Topic K: Chemistry of Carbon Compounds

This part develops organic chemistry by focusing on the oxygen-containing families: alcohols, carboxylic acids and esters. It examines their functional groups, characteristic reactions and the pathways linking them, then introduces polymers, both addition polymers from alkenes and condensation polymers formed with loss of a small molecule.

Functional groups and reaction types

Organic chemistry is organised around functional groups, the reactive atoms or bonds that give each family its characteristic chemistry. The carbon-hydrogen skeleton is fairly unreactive, so reactions occur at the functional group. Key groups include the hydroxyl -OH of alcohols, the carboxyl -COOH of carboxylic acids, and the ester linkage -COO- of esters, as well as the C=C double bond of alkenes met earlier. Recognising the functional group lets you predict both physical properties, such as solubility and boiling point, and the reactions a compound will undergo. The same functional group behaves similarly across a homologous series, so learning the reactions of one member tells you about the whole family.

Alcohols and their reactions

Alcohols contain the hydroxyl group -OH and have the general formula CnH2n+1OH; methanol and ethanol are the first members. Ethanol is made industrially by hydration of ethene with an acid catalyst, or by fermentation of sugars using yeast, C6H12O6 -> 2 C2H5OH + 2 CO2. Alcohols undergo complete combustion to carbon dioxide and water, releasing energy, which makes ethanol a useful fuel. They are also oxidised by warming with acidified potassium dichromate, which turns from orange to green, converting a primary alcohol first to a carboxylic acid. Alcohols are more soluble in water than alkanes of similar size because the -OH group forms hydrogen bonds with water molecules.

Carboxylic acids

Carboxylic acids contain the carboxyl group -COOH and have the general formula CnH2n+1COOH; methanoic and ethanoic acid are the simplest, with ethanoic acid being the acid in vinegar. They are weak acids, ionising only partially in water to release H+, so they show the typical acid reactions but more gently than strong mineral acids. They react with reactive metals to give a salt and hydrogen, with carbonates to give a salt, water and carbon dioxide, and with alkalis to give a salt and water. Carboxylic acids can be made by oxidising a primary alcohol with acidified dichromate. Their salts, the carboxylates, and their reactions with alcohols to form esters are important in the wider pathway.

Esters and esterification

Esters are formed when a carboxylic acid reacts with an alcohol in the presence of a little concentrated sulfuric acid as catalyst, a reaction called esterification. The reaction is reversible and produces an ester and water, for example ethanoic acid plus ethanol gives ethyl ethanoate and water. The functional group is the ester linkage -COO-. Esters are often volatile liquids with sweet, fruity smells, which is why they are used as flavourings and in perfumes. Naming an ester takes the alkyl part from the alcohol and the -oate part from the acid. The reverse reaction, hydrolysis, splits an ester back into its acid and alcohol, completing a clear two-way pathway between the families.

Reaction pathways

The oxygen-containing families are linked by reaction pathways that the DSE expects you to trace. Starting from an alkene, addition of water gives an alcohol; oxidising that alcohol with acidified dichromate gives a carboxylic acid; and reacting the acid with another alcohol gives an ester, with water released. Reading a pathway in reverse, an ester can be hydrolysed back to its acid and alcohol. Being able to give the reagents and conditions for each step, such as acidified potassium dichromate and heat for oxidation, or concentrated sulfuric acid for esterification, is essential. Pathway questions test whether you can connect functional group changes with the correct reagents, conditions and observations across several steps.

Addition polymers

Polymers are very large molecules built by joining many small repeating units called monomers. In addition polymerisation, many unsaturated monomers containing C=C double bonds add together with no other product formed, so the whole monomer becomes part of the chain. Ethene polymerises to poly(ethene), and similar reactions give poly(propene) and poly(chloroethene) (PVC). The repeating unit is drawn by opening the double bond and showing the unit in brackets with a continuation bond. Addition polymers are unreactive and not biodegradable, which makes them durable but causes plastic waste problems. Their properties depend on the monomer used and on chain length and branching, allowing a wide range of useful plastics to be produced.

Condensation polymers

In condensation polymerisation, monomers join together with the loss of a small molecule, usually water, at each link. This requires monomers with two reactive functional groups, such as a diol with a dicarboxylic acid, or a diamine with a dicarboxylic acid. Polyesters form when acid and alcohol groups join with loss of water, creating repeated ester linkages, as in the fibre used for clothing. Polyamides such as nylon form when acid and amine groups join, creating amide linkages. Because a small molecule is lost at each join, condensation differs fundamentally from addition polymerisation, where nothing is lost. Some condensation polymers can be hydrolysed back to their monomers, which has implications for recycling and biodegradability.