Understanding Organic chemistry: OxfordAQA IGCSE Chemistry Explained

A Cleaner Way to Read a Messy Topic

Organic chemistry OxfordAQA IGCSE candidates meet often looks, at first glance, like a long list of unrelated facts: crude oil fractions, alkanes, alkenes, polymers, alcohols, carboxylic acids, esters. Read across a French or German chemistry syllabus and you will find the same molecules, but organised around functional groups from the very start. OxfordAQA IGCSE chemistry organic chemistry takes a different route: it builds the topic outward from crude oil as a raw material, through the hydrocarbons that come from it, and only then into the alcohols and acids that sit one step further along the chain. Understanding that structure, rather than memorising each fact in isolation, is what makes IGCSE 9202 organic chemistry tractable. This deep dive covers three linked areas: carbon compounds as fuels, synthetic and naturally occurring polymers, and organic compounds and their structure and reactions.

Carbon Compounds as Fuels

Crude Oil and Fractional Distillation

Crude oil is a mixture of a very large number of compounds, most of which are hydrocarbons: molecules made up of hydrogen and carbon atoms only. The many hydrocarbons in crude oil are separated into fractions by fractional distillation. The oil is evaporated and allowed to condense at a series of different temperatures, so that each fraction collected contains molecules with a similar number of carbon atoms. You are not expected to name specific fractions or fuels, but you should be able to explain the general process happening inside a fractionating column: hot vapour rises, cools as it goes, and condenses out at different heights depending on the size (and therefore the boiling point) of the molecules.

Alkanes: The Saturated Hydrocarbons

Most of the hydrocarbons in crude oil are alkanes, a homologous series with the general formula C_nH_2n+2. In alkanes, every carbon-carbon bond is a single covalent bond, which is why they are described as saturated. You need to recognise alkanes from displayed, structural or molecular formulae, though you only need the names methane, ethane and propane.

Properties of hydrocarbons change predictably with molecule size, and this drives how they are used as fuels:

• Boiling point increases as the molecule gets longer.
• Viscosity increases as the molecule gets longer (longer hydrocarbons are thicker, more syrup-like).
• Flammability decreases as the molecule gets longer (shorter hydrocarbons ignite more easily).

Combustion Products

Complete combustion of a hydrocarbon fuel in plenty of oxygen produces carbon dioxide and water. Where combustion is incomplete, or where the fuel contains sulfur or the flame reaches a high enough temperature, other products appear: carbon monoxide, sulfur dioxide, oxides of nitrogen, and solid particulates (soot and unburnt fuel). Each of these has a distinct environmental consequence you should be able to link by name: sulfur dioxide and oxides of nitrogen cause acid rain, extra carbon dioxide contributes to climate change, and particulates cause global dimming. A common exam pattern gives you the elements present in a fuel and asks you to predict which pollutants form; work from the elements present in the fuel to the possible oxidation products, rather than trying to recall the list from memory.

Cracking

Longer hydrocarbons can be broken down (cracked) into smaller, more useful molecules by heating them to vaporise them, then passing the vapour over a hot catalyst or mixing it with steam at very high temperature so that thermal decomposition occurs. Cracking produces a mixture of alkanes and alkenes. Alkenes are unsaturated hydrocarbons, general formula C_nH_2n, containing at least one carbon-carbon double bond. You should be able to recognise alkenes by name or formula (ethene and propene are the two you need by name), and know the test for a carbon-carbon double bond: alkenes turn bromine water from orange to colourless, while alkanes do not.

Worked example: An unlabelled hydrocarbon sample decolourises bromine water rapidly. What does this show about its structure, and what is the general formula of that class of hydrocarbon? Answer: the sample contains at least one carbon-carbon double bond, so it is an alkene, general formula C_nH_2n.

Ethanol Production: Two Routes Compared

Ethanol can be manufactured in two distinct ways, and comparing them is a favourite exam format. Reacting ethene with steam, using a phosphoric acid catalyst, is a continuous process; it is efficient but depends on ethene, which comes from crude oil, a non-renewable resource. Fermenting sugar using enzymes in yeast is a batch process; it uses a renewable resource but is typically slower and needs the ethanol to be purified afterwards. You should be able to compare the two routes across raw material, type of process, rate of reaction, conditions and purity of product, since a comparison table like this is exactly how the mark scheme is usually structured.

