points. If parts of this module don’t make sense, we also recommend the CrashCourse Organic Chemistry Series.

Introduction

  • All life on Earth is based on compounds of Carbon, usually in an aqueous environment
  • When scientist refer to “organic compounds”, it usually means compounds of carbon (excluding simpler compounds like $\ce{CO2,CO,CS2,}etc.\newcommand{degC}{^{\circ}C}\newcommand{deg}{^{\circ}}$)
  • Because Carbon is the first element with 4 valence electrons, it can easily covalently bond with many other atoms at once, meaning it can form lots of different compounds
  • In module 1, the concept of Carbon allotropes, molecules of Carbon with have different chemical properties, was explored
  • Module 7 looks at carbon compounds involving other elements (mostly Hydrogen, Oxygen, Nitrogen, and More Carbon™)
  • Just as Modules 5 and 6 were basically harder versions of module 2, this is basically the next level of module 1, with a bit of modules 3 and 4 sprinkled in. If you need to revise these, check out our Preliminary Chemistry Course.

Structure of an Organic Molecule

  • Organic molecules have 3 main components:
    • The Backbone: the longest continuous chain of connected carbon molecules in the molecule
    • The Functional Group: an offshoot from the backbone which gives the molecule distinct properties, also known as side groups
    • The Hydrogens: Hydrogen is used to fill all of the leftover Carbon valence electrons

Types of Organic Carbon Molecules

  • Organic carbon compounds fall into two main categories: cyclic and acyclic (also known as closed and open chain). Each type has subcategories within themselves, but for the main categories, seeing an example Lewis diagram of each type gives away where the name comes from:
Octane, an organic acyclic/open-chain compound (C8H18).
Octane, an organic acyclic/open-chain compound (C8H18).
Benzene, an organic cyclic/closed-chain compound (C6H6). Yes I know they should be capital C's, I'll fix that eventually.
Benzene, an organic cyclic/closed-chain compound (C6H6). Yes I know they should be capital C’s, I’ll fix that eventually.
  • For now, we’ll just look at chains, but don’t worry, it’s still going to hurt your brain.

Types of Organic Chains

Single Bonds (Alkanes)

  • Alkanes are organic carbon molecules where Carbon atoms in the backbone are only connected via single bonds
  • Alkanes have the general form of $\ce{C_{n}H_{2n+2}}$, and are therefore a homologous series
NameMolecular FormulaMelting Point $(\degC)$Boiling Point $(\degC)$
methane$\ce{CH4}$-182-164
ehtane$\ce{C2H6}$-183-84
propane$\ce{C3H8}$-190-42
butane$\ce{C4H10}$-1380
pentane$\ce{C5H12}$-13036
hexane$\ce{C6H14}$-9569
heptane$\ce{C7H16}$-9198
octane$\ce{C8H18}$-57128
  • Alkanes are known as “saturated compounds”, and have an open, chain-like structure
  • The names will end in -ane, for example, Ethane:

Branched Alkanes

  • Branched alkanes are alkane compounts with Alkyl side groups:
Alkyl GroupMolecular Formula
Methyl$\ce{CH3-}$
Ethyl$\ce{C2H5-}$
Propyl$\ce{C3H7-}$
Butyl$\ce{C4H9-}$
Pentyl$\ce{C5H11-}$

Remembering the names is difficult sometimes, but you might have noticed that they actually correspond to the number of Carbon atoms there are in the skeleton:

Longest ChainPrefixNumber of identical groupsPrefix
1Meth-5Pent-
2Eth-6Hex-
3Prop-7Hept-
4But-8Oct-

We’ll look at naming organic compounds later on. Remember that you can always jump around the post as needed.

Naming Branched Chain Alkanes

  1. Find the longest continuous chain of carbon, and assign the parent name based on this number

  2. Find whatever groups that are not part of the longest continuous chain. Name these as prefixes and put

    them in alphabetical order.

  3. Assign numbers to groups by counting from the closest end of the chain

Example
  • There are 6 carbons in the main chain, with the largest bond being a single bond, so the ending is -hexane
  • The side chain has 1 carbon and 3 hydrogen, so methyl- is added (methylhexane)
  • The side chain is at position 3, so the compound is 3-methylhexane
This system works for all 3 types of open-chain organic molecules, just switch out -ane for the correct ending.

Double Bonds (Alkenes)

  • Alkenes are organic compounds where the Backbone carbon has double bonds
  • Alkenes can also have single bonds in the backbone, but there must be at least 1 double bond (BUT NO TRIPLE BONDS!!!)
  • Alkenes are known as “unsaturated compounds”, and are more reactive than alkanes as a result of the double bond
  • The names of alkenes end with -ene, for example 1-pentene:

Variants of Alkenes

  • The location of double bonds can be different, even if the empirical formula is the same
  • For example, Butanol has 2 variants:

Triple Bonds (Alkynes)

  • Alkynes are the last variant of open-chain compounds, and have at least one triple bond in their backbone
  • Alkynes can also have single and double bonds in the backbone, but must have at least 1 triple bond
  • Alkynes are also “unsaturated compounds”, and are the most reactive organic group
  • The names of alkynes ends with -yne, for example propyne:
INTRODUCING THE SHORTCUT! If you didn’t notice the pattern, look at the letter which is different between the three. It goes in alphabetical order! Single bond has A, double bond has E, triple bond has Y.

Physical Properties

Properties of Homologous Series

Members of the same homologous series have:

  • A similar structure and the same general formula and functional group (each member of a homologous series differs by a −𝐶𝐻2 − unit from the previous member)
  • A pattern to their physical properties
  • Similar chemical properties

Melting Points and Boiling Points

  • The melting and boiling points are measures of the thermal energy required to overcome intermolecular forces.

As ↑ intermolecular forces, ↑ $E_{Heat},\therefore$↑ MP and ↑ BP

  • The packing of molecules also affects the boiling and melting point.Packing depends on molecular shape. Molecules that are small, symmetrical or unbranched tend to be able to pack more closely together. This results in stronger intermolecular forces.

  • The effect of packing on intermolecular forces strength is more significant for molecules in solid states (melting point).

Melting Points and Boiling Points of Alkanes

Alkane molecules are nonpolar, the only intermolecular force is dispersion forces. As the length of the carbon chain increases, the overall force of attraction between molecules also increase. (Dispersion forces is proportional to molar mass). Because boiling and point is determined by the strength of intermolecular forces, boiling point increases as alkane chain length increases.

Molecular shapes also influence the strength of dispersion forces. Straight-chained alkanes are able to fit together more closely and tend to have higher boiling points than their corresponding chain isomers.

Melting point is affected by the strength of the dispersion forces, size and shape of the molecule.

Melting points of hydrocarbons follow the same general patterns as boiling points, with a few exceptions. The melting points of straight-chain hydrocarbons increase as the number of carbon increases. However, there are deviations in this trend, relating to whether the molecules have an even or odd number of carbon atoms.

Chains with even numbers of carbon atoms pack slightly more efficiently in the solid state than chains with odd numbers. The more efficient packing requires more energy to melt the compound resulting in a higher melting point.

NameMolecular MassMelting PointBoiling Point
Methane16.04-182-164
Ethane30.07-183-89
Propane44.09-190-42
Butane58.12-1380
Pentane72.15-13036
Hexane86.17-9569
Heptane100.2-9198
Octane114.2-57126

That took longer than it should have, all because we wanted a scaleable graph. Is that too much to ask, internet?

Melting Points and Boiling Points of Alkenes and Alkynes

  • Alkenes and alkynes, like alkanes, are nonpolar hydrocarbons.

  • Their molecules are also nonpolar so the forces of attraction between them are only weak dispersion forces.

  • Members of these homologous series have relatively low boiling points similar to those observed for alkanes with the same number of carbon atoms.

  • As with alkanes, the boiling points of alkenes and alkynes increase with molecular size as the strength of dispersion forces between molecules increase.

  • The boiling points of alkenes are slightly lower than alkanes with the same number of carbon atoms.

