Carboxylic acids
The −COOH group, reactions of ethanoic acid, and ester formation.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.7.1Describe the formation of ethanoic acid by oxidation of ethanol and its reactions as a weak acid.Supplement
5 minute read
Carboxylic acids
Carboxylic acids carry the −COOH functional group and behave chemically as acids, which means most of what you already know about acid reactions (from Topic 7) transfers directly — you just need to recognise ethanoic acid as "an acid that happens to be organic."
Making ethanoic acid
Ethanoic acid forms by the oxidation of ethanol, either using an oxidising agent (such as acidified potassium dichromate) in the lab, or slowly and naturally when ethanol is exposed to air (microorganisms/bacteria oxidise it) — which is why wine left open turns sour as it slowly becomes vinegar.
Ethanoic acid is a weak acid
Unlike strong acids such as hydrochloric acid, ethanoic acid is a weak acid: it only partially dissociates into ions in aqueous solution (CH₃COOH ⇌ CH₃COO⁻ + H⁺), so a solution of ethanoic acid has a higher pH (is less acidic) than a strong acid of the same concentration, and reacts more slowly with, e.g., magnesium or carbonates.
Typical acid reactions
Carboxylic acids undergo the standard acid reactions: with metals → salt + hydrogen (e.g. with magnesium → magnesium ethanoate + hydrogen); with carbonates → salt + water + carbon dioxide; with alkalis → salt + water (neutralisation). The salts formed are named "-anoate", e.g. sodium ethanoate.
Esterification
Carboxylic acids react with alcohols, warmed with a concentrated sulfuric acid catalyst, to form an ester and water — e.g. ethanoic acid + ethanol → ethyl ethanoate + water. Esters are typically sweet-smelling and used in flavourings and perfumes.
Think of it like this
Ethanoic acid is a weak acid the way a half-open tap is a weak flow: the "water" (H⁺ ions) is available, but only a fraction is let through at once, compared with a strong acid where the tap is fully open (fully dissociated).
Worked examples
Method, step by step
Propanoic acid reacts with methanol in the presence of concentrated sulfuric acid. Name the organic product and write a word equation.
- 1This is an esterification reaction between an acid and an alcohol.
- 2The alcohol's name (methan-) becomes "methyl"; the acid's name (propanoic) becomes "propanoate".
- 3Word equation: propanoic acid + methanol → methyl propanoate + water.
Methyl propanoate; propanoic acid + methanol → methyl propanoate + water.
Common misconceptions
- Assuming "weak acid" means "dilute acid" — weak refers to the *degree of dissociation* (only partly ionises), while dilute refers to *concentration*; a concentrated weak acid and a dilute strong acid are entirely different things.
- Forgetting esterification needs a catalyst and heat — simply mixing an acid and an alcohol at room temperature does not form an ester at a useful rate.
In the exam
- When comparing a weak and a strong acid of equal concentration, three comparisons are commonly tested: pH (weak acid has a higher pH), rate of reaction with a metal/carbonate (weak acid reacts more slowly), and electrical conductivity (weak acid conducts less, fewer ions present).
- Ester names swap the order compared with acid + alcohol: "ethanoic acid + ethanol" gives "ethyl ethanoate" — the alcohol part becomes the first word (with -yl) and the acid part becomes the second (with -oate).