Formulae, functional groups and the naming of organic compounds

A-level organic chemistry introduces several functional groups beyond the AS set. Be able to recognise and name each:

• Arene — contains a benzene ring (e.g. benzene, methylbenzene).
• Phenol — –OH bonded directly to a benzene ring (C₆H₅OH).
• Acyl chloride — –COCl (e.g. ethanoyl chloride, CH₃COCl).
• Amide — –CONH₂ (e.g. ethanamide, CH₃CONH₂).
• Nitrile — –C≡N (e.g. propanenitrile, CH₃CH₂CN).
• Amine — –NH₂ (e.g. ethylamine, C₂H₅NH₂).
• Ester — –COO– (e.g. methyl ethanoate).

The arenes have the general formula $C_nH_{2n-6}$ for a single ring (so methylbenzene is C₇H₈).

For a single substituent, the benzene ring is the parent and no number is needed: nitrobenzene (C₆H₅NO₂), methylbenzene (C₆H₅CH₃), chlorobenzene, phenol, phenylamine.

For two or more substituents, number the ring carbons to give the lowest set of locants, and list the substituents alphabetically. For example, a benzene ring with a methyl on C1 and a nitro on C2 is 1-methyl-2-nitrobenzene.

When a higher-priority functional group is present, it becomes the parent and takes the lowest locant — e.g. a ring with –COOH and –OH at the 4-position is 4-hydroxybenzoic acid (the acid outranks the phenol).

Many A-level molecules contain more than one functional group. The highest-priority group takes the suffix (and the lowest locant); the others become prefixes (substituents).

A useful order (high → low): carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol/phenol > amine.

Worked. HOOC–C₆H₄–OH (a benzene ring with –COOH and –OH at the 4-position): the carboxylic acid is the principal group, so the parent is benzoic acid (–COOH at C1) and the –OH becomes a 'hydroxy' prefix → 4-hydroxybenzoic acid.

Also keep the five formula representations clear: empirical (simplest ratio), molecular (actual atoms), structural (CH₃CH₂OH), displayed (every bond), and skeletal (lines, H on C implied).