  • Alkynes have higher boiling points than both alkenes and alkanes that have the same number of carbon atoms. This is due to the increase packing because of its linear shape.

  • Since alkenes have lower molecular mass compared to alkanes with the same number of carbon atoms, the strength of the dispersion forces is weaker. Hence the boiling points are lower.

  • Alkenes, alkynes and haloalkanes follow the similar pattern to alkanes.

  • As the length of the carbon chain increases, the melting point increases.

  • The alkenes ethene, propene and butene are all gases at room temperature, alkenes with 5-14 carbons are liquid, and longer-chained molecules are solid.

  • Alkynes follow the same general trend for melting points seen in alkanes and alkenes.

  • However, the position of the triple bond can greatly affect the melting points as the shape of the molecule changes.

Solubility

  • The solubility of a substance depends on the strength of the InterMolecular Forces within the solute and within the solvent (cohesive forces), in comparison to the IMF between the solute and solvent (adhesive forces).

  • The generalisation is that polar compounds tend to only dissolve well in polar compounds and non-polar compounds only dissolve in non-polar compounds.

  • Hydrocarbons are soluble in each other as well as in non-polar organic compounds such as benzene, diethyl ether and carbon tetrachloride (tetrachloromethane).

  • This is because the cohesive forces within the solvent are also weak dispersion forces that are similar in strength to the dispersion forces within hydrocarbons.

Density

  • Density is the measure of mass per unit volume ${g/mL\text{ or }g/cm^{3}}$
AlkaneFormulaDensity $(g/cm^{3}\text{ at }20\degC)$Molecular Mass $(g/mol)$
Pentane$\ce{C5H12}$0.62672.15
Decane$\ce{C10H22}$0.730142.3
Pentadecane$\ce{C15H32}$0.768212.4
Isosane$\ce{C20H42}$0.789282.6
  • Alkanes cannot mix with water → If you try, the alkane will float on top of the water
  • This is because all alkanes have a lower density than water

Volatility

Volatility is the ability of a liquid (or solid) to escape and form a vapour. ↑ BP→↑ IMF Strength→↑ Volatility Volatility is measured by vapour pressure, which is a measure of concentration in the gas phase above the liquid. It is constant at a constant temperature.

Hydrocarbons are non-polar and hence have dispersion forces as their only intermolecular force. Since the intermolecular forces are relatively weak, their bonds are easily overcome and hence the BP is relatively low. Therefore, hydrocarbons will be volatile.

↑ Molecular Mass → ↑ Strength of dispersion forces → ↑ 𝐸h𝑒𝑎𝑡 → ↓ Volatility at a constant temperature.

Viscosity

Viscosity refers to a substance’s resistance to fluid flow. Liquids with a relatively high resistance to flow have high viscosity. For a substance to flow, particles must flow over each other. Viscosity depends on:

  • Strength of intermolecular forces. ↑ IMF→↑ Viscosity

  • Size of molecules. ↑ Size→↑ Viscosity

  • Temperature. ↑ Temperature→↓ Viscosity

    Viscosity decreases with increasing temperature. At higher temperatures, molecules have greater kinetic energy. Thus, the molecules move around more which increases the space between molecules. This causes the intermolecular forces to be weaker and hence viscosity decreases.

Functional Groups

A functional group is an atom or group of atoms which give a compound some characteristic physical and chemical property.

A homologous series is a family of organic compounds with the same general formula or functional groups with similar chemical properties.

Alcohols (Alkanols)

Alcohols are formed when one of the hydrogens on the end of the chain has been replaced with an -OH group

Naming

  1. Find the longest continuous chain of carbon that contains the functional group
  2. Replace the ane/ene/yne with -ol
  3. Name the substituents as prefixes in alphabetical order
  4. Number the chain so that the functional group has the lowest possible number

Alcohols can also be classified according to the number of carbon atoms attached to the carbon bearing the -OH group.

  • A primary (1°) alcohol is one in which the carbon bearing the -OH group is bonded to one other carbon atom.
  • A secondary (2°) alcohol is one in which the carbon bearing the -OH group is bonded to two other carbon atoms.
  • A tertiary (3°) alcohol is one in which the carbon bearing the -OH group is bonded to three other carbon atoms.

Aldehydes and Ketones (Carbonyl Group)

A carbonyl functional group consists of carbon attached to an oxygen atom by a double bond. The difference between aldehydes (alkanals) and ketones (alkanone) is the position of the carbonyl group along the carbon chain:

  • If the carbonyl group is on a terminal carbon (at the end of a chain), it is called an aldehyde, which is given the suffix “-al”.
  • If the carbonyl group is in the middle of the carbon chain, it is called a ketone, which is given the suffix “- one”.

Naming

  • When the aldehyde is the suffix, the aldehyde carbon is assigned the number 1. Since it is always carbon 1, “1” (terminal carbon) is omitted from the name. (“-al”) e.g. 2-methylbutanal
  • When the ketone is the suffix, the carbon chain is numbered so that the ketone is assigned the lowest number. (“-one”) e.g. Pentan-3-one.

Carboxylic Acids (Alkanoic Acids)

Carboxylic acids contain the carbonyl group, C=O, connected to a hydroxyl group. The carboxyl group is abbreviated as -COOH, and is given by the suffix-oic acid”.

Carboxylic Acid Functional Group, where R represents the rest of the molecule.
Carboxylic Acid Functional Group, where R represents the rest of the molecule.

Since the carboxylic acid uses up 3 bonds on the carbon atom, it must be situated at the end of the chain (terminal carbon).

Naming

Unfortunately, the carboxylic acids have different names each. However, some of them should be familiar.

IUPAC NameFormulaCommon Name
Methanoic Acid$\ce{HCOOH}$Formic Acid
Ethanoic Acid$\ce{CH3COOH}$Acetic Acid
Propanoic Acid$\ce{CH3CH2COOH}$Propionic Acid
Butanoic Acid$\ce{CH3CH2CH2COOH}$Butyric Acid
Pentanoic Acid$\ce{CH3CH2CH2CH2COOH}$Valeric Acid
2-Hydroxypropanoic Acid$\ce{HC3H5O6}$Lactic Acid
2-Hydroxypropane-1,2,3-Tricarboxylic Acid$\ce{H2C6H5O7}$Citric Acid
2-Hydroxypropane-1,2,3-Tricarboxylic Acid, more often referred to as Citric Acid, with each functional group coloured. You don't need to know this for your HSC, relax.
2-Hydroxypropane-1,2,3-Tricarboxylic Acid, more often referred to as Citric Acid, with each functional group coloured. You don’t need to know this for your HSC, relax.

Amines and Amides

  • Amines and Amides both contain nitrogen
  • Amines contain $\ce{NH2}$, and use the suffix “-amine” or the prefix “amino-”
  • Amides are carboxylic acids where the OH group has been replaced with an amine. They use the suffixamide”.
  • When they are quoted as the prefix, the carbonyl and the amine are named separately (using the prefixes “oxo” and “amino” respectively).

Halogenated Organic Compounds

  • Halogenated organic compounds have had hydrogen atoms replaced with a halogen (group 17 element)
  • They are named using prefixes placed in front of the name of the alkane
    • e.g. difluoropentane

Naming Priorities of Function Groups

  • The highest priority functional group takes the suffix, and the other functional groups are placed earlier in the name.

  • Aldehyde > Ketone > Alcohol > Alkyne = Alkene

NameStructureSuffixPrefix
Carboxylic/Alkanoic Acid$\ce{R-COOH}$-oic acid
Amide$\ce{R-CONH2}$-amideOxo- & Amino- (The carbonyl & amine groups are named as if they were separate)
Aldehyde (Alkanal)$\ce{R-COH}$-alOxo-
Ketone (Alkanone)$\ce{R\prime-CO-R}$-oneOxo-
Alcohol (Alkanol)$\ce{R-OH}$-olHydroxy-
Amine$\ce{R-NH2}$-amine-amino-
Alkene$\ce{R - CH = CH -R\prime}$-ene-ene-

Structural Isomers

Isomers

  • Structural isomers are molecules that have the same molecular formula, but their atoms are arranged in different ways, giving rise to different structural formulae.

