Formulas, functional groups and the naming of organic compounds

Chemistry A2 · An introduction to A Level organic chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The functional group of a compound decides how it reacts and affects its physical properties. Compounds with the same functional group react in the same way. The A Level course adds arenes, halogenoarenes, phenols, acyl chlorides, amides, secondary and tertiary amines and amino acids.

In a skeletal formula every line end and every corner is a carbon atom. Hydrogen atoms on carbon are not drawn, so add enough to give each carbon four bonds.

To name a compound, find the main functional group and the longest carbon chain that contains it, then number so that the groups get the lowest numbers. In an aromatic compound the ring carbon that carries the main group is carbon 1.

Rules to remember

Acyl chloride, RCOCl: name ends in -oyl chloride, e.g. CH₃COCl is ethanoyl chloride. The COCl carbon is carbon 1.
Amide, RCONH₂: name ends in -amide. A group on the nitrogen is shown with N-, e.g. CH₃CONHCH₃ is N-methylethanamide.
Ester, RCOOR′: the alkyl group R′ (from the alcohol) comes first, then the -oate part (from the acid), e.g. HCOOCH₂CH₃ is ethyl methanoate.
Ring positions: carbon 1 has the main group (–OH in phenol, –COOH in benzoic acid, –NH₂ in phenylamine); position 2 is next to it, 3 is one further on, 4 is directly opposite.
Each group on a benzene ring replaces one H: C₆H₅– with one group, C₆H₄ with two, C₆H₃ with three.
Amino acids are named as amino-substituted acids, e.g. H₂NCH₂COOH is 2-aminoethanoic acid.
General formula of a homologous series: alkyl group + functional group, e.g. acyl chlorides C_nH_2n+1COCl.

Common mistake

Naming an ester the wrong way round. The part joined to the single-bonded O comes from the alcohol and gives the first word; the part with C=O comes from the acid and gives the -oate word.

Worked example

A benzene ring carries an –NH₂ group. A bromine atom is on the ring carbon directly opposite the –NH₂ group. What are the name and molecular formula of the compound?

The parent compound is phenylamine, so the carbon with –NH₂ is carbon 1.
Directly opposite carbon 1 is carbon 4, so the name is 4-bromophenylamine.
The ring has two groups, so it keeps 4 H atoms: C₆H₄, plus Br, plus NH₂ (2 more H atoms).
Count: C 6; H 4 + 2 = 6; Br 1; N 1.

Answer: 4-bromophenylamine, C₆H₆BrN

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