Biofuels, including biodiesel and ethanol produced from plant material by fermentation between 20 °C and 35 °C, are presented as possible alternatives to hydrocarbon fuels; you should be able to evaluate their use of renewable resources, their impact on land use (competing with food crops) and their carbon footprint, rather than simply asserting that biofuels are automatically better.

Synthetic and Naturally Occurring Polymers

Alkenes are the starting point for addition polymerisation: many small monomer molecules join together to form one very large polymer molecule. Poly(ethene) from ethene and poly(propene) from propene are the two you should be able to represent, showing the monomer's double bond opening up to form single bonds in a long repeating chain.

The properties of a polymer depend on what it is made from and the conditions of its manufacture; low-density and high-density poly(ethene), for instance, are made using different catalysts and reaction conditions and have different properties as a result. A further distinction worth holding onto:

• Thermosoftening polymers consist of individual, tangled chains with only weak intermolecular forces between them, so they soften and melt on heating.
• Thermosetting polymers have cross-links between the chains, which hold the structure together even when heated, so they do not melt.

You should be able to explain the thermosoftening behaviour in terms of intermolecular forces between chains being overcome by heat, rather than describing it as "the plastic just melts" without a mechanism.

Disposal is the other exam angle on polymers: most are not biodegradable, meaning microbes cannot break them down, which causes problems in landfill and litter. Plastic bags made from a blend of polymer and cornstarch have been developed specifically to break down more easily, an example worth having ready if a question asks for a way manufacturers are addressing plastic waste.

Organic Compounds: Structure and Reactions

Alcohols

Alcohols contain the functional group -OH. Methanol, ethanol and propanol are the first three members of the homologous series, and you need to recognise them from displayed or structural formulae. All three: dissolve in water to give a neutral solution; react with sodium to produce hydrogen gas; burn in air; and are used as fuels and solvents. Ethanol specifically is the alcohol present in alcoholic drinks.

Ethanol can be oxidised to ethanoic acid, a carboxylic acid, either using a chemical oxidising agent or by microbial action; ethanoic acid is the acid responsible for the sourness of vinegar.

Carboxylic Acids

Carboxylic acids carry the functional group -COOH. You need methanoic, ethanoic and propanoic acid by name. Carboxylic acids: dissolve in water to give acidic solutions; react with carbonates to give carbon dioxide, along with a salt and water; and react with alcohols, in the presence of an acid catalyst, to produce esters. A key exam distinction: carboxylic acids do not ionise fully in water, which makes them weak acids, so a carboxylic acid solution has a higher pH than a strong acid of the same concentration.

Esters

Esters carry the functional group -COO-. Ethyl ethanoate, made from ethanol and ethanoic acid, is the only named ester you need to recognise:

C2H5OH + CH3COOH → CH3COOC2H5 + H2O (ethanol + ethanoic acid → ethyl ethanoate + water)

Esters are volatile, distinctively scented compounds, which is why they are used as flavourings and perfumes.

Common Mistake: Mixing Up the Functional Groups

Students under exam pressure frequently swap -OH (alcohol) with -COOH (carboxylic acid) when labelling a formula, or forget that an ester's functional group, -COO-, sits between two carbon chains rather than at the end of one. Practise identifying all three functional groups from formulae presented in random order, not always in the same alcohol-then-acid-then-ester sequence the textbook uses, since that is exactly how the exam will present them. Once each functional group is OxfordAQA IGCSE chemistry explained clearly on its own terms, telling them apart under time pressure stops being a guessing game.

Self-Check Questions

1. Give the general formula for the alkanes and for the alkenes, and state the type of bond that distinguishes them.
2. Describe the test that distinguishes an alkene from an alkane, including the observation.
3. Compare the production of ethanol from ethene with production by fermentation, across raw material and process type.
4. Explain, in terms of intermolecular forces, why a thermosoftening polymer melts on heating but a thermosetting polymer does not.
5. Write the word equation and name the products for the reaction of ethanol with ethanoic acid.

These OxfordAQA IGCSE chemistry revision notes are worth returning to whenever a past-paper question on this unit surprises you, since the underlying set of ideas is small even though the vocabulary looks large. Working through OxfordAQA IGCSE chemistry practice questions on organic chemistry after each section above, rather than only at the end, is the most reliable way to convert these OxfordAQA IGCSE chemistry notes into marks under exam conditions.