  • Although isomers have the same molecular formula, they are different compounds with different chemical and physical properties, as well as different names.

Chain Isomers

Chain isomers involve rearrangement of the carbons in the backbone, resulting in a different number of carbons in the longest chain or different branching in the carbon chain.

Example: Hexane
  • An alkane with the molecular formula $\ce{C6H14}$ has 5 chain isomers
  • Each isomer has the same molecular formula, but a different name
The 5 hexane chain isomers: hexane, 2 methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.
The 5 hexane chain isomers: hexane, 2 methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.

Position Isomers

  • Position isomers occur when molecules have the same carbon chain, but the functional group is at a different location.

  • Position isomers only exist for molecules that contain functional groups, where the chain is long enough for the functional group to occupy different positions. (Eg. Not possible for ethene or ethane).

Example: Butene
But-1-ene
But-1-ene
But-2-ene
But-2-ene

Functional Group Isomers

Functional group isomers result when the atoms in the molecules are arrange in different ways that lead to the isomers having different functional groups.

Example: $\ce{C3H6O2}\newcommand{orange}{\color{orange}}$
  • Propanoic Acid and 1-hydroxypropan-2-one are functional group isomers, with the same molecular formula $(\ce{C3H6O2}),$ but different arrangements of atoms:
  • Propanoic Acid contains a carboxyl functional group, while 1-hydroxypropan-2-one contains hydroxyl and carbonyl functional groups
Propanoic Acid
Propanoic Acid
1-hydroxypropan-2-one
1-hydroxypropan-2-one

Hydrocarbon Reactions

Combustion Reactions

This is what chemistry is all about, isn’t it? Setting things on fire and seeing what happens.

  • All combustion reactions are exothermic. It may be complete or incomplete.
  • For complete combustion to occur, excess oxygen must be readily available, and the products are always carbon dioxide and water.

$\orange{\ce{C8H18(l) + 25/2O2(g)→8CO2(g) +9H2O(l)}}$

  • Incomplete combustion occurs when there is insufficient oxygen. The products are water and three different oxidations of carbon, soot, carbon monoxide and carbon dioxide.

  • Incomplete combustion results in less energy being produce per mole of fuel combustion, making it less efficient.

  • This is due to the reduction of C=O bonds being formed. The formation of C=O releases a large amount of energy.

  • Alkenes and alkynes tend to burn with a more sooty flame compared to alkanes due to the higher percentage of carbon atoms. Some of the carbon may not combine with oxygen.

  • Standard molar heat of combustion is the energy released. Therefore, it is positive.

  • Standard enthalpy of combustion is always negative as combustion is exothermic. It is the change in enthalpy.

Reactions of Unsaturated Hydrocarbons

Stability of Carbon Bonds

  • The bond energies dictate the overall reactivity of hydrocarbon compounds. Subsequent carbon-carbon bonds in a multiple bond are less stable and weaker than the original single covalent bond.
  • This means that double and triple bonds are highly reactive and can break open more easily and allow atoms to join (saturate).
  • This makes the alkenes and alkynes highly reactive compared to alkanes.
  • Alkenes and alkynes are able to react with a number of chemical reactions called addition reactions

Addition Reactions of Alkenes - Hydrogenation

  • Alkenes react with hydrogen gas in the presence of a metal catalyst to form a saturated alkane.

$\orange{\text{General Formula: }\ce{Alkene +H2(g)→[Ni]Saturated Alkane}}$

Addition Reactions of Alkenes - Bromination and Chlorination

  • Bromine and chlorine add to almost all alkenes very rapidly at STP, creating a compound that has 2 bromines/chlorines on adjacent carbons
  • This occurs spontaneously at STP, and can be used as an indicator for alkenes and alkynes
1,2-Dibromothane
1,2-Dibromothane

Addition of HX (Hydrohalogenation)

  • Any of the hydrogen halides (HF, HCl, HBr and HI) can add to the double bond of an alkane to give the corresponding alkyl halide.
  • The double bond is converted into a single bond.

Predicting a Major Product

  • When an asymmetric reagent, such as HBr is added to an asymmetric alkene, more of one isomer is produced than the other.

  • The predominant isomer is called the major product and the other isomer(s) is called the minor product(s).

  • In some reactions, only the major product will be formed.

  • The major product obtained from an addition reaction can be predicted using Markovnikov’s rule.

  • In addition, for reactions involving unsymmetrical alkenes, the hydrogen atom will predominantly bond to the carbon atom bearing the greater number of hydrogen atoms.

Addition of Water (Hydration)

  • Water alone does not react with alkenes, but if an aqueous acid catalyst (eg. Dilute H2SO4) is added and the mixture is heated, water adds to the C=C double bond to give an alkanol.

Addition Reactions of Alkynes

  • Alkynes undergo addition reactions in a similar fashion to alkenes.

    • One of the three bonds in the triple bond is broken, and two new bonds form. The original triple bond is converted into a double bond.
    • The second addition reaction can be stopped by controlling equivalents of reagent used or by using a specialised reagent.

Hydration of Alkynes

  • Alkynes do not react with water with only an acid catalyst. A mercury(II) catalyst, such as mercury(II) sulfate, must also be present.
  • A carbonyl (ketone or aldehyde) is produced instead of an alcohol. The alcohol forms as an intermediate which quickly rearranges to form the carbonyl.

Substitution Reactions of Alkanes

  • A substitution reaction occurs when an atom or functional group in a molecule is replaced or substituted by another atom or group.
  • Alkanes are far less reactive than alkanes and alkynes as C-C single bonds are relatively strong.
  • However, their hydrogen atoms can be substituted by halogens.
  • These reactions do not occur spontaneously at RTP.
  • Substitution of alkanes can only be carried out with chlorine and bromine (fluorine reacts too explosively, and iodine does not react) and required energy in the form of ultraviolet (UV) radiation.

$\orange{\ce{CH4(g) +Cl2(g)→[UV Light]CH3Cl(g) +HCl(g)}}$

Physical Properties of Alkanols

In case you forgot, alkanols and alcohols are the same thing, its just that the names tend to be -nol, its just easier to remember if we consistently use Alkanol (i.e. Alkane + nol)

Intermolecular and Intramolecular Bonding Forces of Alkanols

  • Alkanols contain highly electronegative oxygen atoms, creating a polar bond in the molecule.
  • The hydrocarbon chain is still non-polar, and has dispersion/Van Der Waals forces.
    • The hydrocarbon end of the molecule repels water.
  • The -OH functional group is polar, forming hydrogen bonds, dipole-dipole bonds, ion-dipole bonds, and dispersion bonds.
    • This end is attracted to water.
  • As the chain length of an alcohol increases, the strength of the dispersion force increases. However, the extent of hydrogen bonding does not change.
  • The strength of the hydrogen bonding depends primarily on the molecule’s shape and the number of hydrogen bond donors and acceptors available.
  • Thus the length of the hydrocarbon chain influences the properties within the homologous series of alcohols, and the hydroxyl group influences the differences in properties between alcohols and other homologous series.

Melting and Boiling Points of Alkanols

AlkanolMolecular mass (g/mol)Melting Point $(\degC)$Boiling Point $(\degC)$
Methanol32.04-9865
Ethanol46.07-11478
Propan-1-ol60.09-12697
Butan-1-ol74.12-90118
Pentan-1-ol88.15-79138
Hexan-1-ol102.17-52157
Heptan-1-ol116.20-34176
Octan-1-ol130.22-16195
  • ↑ Chain length→↑Molecular mass→↑ Strength of dispersion forces→↑BP and ↑ MP.
  • However, the shape of ethanol and methanol allow them to pack more closely together than propan-1-ol, thus resulting in stronger intermolecular forces and a higher MP.
  • The position of the hydroxyl group within the molecule can also affect the melting and boiling point of alcohols
  • For example, butan-2-ol has a lower boiling point than butan-1-ol
  • Secondary and tertiary alkanols have lower boiling points than their primary isomers
  • The dispersion forces will be the same for all three isomers
  • However, hydrogen bonding is weaker for secondary and tertiary alkanols, as the alkyl group adjacent to the OH group hinders the OH group’s ability to get close to another molecule, restricting their ability to form strong hydrogen bonds
  • As a result, the lower boiling points arise from weaker hydrogen bonding in secondary and tertiary alkanols.

