The carboxylic acid family
Carboxylic acids are a homologous series with the general formula CnH2n+1COOH. Every member contains the carboxyl functional group, -COOH. The most important example is ethanoic acid, CH3COOH, the acid in vinegar. Methanoic acid (HCOOH) is the first member.
Key idea / 要点
Functional group: -COOH (carboxyl). Carboxylic acids are weak acids: in water they are only partially ionised, so they have a lower H+ concentration than a strong acid of the same concentration.
Typical acid reactions
Although weak, carboxylic acids show all the usual acid reactions:
- With a reactive metal → salt + hydrogen. e.g. magnesium gives magnesium ethanoate + H2.
- With a carbonate → salt + water + carbon dioxide (fizzing).
- With an alkali (neutralisation) → salt + water, e.g. NaOH gives sodium ethanoate + water.
The salts are named after the acid: ethanoic acid forms ethanoates. Carboxylic acids can be made by oxidising alcohols (ethanol → ethanoic acid). They also react with alcohols, in the presence of an acid catalyst, to form sweet-smelling esters plus water.
Worked example / 例题
Find the relative molecular mass of ethanoic acid, CH3COOH (Ar: H=1, C=12, O=16). It contains 2C, 4H and 2O: (2×12) + (4×1) + (2×16) = 24 + 4 + 32 = 60.
Remember / 记住
- -COOH is the carboxyl group; acids are weak (partly ionised).
- acid + carbonate → salt + water + CO2.
- acid + alcohol → ester + water.