Alcohols and Carboxylic Acids: GCSE Chemistry Guide

Alcohols contain the hydroxyl functional group (–OH) and carboxylic acids contain the carboxyl group (–COOH). Both form homologous series with gradually changing properties. They are connected by oxidation — ethanol can be oxidised to ethanoic acid — and by esterification, the reaction of an alcohol with a carboxylic acid to form an ester and water.

What are alcohols?

Alcohols are a homologous series of organic compounds, each containing one or more hydroxyl (–OH) functional groups. The general formula for simple alcohols is CₙH₂ₙ₊₁OH.

The first four members of the series are:

Methanol is highly toxic — even small amounts can cause blindness or death. Ethanol is the alcohol in alcoholic drinks, but can also be used as a fuel and is produced industrially by fermentation of sugars.

What are the key reactions of alcohols?

1. Combustion

Alcohols burn in oxygen to produce carbon dioxide and water (complete combustion):

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Ethanol burns with a pale blue flame and is used as a biofuel in Brazil and in racing cars.

2. Oxidation to carboxylic acids

Alcohols can be oxidised to form carboxylic acids. In everyday life this occurs when wine is left exposed to air — bacteria oxidise ethanol to ethanoic acid (acetic acid), making the wine taste sour:

C₂H₅OH + O₂ → CH₃COOH + H₂O (ethanol → ethanoic acid)

In the laboratory, oxidising agents such as acidified potassium dichromate(VI) carry out this reaction; the solution changes from orange to green as the dichromate is reduced.

3. Reaction with sodium

Alcohols react with sodium metal to produce a sodium alkoxide salt and hydrogen gas — the reaction is similar to water with sodium, but less vigorous:

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

What are carboxylic acids?

Carboxylic acids contain the carboxyl functional group (–COOH). The general formula is CₙH₂ₙ₊₁COOH for simple carboxylic acids.

Carboxylic acids are weak acids — they only partially dissociate in water. This means a solution of ethanoic acid has a higher pH (less acidic) than the same concentration of hydrochloric acid, a strong acid that fully dissociates.

What are the key reactions of carboxylic acids?

1. With carbonates (neutralisation)

Carboxylic acids react with sodium carbonate to produce a salt, water, and carbon dioxide:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

The fizzing confirms CO₂ is released and is a test for a carboxylic acid.

2. With alcohols — esterification

Carboxylic acids react with alcohols in the presence of an acid catalyst (typically concentrated sulfuric acid) to form an ester and water:

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

(ethanoic acid + ethanol → ethyl ethanoate + water)

The reaction is reversible (note the ⇌ sign), so the yield of ester is less than 100%. Esters have distinctive, often pleasant fruity or floral smells and are used in perfumes, food flavourings, and plasticisers.

How are esters named?

The ester's name comes from its two parent compounds:

1. The alcohol part gives the first word: ethanol → ethyl
2. The acid part gives the second word, with "-anoic acid" replaced by "-anoate": ethanoic acid → ethanoate

So: ethanol + ethanoic acid → ethyl ethanoate And: methanol + butanoic acid → methyl butanoate (smells like pineapple)

Frequently asked questions

What is the difference between ethanol and ethanoic acid?

Ethanol (C₂H₅OH) is an alcohol — it contains a hydroxyl (–OH) group and is the compound found in alcoholic drinks. Ethanoic acid (CH₃COOH) is a carboxylic acid — it contains a carboxyl (–COOH) group and is the main component of vinegar. Ethanol can be converted to ethanoic acid by oxidation: bacteria or acidified potassium dichromate(VI) remove two hydrogen atoms, converting –OH into –COOH. The two compounds belong to different homologous series and have different chemical properties.

Why are carboxylic acids weak acids?

A strong acid, such as hydrochloric acid (HCl), fully dissociates in water — every molecule releases an H⁺ ion, so the solution contains very high concentrations of H⁺. A weak acid, such as ethanoic acid (CH₃COOH), only partially dissociates in water — in a typical solution, less than 1% of molecules release their H⁺ at any one time. The rest remain as intact acid molecules. This means a given concentration of a weak acid produces a lower concentration of H⁺ than the same concentration of a strong acid, giving a higher pH (less acidic).

What is esterification and how do you carry it out in the lab?

Esterification is the reaction between a carboxylic acid and an alcohol to form an ester and water, in the presence of a concentrated acid catalyst (usually concentrated sulfuric acid, which acts as a catalyst only). In the laboratory, the acid and alcohol are warmed together with a few drops of concentrated sulfuric acid under reflux. The product ester is separated by distillation. Because the reaction is reversible, the yield is typically 50–70% under simple conditions; adding excess of one reagent or removing the water as it forms can shift the equilibrium to increase yield.

What are esters used for?

Esters are used widely because of their pleasant fragrances and because they are generally safe, low-toxicity solvents. They are found in artificial food flavourings — many fruit-flavoured sweets contain methyl and ethyl esters — and in perfumes. Esters are also used as plasticisers (added to PVC to make it flexible), as solvents in glues and nail-varnish remover (ethyl ethanoate), and biodiesel is a mixture of esters produced by reacting vegetable oils with methanol.

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