Solubility in Water

AlkanolSolubility in Water at STP (g/100mL)
MethanolMiscible
EthanolMiscible
Propan-1-olMiscible
Butan-1-ol7.9
Pentan-1-ol2.3
Hexan-1-ol0.59
  • Small alcohols dissolve well in water.

  • However, solubility in water decreases with increasing carbon chain length.

  • The solubility of alcohols in water is dictated by size because the opposing effects of the polar and non-polar portions of the molecule.

  • The polar hydroxyl group is hydrophilic (“water loving”) and the non-polar hydrocarbon chain is hydrophobic (“water-hating”)

  • Water molecules cannot solvate the large non-polar carbon chains in long alcohols.

  • The -OH group can form hydrogen bonds with water, allowing solvation of this portion of the molecule.

  • However, to solvate the large non-polar carbon chain, many strong hydrogen bonds between water

    molecules need to be broken.

  • Since the alkyl chain has no strong attraction to water, these cohesive hydrogen bonds cannot be broken.

  • Thus, the alkyl end remains unsolvated, and solubility in water is drastically decreased.

  • A very general rule for solubility in water, if it has less than a 4:1 carbon:oxygen ratio, it is soluble.

  • Small alcohols are good solvents for dissolving both polar and non-polar substances due to the presence of both polar and non-polar areas in the molecule.

  • The polar hydroxyl group can form polar interactions such as ion-dipole, hydrogen bonding and dipole- dipole forces with other polar and ionic substances

  • The non-polar hydrocarbon chain can form dispersion forces with other non-polar substances.

Solubility in Organic Solvents

Alcohols become more soluble in non-polar organic solvents such as hexane, benzene and toluene $(\ce{C7H8})$, as the length of the carbon chain increases.

    • The large alkyl chain can form strong dispersion forces with other non-polar substances. These are strong enough to disrupt hydrogen bonds holding the alcohol together.
    • Therefore, the alcohol molecules separate and disperse throughout the solvent.

Reactions of Alkanols

Dehydration Reactions

When alcohols are heated with concentrated sulfuric or phosphoric acid as a catalyst, an OH and a H atom on the adjacent carbon will be eliminated from the alcohol to give an alkene and water.

$\orange{\ce{CH3CH2OH→[H2SO4][180\degC]CH2CH2 + H2O}}$

$\orange{\text{Ethanol in Sulfuric acid at 180}\degC\rightarrow\text{Ethylene+Water}}$

  • The reactivity and rate of reaction varies depending on the type of alcohol.
    • Tertiary alcohols are the most reactive and always react the fastest. Dehydration occurs readily at room temperature.
    • Primary and secondary alcohols require higher temperatures. Primary alcohols are less reactive and react slower than secondary alcohols.

Substitution Reactions of Haloalkanes

  • Alkanols undergo substitution in the presence of a hydrogen halide to form the corresponding Alkyl Halide and Water.
  • The trend in reactivity is ths same as for dehydration
  • For primary alcohols, larger halides react faster (chlorine is slowest, iodine is fastest)

Oxidization

  • Alcohols can be oxidised with strong oxidizing agents such as acidified solutions of permanganate $(\ce{MnO4-})$ or dichromate $(\ce{Cr2O7^{2-}})$ to give carbonyl compounds
  • A change in oxidization states can be seen for the carbon atoms

Oxidisation of Primary Alcohols

  • Primary alcohols are oxidised into carboxylic acids.

  • The oxidation of alcohols is driven by a simultaneous reduction reaction, usually of inorganic reagents. Either acidified permanganate (MnO4- purple in colour) or dichromate (Cr2O72- orange in colour) can be used as the oxidant.

  • The oxidation occurs stepwise: the alcohol is first oxidised to the aldehyde, which is then oxidised into the carboxylic acid.

  • Aldehydes are very reactive and the second oxidation generally occurs too rapidly for it to be separated practically.

    • $\ce{MnO4-}$ goes from purple to colorless
    • $\ce{Cr2O7^2-}$ goes from orange to green

Oxidisation of Secondary Alcohols

  • Secondary alcohols can be oxidised to produce ketones.

  • Either acidified permanganate (MnO4- purple in colour) or dichromate (Cr2O72- orange in colour) can be used as the oxidant.

  • Since there are no hydrogen atoms attached to the ketone carbon, no further oxidation can occur.

  • Secondary alcohols are generally less reactive than primary alcohols, thus require higher temperatures and longer reaction times to be oxidised.

Oxidisation of Tertiary Alcohols

  • Tertiary alcohols cannot be oxidized to form a carbonyl compound.
  • The carbon bearing the hydroxide group has no hydrogen atoms, and therefore cannot be oxidised.

Combustion of Alcohols

  • All alcohols can readily combust, either completely or incompletely.
  • Because they are oxygenated, they are prone to complete combustion, and the reaction is highly exothermic as a result of the oxygen and hydrogen combustion reaction which occurs.

Production of Alkanols

Why is there so much about alcohols in this module???

Production by Hydration (Production by Addition)

  • Aqueous catalysts such as dilute sulfuric or phosphoric acid allow water to react with alkenes.
  • HO is added to the C=C double bond, forming an alkanol
  • For example:

$\orange{\ce{C2H4 +H2O →[H3PO4 Catalyst][300\degC] C2H5OH}}$

Production by Substitution

  • Alkanols can also be produced from the substitution of haloalkanes.

$$\begin{gather*}\orange{\ce{RX +OH- →[H2O][100\degC]ROH +X-}} \\ \text{Halocarbon + Hydroxide Ion}\rightarrow\text{Alcohol + Halide Ion}\end{gather*}$$

  • This reaction occurs by heating the haloalkane with a solution of sodium hydroxide or potassium hydroxide.

  • The hydroxide ion from the aqueous base replaces the halogen atom to generate an alcohol and halide salt.

  • X: Cl, Br, I. Fluoroalkanes will not react as the C-F bond requires too much energy to break.

  • This reaction occurs due to the highly polarised carbon-halogen bond, which produces a partial positive charge on the carbon atom.

    • The partially positive carbon atom can be easily “attacked” by a negatively charged hydroxide ion.
    • This results in the formation of a covalent carbon-oxygen bond. In the process, the negative charge is donated to the electronegative halogen atom, which leaves as a halide ion.
  • The rate of this reaction is dependent on the type of haloalkane and the halogen atom that leaves the molecule.

  • Haloalkanes can be categorised as primary, secondary and tertiary. The reactivity is highest for primary, followed by secondary and then tertiary.

  • This is because the presence of alkyl groups greatly hinders the ability of the hydroxide ion to approach the partially positive carbon and thus slows the reaction.

  • The type of halogen atom leaving the molecule also has an effect on the rate. The reaction occurs the fastest with iodide, followed by bromide and then chloride.

  • This is because the carbon-halogen bonds are of different strengths.

  • The lower the bond energy, the easier it is to break the bond.

  • It is also possible for haloalkanes to undergo substitution reactions with water to form alcohols. This reaction occurs much more slowly, and the reactivity is highest for tertiary.

Production by Fermentation

  • Fermentation is a process that involves the conversion of carbohydrates into simple alcohols by the action of enzymes.
  • This is a natural process used by microorganisms to extract energy.
  • Carbohydrates have the molecular formula of $\ce{C_{x}(H2O)_{y}}$.
  • Carbohydrates are abundant in plant material.
  • They are also called saccharides.
  • The simplest carbohydrates are monosaccharides.
  • They are the building blocks of more complex carbohydrates, such as disaccharides like sucrose and polysaccharides like cellulose.
  • The fermentation of monosaccharides, such as glucose and fructose is the simplest form of fermentation which process ethanol and carbon dioxide.

$$\orange{\ce{C6H12O6(aq)→[Yeast]2Ch3CH2OH(aq) +2CO2(g)}}$$

  • This process relies on the presence of zymase, an enzyme found in yeast.

Conditions for Fermentation

  1. Zymase is present – found in yeast
  2. Warm temperatures (30 − 40°C but depends on the yeast strain)
  3. Anaerobic environment (oxygen limited environment)
  4. Aqueous solution of sugar
  • Fermentation of monosaccharides must be catalysed by zymase. Since zymase is a biological catalyst, it is sensitive to temperature.
  • Yeast can produce ethanol to concentrations up to only about 15% v/v. This is due to ethanol being toxic and around 15% v/v, yeast will start to die.
  • To produce higher alcohol contents, it is necessary to distill the liquid
  • If the aqueous mixture from a fermentation process is subjected to fractional distillation, 95% ethanol can be obtained.
  • To obtain 100% ethanol, more elaborate procedures such as molecular sieving are required.

Environmental Impacts of Hydrocarbons

Sources of Hydrocarbons

  • The primary source of hydrocarbons is from petroleum. Petroleum is a mixture of hundreds and thousands of different alkanes, ranging from methane up to alkanes with 40 or more carbons.

  • The mixture of gases found in petroleum is called natural gas and the mixture of liquid components is called crude oil.

  • Petroleum is found within pores of rocks deep in the ground.

  • The complex mixture is separated into fractions according to their boiling points using fractional distillation.

  • The petroleum is heated to about 400°C to produce hot liquid/vapour mixture that enters the fractioning tower.

  • Inside the tower are horizontal trays, each which contains many bubble caps upon which alkanes condense.

  • Fractions which have lower boiling points will rise higher in the column before condensing.

  • Fractions which have higher boiling points will not rise as high and will condense towards the bottom of the column.

  • Differences in physical and chemical properties of each petroleum fraction mean that they are suitable for different purposes.

  • Generally, light fractions (LPG, petrol, naphtha) are more useful and are in higher demand than heavy fractions (heavy fuel oil, lubricating oil, wax and asphalt).

  • Some of the longer alkanes are further processed through cracking, which involves heating alkanes to high temperatures in the absence of oxygen.

  • This causes them to split and form shorter, more useful alkanes as well as alkenes.

  • A zeolite catalyst, which consists of Al, Si and O, may be employed to allow this reaction to be carried at lower temperatures.

Uses of Hydrocarbons

  • The major use of petroleum is transport. Hydrocarbons are excellent fuels and the combustion of hydrocarbons is the primary source of energy production globally.
  • Unsaturated hydrocarbons are highly reactive and can undergo addition reactions.
  • This makes them extremely important as raw materials for the production of other organic chemicals, such as haloalkanes and alcohols, and commercially valuable goods such as plastic.

Issues with using Petroleum

  • Petroleum deposits in the ground are formed by the burial and decomposition of prehistoric living organisms over millions of years. Thus, petroleum is a finite and non-renewable resource.

  • As the world’s crude oil diminishes, there will be enormous negative economic and sociocultural consequences. Such as the instability of world markets and increase costs of goods.

  • Another huge problem that arises is that the combustion of petroleum releases huge amounts of carbon dioxide into the atmosphere.

  • Carbon dioxide is a greenhouse gas, so it absorbs infrared radiation from the atmosphere and keeps our Earth warm.

  • The extra carbon dioxide produces through combustion is a major contributor to the enhanced greenhouse effect which causes global warming.

  • The consequences for global warming include rising sea levels which in the long term will result in land loss and flooding, more frequent and intense extreme weather events, warming of the oceans and disruptions to the feeding behaviour of wildlife.

  • The higher concentrations of carbon dioxide has also resulted in the acidifications of oceans which is threatening the survival of aquatic life.

Biofuels

Alternative Fuel Sources

  • A possible solution to the reliance on non-renewable crude oil is to use biofuels.
  • Biofuels are fuels derived from biomass, which is biological material from living or recently living organisms such as wood, crops, wet waste and animal waste.

Bioethanol

  • Bioethanol is ethanol produced from the fermentation of monosaccharides such as glucose and fructose.
  • Monosaccharides are the building blocks of more complex carbohydrates which make up plant material.
  • Monosaccharides can be sourced from:
    • Sucrose based feedstock (eg. Sugarcane and fruits)
    • Starch based feedstock (eg. Grains like wheat and corn)
    • Cellulose based feedstock (eg. Wood residues)
  • Fermentation usually involves mashing up grains, sugarcanes or fruits with water to create an aqueous solution of sugar to which yeast is added:
    • Sucrose $(\ce{C11H22O11})$ and starch are readily hydrolysed into monosaccharidesduring the fermentation process, as yeast contains the necessary enzymes required to catalyse the breakdown.
    • Hydrolysis: $\ce{C11H22O11(aq) +H2O(l) →2C6H12O6(aq)}$
    • Fermentation: $\ce{C6H12O6(aq) →[Yeast]2C2H5OH(aq) +2CO2(g)}$
  • Cellulose is difficult to break down to its component sugars as the enzyme cellulase is not readily available for industrial use.
  • Bioethanol has been developed as a substitute for petroleum-based ethanol and as an alternative to petrol. It has the potential to be used as standalone fuel to completely replace petrol but is usually used as an additive to petrol.
  • In Australia, most cars post-1986 can use up to 10% ethanol (E10 fuel).

Biogas

  • Biogas consists of mixtures of gases, such as methane, carbon monoxide and hydrogen, released from the natural breakdown of organic matter by anaerobic bacteria.
  • The organic matter is sourced from natural wastes from agriculture and households, such as manure, human sewage, food processing wastes and crop wastes.
  • To produce biogas, the waste is placed in a large enclosed tank, called the digester, containing anaerobic bacteria. The gas released from the decay is collected by gas outlets.
  • The biogas collected can be combusted as a fuel to generate electricity or to heat boilers from industrial processes and for cooking and heating water in homes.
  • The biofuel production represents a carbon cycle, where plants absorb carbon dioxide during growth, recycling the carbon dioxide released during combustion.
  • The use of biogas also helps reduce the enhanced greenhouse effect as methane is a greenhouse gas that has a larger effect than carbon dioxide.
  • By collecting it and using it to produce electricity, less is released into the atmosphere.
  • Another environmental benefit is that bioethanol and biodiesel are cleaner fuels than petroleum fuels.
  • They are oxygenated, so complete combustion is more likely to occur and they do not contain sulfur impurities.
  • They are non-toxic and biodegradable, thus do not pose as severe a threat to the environment in the event of a spill.

Obstacles in the Production of Biofuels

  • In Australia, biomass sources currently being used are waste residues.

    • Bioethanol is made from sugar cane molasses (waste) and waste from starch and red sorghum production.
    • Biogas is generated from the treatment of waste water
    • Biodiesel is produced from waste vegetable oil from restaurants and industrial food producers
  • However, this only produces a small percentage of Australia’s fuel needs. It is not possible to manufacture enough biofuel from these sources to replace all petroleum fuels used today.

  • Large amounts of fertile land would be required to grow the crops.

    • This requires clearing of forests and bushland which will contribute to an increase in carbon dioxide levels in the atmosphere and to habitat loss.

    • Fertile land is also limited thus it competes with food crops, resulting in increase in food prices.

    • Intensive farming can lead to land degradation and erosion.

    • A large amount of fertiliser would be required to replace the nutrients taken from the ground.

    • A large amount of water would be required.

    • There is also a large amount of energy required for harvesting of crops and processing biofuels, energy that is currently derived from fossil fuels.

    • Large scale commercial agriculture leads to the reduction of biodiversity due to the loss of important

      organic matter from that crop.

  • The potential of biofuels lies in making it financially viable compared to conventional petroleum-based fuels.

Organic Acids and Bases

  • Organic acids are acids that have a carbon-based structure. The most common type of carboxylic acids are alkanoic acids. They occur abundantly in nature.

    • Propanoic acid is found in cheese
    • Methanoic acid is produce in ant venom
    • Lactic acid is in milk
    • Citric acid is in citric fruits
  • All carboxylic acids are weak acids thus they will partially ionise in water to produce hydrogen or hydronium ions.

  • Each carboxylic acid group is monoprotic.

  • The covalent bond between oxygen and hydrogen is highly polar. The bonding electrons are strongly

    attracted to oxygen so the hydrogen can be drawn easily by a base, leaving behind a negative charge on the oxygen.

  • If more than one carboxylic acid group is present in the molecule, these acids will be polyprotic. For

    example, citric acid is triprotic.

  • The strength of a carboxylic acid is influenced by the length and substitution of the hydrocarbon chain.

    • As the chain length increases, the strength of the acid decreases.
    • This is because the alkyl groups are capable of donating electron density. Carbon is more electronegative, so it pulls electron density towards itself and away from hydrogen.
    • The additional electron density allows carbon to “donate” additional negative charge to neighbouring carbon atoms.
    • Substituting a highly electronegative atom, such as a halogen, onto the hydrocarbon chain increases the strength of the acid.
    • The strong electron-withdrawing power of the substituent helps weaken the oxygen- hydrogen bond. This makes it easier for the hydrogen to be dissociated.
    • As the number of electronegative atoms increases, so does the strength of the acid.
    • Fluorine has the biggest electron-withdrawing effect, then chlorine, bromine and iodine.

Amides (Weak BL Bases)

  • Organic bases are organic compounds that are characterised by the presence of an atom with a lone pair of electrons that can accept an $\ce{H+}$
  • Nitrogen containing compounds such as amines are the most common organic bases.
  • Many amine bases exist in nature.
  • The most important being the four nitrogenous DNA bases: adenine, cytosine, guanine and thymine, and the 20 natural amino acids used to make proteins in living organisms.
  • Amino acids are molecules that contain both a carboxylic acid and a basic amine group.
  • Simpler amines are made by substitution reactions of haloalkanes with ammonia.
  • Amines act as a base in an analogous manner to ammonia. The lone pair of electrons on the nitrogen can accept a proton, forming an ammonium ion. Like ammonia, they are weak bases.
  • ↑ Chain length→↑ Partial negative→↑ H+ acceptor→↑ Base strength
  • Alkyl groups are capable of donating electron density.
  • This results in a build-up of partial negative charge on the electronegative nitrogen atom, allowing it to pick up H**+** more readily and also stabilise the positive charge on the ammonium ion.
  • The longer the chain, the greater the electron density donate.
  • Amides are also nitrogen containing compounds with a lone pair of electrons.
  • Amides are neutral compounds.
  • The presence of the highly electronegative oxygen atom in the C=O group pulls electron density away from the nitrogen atom, which makes it more difficult to accept a H+ and stabilise a positive charge if nitrogen was to accept it.

Reactions of Carboxylic Acids and Amines

  • Carboxylic acid + reactive metal → salt + hydrogen gas
  • Carboxylic acid + metal hydroxide/oxide → salt + water
  • Carboxylic acid + metal carbonate/bicarbonate → salt + carbon dioxide + water
  • Amine + acid → ammonium salt
  • When carboxylic acids react with amines, the product formed will depend on the conditions.
  • At low temperatures, a proton transfer (neutralisation reaction) proceeds between the carboxylic acid and amine, forming a carboxylate ion (acid) and an alkyl (base).
  • At higher temperatures or in the presence of a suitable catalyst, an amide is produced with the elimination of water (condemnation reaction), which forms the OH group on the acid and a hydrogen from the amine.
  • Condensation reaction: Where two or more molecules combine to form a larger molecule with the simultaneous elimination of a small molecule such as water or methanol.

Physical Properties of Carboxylic Acids, Amines, and Amides

Boiling Point

  • Carboxylic acids, amines and amides are all polar molecules. They can form hydrogen bonds with other molecules.
  • ↑ Alkyl chain→↑ MM→↑ Dispersion forces→↑ BP Amides do not exhibit a linear relationship between boiling point and molecular weight.
  • This is because the hydrogen bonding exhibited by the amides is extensive and are more complex.
  • Amides exhibit the highest boiling points compared to carboxylic acids and amines.
  • Carboxylic acids have higher boiling points than amines.
  • Carboxylic acids and amines have less atoms that can form hydrogen bonds.
  • Each carboxylic acid molecule can form two hydrogen bonds with another acid molecule through the double bonded oxygen and OH group, forming a dimer (existing in pairs).
  • Each amine molecule can only form one hydrogen bond with another amine molecule.
  • Amines and amides can be classified into primary, secondary and tertiary depending on the number of alkyl groups attached to the nitrogen.
  • Primary amines have the highest boiling points, followed by secondary and then tertiary amines. The same trend occurs with amides.

Solubility

  • Small amines, amides and carboxylic acids dissolve completely in water. However, solubility decreases as the hydrocarbon chain increases.
  • ↑ Carbon chain length→↑ Non-polar nature of compound→↑ Dispersion forces domination→Water cannot solvate the long hydrocarbon chain due to cohesive bonds→↓ Solubility
  • Similar to alcohols, the trend is reversed for their solubility in organic solvents.

Esters

  • Esters are organic compounds with the functional group $\ce{COO-}$
  • Esters are formed from the reaction of a carboxylic acid and an alcohol.
  • The OH group on the acid is replaced with an OR group from the alcohol. This reaction is called esterification (a condensation reaction).
  • The reaction is extremely slow.
  • Concentrated $\ce{H2SO4}$ (dehydrating agent) is used to remove the water and catalyse the reaction.
  • Becaise they are all liquids, $K_{eq}=\frac{[\ce{H2O}][\text{Ester}]}{[\text{Alkanoic Acid}][\text{Alkanol}]}$
  • Esters are everywhere in nature and are used in a number of industrial applications.
  • Short chain esters are known for their distinctive, fruit like odours and many occur naturally in fruits and the essential oils for plants.
  • Due to their pleasant odours, they are commonly used as flavouring (banana lollies) agents in processed foods, as well as fragrances in perfumes and cosmetics.
  • Fats and oils are also naturally occurring triesters derived from glycerol and fatty acids.

Nomenclature of Esters

  1. The alkanol is changed to “alkyl” and is the first word of the esters name
  2. The alkanoic acid becomes the alkanoate and is the second word of the esters name

Esterification

To increase rate of reaction:

  • ↑ Temperature
  • ↑ Acid catalyst

To increase yield:

  • Remove water
  • Concentrated $\ce{H2SO4}$ (catalyst and dehydrating agent)

Reflux

  • Esterification must be carried out under reflux.
  • Reflux is a technique that involves heating a reaction mixture in a vessel fitted with a cooling condenser so that the volatile reactants and products are returned to the reaction mixture without any loss.
Reflux lab diagram.
Reflux lab diagram.
ComponentPurpose
Reaction FlaskContains volatile reactants and products.
CondenserPrevents the volatile reactant or product from escaping before the reaction has reached equilibrium by cooling the reactant vapour into a liquid. Water enters at the base and leaves from the top.
Boiling ChipsTo provide a surface upon which bubbles form, promoting even boiling.
Open TopTo avoid dangerous pressure build-up inside the apparatus.

Soaps

Structure of Soaps

  • Soaps are surfactants which, when dissolve in water, help to remove dirt, oil and foreign matter from surfaces.

  • Soaps are salts of fatty acids:

    • Fatty acids are carboxylic acids with long hydrocarbon chains (10+)
    • The salt of a fatty acid consists of a negatively charged carboxylate ion (called the head) with a long hydrocarbon chain (called the tail) and a positively charged ion.
  • In water, the sodium or potassium ions float free and do not play a part in the cleaning actions of soaps.

  • It is the negatively charged fatty acid ion which is responsible for the cleaning action.

    • The charged head is hydrophilic (water attracting) due to its polar nature
    • The tail is hydrophobic (water repelling) due to its non-polar nature

Saponification

  • Soaps are produced from the hydrolysis of fats and oils (lipids) in a basic solution such as NaOH.
  • Fats and oils are known as fatty esters, triglycerides or triesters.
  • When esters are heated in the presence of a strong base such as NaOH or KOH, the ester is broken down to give the alcohol and a carboxylate ion.
  • This reaction is known as saponification. It is a type of hydrolysis reaction, involving the breaking of a chemical bond by the addition of water

How Soaps Work

  • Soap is a surfactant (surface active agents)
  • This means it functions by reducing the surface tension of water and binding to grease and dirt to emulsifies them.
  • The hydrophobic part of the soap molecule is long, non-polar hydrocarbon chain. It is strongly repelled by water molecules.
  • When soap molecules are added into water, they form an oriented monolayer (with tails sticking out of the water) at the surface in order to satisfy the interaction of both the hydrophobic and hydrophilic portions of the soap molecule.
  • This effectively breaks the hydrogen bonding between molecules of water and thus reduces the surface tension of water.
  • The components self-assemble into the most stable arrangement, which consists of spherical structures with the carboxylate groups forming a negatively-charged spherical surface, with the hydrocarbon chains inside the core of the sphere.
  • The spheres are called micelles. The hydrophobic tails are shielded from the water by the polar heads, which minimises the repulsive forces in the system.

1. Dissolution

Soap molecules must be first dissolve in water. The hydrophilic head of a soap ion interacts with water molecules via ion-dipole interactions and hydrogen bonding.

2. Adsorption

  • Grease and oil consists of non-polar molecules.

  • The hydrophobic tails of the soap ions dissolve in the grease due to dispersion forces and orientates themselves.

  • Surfactant molecules continue to absorb into the grease, decreasing the surface tension of water at the interface between the grease and water.

  • The hydrophilic heads interacting with water via ion-dipole forces effectively pull the grease off the surface.

3. Emulsification

With agitation, the grease layer breaks into smaller, spherical droplets (micelles), with the hydrophilic surfactant head groups interacting with the water via ion-dipole forces, and the hydrophobic surfactant tails adsorbed into the grease. This forms a dispersion of grease droplets in water (an emulsion).

The negative charged heads on the soap repel each other, preventing the grease and dirt from joining together and keeping them dispersed throughout the solution. Therefore, the grease and oil can be simply rinsed away, leaving a clean surface.

Synthetic Detergents

Problems with Soaps

  • The soap ion must be dissolved in water to have a cleaning effect
  • As salts of weak acids, in low pH, soaps are converted into uncharged fatty acids

$$\begin{gather*}\orange{\ce{CH3(CH2)16COONa(aq) +HCl(aq) →CH3(CH2)16COOH(s) +NaCl(aq)}} \\ \text{Soluble fatty acid ion + acid → Insoluble fatty acid molecule + ionic salt}\end{gather*}$$

  • Soaps can also form insoluble salts (scum) in hard water (water with high concentration of divalent metal ions such as $\ce{Ca^{2+}}$ and $\ce{Mg^{2+}}$)

Synthetic Detergents

  • All such synthetic surfactants are known as detergents
  • They have a similar structure to soaps, with a long non-polar hydrocarbon tail and a polar head. However, they vary in the structure of the polar head group.

Anionic Detergent

  • All anionic detergents have a negatively charged polar head.
  • The main structural difference between anionic detergents and soap is the presence of a sulfate $(\ce{R-SO2-O-})$ or sulfonate group $(\ce{R-O-SO2-O-})$ instead of a carboxylate $(\ce{R-COO-})$.

Effectiveness in Hard or Acidic Water

  • Anionic detergents don’t form insoluble precipitates.
  • They form salts that are all soluble.
  • However, the effectiveness is still reduced in hard and acidic water.
  • The positive ions in hard water/acidic water will attract the negative head of the detergent.
  • Phosphate is an example of a builder that can be added to soften the water.

Cationic Detergents

  • All cationic detergents have a positively charged polar head. The positive head is usually a quaternary ammonium ion. They are also known as fatty amine salts.
  • Generally cationic detergents are not very good cleaning agents due to the strong attraction of cationic detergents to negatively charged surfaces (Most surfaces are negatively charged).
  • This attraction however can be beneficial in certain situations.
    • Many fabrics acquired acquire a negative charge when they become wet.
    • Cationic detergents are used in fabric softeners and hair conditioners. The strong attraction of cationic detergents to negatively charged surfaces can be detrimental in other situations.\
  • Cationic detergents are particularly toxic to microorganisms.
  • They are attracted to the negative surface of bacteria and damage or kill bacteria that are involved in their decomposition.
  • They therefore have very low biodegradability.

Non-Ionic Detergents

  • All non-ionic detergents have uncharged polar heads. The polar heads consists of polar groups such as ethoxylates.
  • Although these surfactants are uncharged, the polar head groups are still attracted to the highly polar water molecules forming numerous hydrogen bonds.
  • Non-ionic detergents don’t foam as much as other detergents, hence they can be used in dishwashers.

Environmental Impacts of Surfactants

  • Detergents are synthetic, whereas soaps are made from naturally occurring biological materials (fats and oils). Thus detergents are less biodegradable.
  • The enhanced stability of detergents means they persist in the environment, causing damage to the mucus membrane in wildlife and resulting in excessive frothing in the water ways.
  • This leads to less sunlight penetration.
  • Toxic to aquatic life.
  • Anionic detergents often contain builders such as sodium triphosphate $(\ce{Na5P3O10})$.
  • The builders react with minerals in hard water and form soluble molecules.
  • High levels of phosphates entering the rivers and waterways can lead to eutrophication (turning a lake into swamp, algae, etc.)

Polymers

  • A polymer is a long chain molecule made up of repeating units, called monomers joined by covalent bonds.

  • The process of linking monomer units is called polymerisation.

  • Polymers may be natural or synthetic:

    • Natural polymers are made by living organisms. Eg. Hair, starch, cellulose, DNA and silk
    • Synthetic polymers are manufactured. Eg. Plastics like polyethylene, polyvinyl chloride and nylon.

Addition Polymers

  • Addition polymers are polymers made by adding unsaturated molecules to each other, without the elimination of any atoms.
  • Additional polymerisation is a type of addition reaction, in which one of the bonds in the C=C double bond is broken to form two new single bonds.
  • A simple way of representing polymers is by writing the repeat units in square brackets followed by the subscript n where n is the number of monomer units in the polymer.
  • Addition polymers are all synthetic, they do not exist in nature.

Condensation Polymers

  • Condensation polymers are polymers formed through the condensation reaction of difunctional monomers with the elimination of a small molecule such as water or methanol in the process.
  • Monomers used to synthesise condensation polymers usually contain groups such as alcohol, carboxylic and amine.
  • Condensation polymers are found everywhere in nature.
  • Polysaccharides such as cellulose and starch are condensation polymers made from glucose monomers.

Polyesters

  • Polyesters are condensation polymers in which the repeating units are joined by ester links.
  • Polymers made from two different monomers are called copolymers.

Polyamides

  • Polyamides are condensation polymers in which the repeating units are joined by amide links.
  • The nylon class refers to polyamides that have linear carbon chains in the repeating units.

Properties of Polymers

  • Physical properties of polymers are important in determining their uses. These include:
    • Melting point (softening point)
    • Mechanical strength
    • Flexibility.
  • This is due to the chemical structure of the polymers which leads to different strengths of intermolecular forces that is dependent on:
    • Molecular weight or chain length
    • Extent of chain branching
    • Presence of side groups (eg. -OH)

Chain Length

  • Polymers are extremely large covalent molecules. The dominant intermolecular force is dispersion.
  • The length of a polymer (and its molecular weight) depends on the number of monomers the polymer contains.
  • The melting point, rigidity and hardness of a polymer increases with an increase in chain length.

Chain Branching

  • Polymers are able to form branched and unbranched chains.
  • Unbranched chains are able to pack more closely in an orderly fashion, forming a rigid crystalline solid.
  • Branched polymer chains are unable to align with each other, forming an amorphous solid that has weak intermolecular forces between the chain.

Crystalline and Amorphous Solids

If the polymer chains have few branches, as in the case with HDPE, the molecules can sometimes line up in a regular arrangement, creating crystalline regions. The regular arrangements brings the polymer chains closer together. The IMF between closely packed chains are stronger, and the presence of crystalline regions strengthens the material overall.

Crystalline regions in a polymer prevents the transmission of light through the material, making it appear cloudy or opaque.

Amorphous region will form where the polymer chains are randomly tangled and unable to pack very closely. In some polymer materials, the entire solid is amorphous. Amorphous polymers are usually more flexible and weaker and are often transparent. (LDPE) Increasing the percentage crystallinity of a material makes it stronger and less flexible. This also makes the material less transparent because crystalline regions scatter light. There are more crystalline regions in unbranched polymers.

There are crystalline and amorphous regions in all polymers.

Side Groups

  • Side groups make a material more rigid and brittle, resulting in harder polymers.

Cross-Linking

  • Polymer chains are held together by intermolecular forces or they can be linked by covalent bonds called cross-links to form a large extended network.
  • Polymers with only intermolecular forces between their chains are called thermoplastic polymers.
  • They soften when heated as the intermolecular forces are relatively weak and easily broken to allow the chains to move between one another.
  • This property allows polymers to be remoulded.
  • Polymers with cross-links are called thermosetting polymers.
  • Since covalent bonds are very string, cross links limit movement between polymer chains, making the polymer more rigid, hard and heat resistant.
  • These polymers cannot be remoulded (like XLDPE).

Types of Polymers

Polyethylene

  • Polyethylene (polyethene) is the most popular plastic in the world. It has a very simple structure.
  • Ethene is an unsaturated molecule because of a double carbon-carbon bond.
  • When ethene polymerases, the double bond breaks and new covalent bond are formed between carbon atoms on nearby monomers.
  • The polyethene formed does not contain any double bonds.
  • Ethene is one of the most simple and versatile monomers.
  • It is easily able to undergo addition polymerisation.

LDPE (Low-Density Polyethylene)

  • LDPE is produced under high temperatures and pressures.
  • Under these harsh conditions, the polymer is formed too rapidly for the molecules to be neat and symmetrical.
  • The products usually contain too many small chains (branches) that divide off the main polymer.
  • The molecules in the polymer cannot pack closely together thus reducing the dispersion forces.
  • The arrangement of the polymer molecules can be described as disordered or non-crystalline.
Properties of LDPE
  • Flexible, chemically inert, good elongation

  • Low melting point/thermoplastic

  • Lightweight, good puncture resistance

  • Waterproof

  • Used in: Milk carton lining, bowls, flexible water pipes, bottles

HDPE (High-Density Polyethylene)

  • Highly specialised transition metal catalyst, known as Ziegler-Natta catalysts are used to avoid the need for high pressure.
  • Due to the polymer being produced under a lower pressure, the conditions are milder and there are fewer branches.
  • The lack of branches allows the molecules to pack together tightly increasing the density and the hardness of the polymer formed.
  • The arrangement of the polymer molecules is more ordered, resulting in crystalline sections.
  • Used in: Food packaging, dustbins, crates, drums, water pipes

Polyvinyl Chloride (PVC)

  • PVC (polychloroethene) is made out of vinyl chloride monomers.
  • The chlorine atoms introduce dipoles into the long molecules.
  • This increases the IMF between molecules, which leads to a higher melting point.
  • A PVC item burning in a flame will not continue to burn when it is removed from the flame.
  • It is used in products such as conveyor belts, cordial bottles, water pipes and the covering of electrical wires.
  • Pure PVC is very hard and brittle.
  • Additives are incorporated into PVC to improve its flexibility, thermal stability and UV stability.
  • In a fire, PVC decomposes to form toxic and corrosive hydrogen chloride.

Polystyrene

  • First commercial production by IG Farben; used in disposable household products, plastic model kits, laboratory containers, insulation and packaging.
  • Benzene rings are covalently bonded to every second carbon atom in the polymer chain.
  • This causes polystyrene to be a hard but quite brittle plastic with a low density.
  • It is used to make food containers, picnic sets, refrigerator parts, and CD and DVD cases.
  • Polystyrene is made from styrene (ethylbenzene monomers).
  • Polystyrene (polyethylbenzene) is commonly manufactured as a foam.
  • Foamed polymers are formed by blowing a gas through melted polymer materials, making them 95-98% air by volume.
  • Foaming can drastically change the physical properties of a polymer material.
  • Polystyrene foam is produced by introducing pentane into melted polystyrene beads.
  • The beads swell up to produce the lightweight, insulating, shock-absorbing foam that is commonly used for takeaway hot drink containers, bean bag beans, packaging materials and safety helmet linings.
  • Once polystyrene has been converted to a foam, it is difficult to recycle.

Polytetrafluoroethylene (PTFE)

  • Polytetrafluoroethene is used in cookware fabrics, wiper blades, nail polish, industrial coatings, also known as Teflon, or Fluon.
  • Made out of tetrafluoroethene monomers.
  • Tetrafluoroethene is formed when all the hydrogen atoms in ethene are replaced by highly electronegative fluorine atoms.
  • It has quite exceptional properties that are very different from those of polyethene. It can be used to make non-stick frying pans, medical implants, gears and clothing.
  • The electronegative fluorine atoms reduce the strength of intermolecular bonds with other substances.

Properties of PTFE

  • Non-stick
  • Heat resistant
  • Chemical resistant
  • Good mechanical properties
  • Low friction coefficient
  • Flame resistant
  • High melting point

Polyethylene Terephthalate (PET)

  • Polyesters are a class of polymers that are formed through the process of condensation polymerisation.
  • Polyesters are formed by combining monomers that contain carboxylic acid and hydroxyl functional groups.
  • They are typically formed by reacting a dicarboxylic acid monomer with a diol monomer.
  • This is the most often polymer used to make polyester fabric.
  • PET is synthesised by reacting benzene-1,4-dioic acid monomers with ethane-1,2-diol monomers.
  • PET has a range of uses including recyclable drink bottles and food packaging.
  • PET is a strong material because the ester groups are polar, so that there are dipole-dipole attractions between the polymer chains.
  • Benzene rings make it stiff and strong, resistant to deformation.

Nylon

  • Nylon stockings created shopping frenzy in the USA in the 1940s.
  • It is also used in clothing, parachutes, kitchen utensils, toothbrushes, fishing lines, guitar strings, seatbelts.
  • Nylon is formed when a monomer containing an anime group on each end reacts with a monomer with a carboxyl group on each end, a polyamide can form.
  • The term ‘nylon’ refers to the group of polyamides, in which the monomers are linear carbon chains.
  • A common example is nylon-6,6 which is named so because the dicarboxylic acid monomer has a chain of 6 carbons and the diamine monomer also has a chain of 6 carbon atoms.
  • Nylon can be easily drawn into fibres that have high tensile strength.
  • These fibres are used to produce strong lightweight material for clothes.

A few years after the release of nylon, a movie called “The Man in the White Suit” came out, portraying the tale of a scientist who had invented an invincible fabric, so stain resistant it couldn’t even be dyed (hence white suit). The product is an instant hit until labour unions and corporate overlords realise that as soon as everyone has the amount they need, they will stop buying more (because it’s invincible), and so they order him to make it weaker. This is almost identical to the invention of nylon, which replaced more fragile fabrics like silk. Over the last few decades, nylon has been weakened intentionally by manufacturers so that it wears out faster than it naturally would, a technique known as “planned obsolescence”. You can read more about it here.

And that’s it! The longest module is over!

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