Lesson 1 of 12 · 5 min

Structure, classes and names

NCERT §9.1–9.3

On his first morning Ravi reads the drum labels in the store: aniline, N,N-dimethylaniline, ethane-1,2-diamine, triethylamine. The chemist asks him what all four have in common.

The story this chapter follows: Ravi's month at the dye works

Ravi, a Class 12 student, spends a month in the lab of a small textile dye works. The plant runs one route all month: 12.3 g of nitrobenzene is reduced to 9.30 g of aniline, diazotised in an ice bath with 6.90 g of NaNO₂, and coupled with 9.40 g of phenol to give 19.8 g of an orange azo dye. Every step of that route is amine chemistry.

The lesson in notes

In short

Amines are ammonia with one, two or all three hydrogens swapped for alkyl or aryl groups. They turn up in proteins, vitamins, alkaloids and hormones, and in synthetic polymers, dyes and drugs.

Examples from medicine: adrenaline and ephedrine, both secondary amines, raise blood pressure; novocain, a synthetic amino compound, is a dental anaesthetic; the antihistamine benadryl has a tertiary amino group. Quaternary ammonium salts serve as surfactants.

As in ammonia, the nitrogen is trivalent and keeps one unshared electron pair. Its orbitals are sp³ hybridised: three form bonds to H or C and the fourth holds the lone pair, so the molecule is pyramidal.

The lone pair squeezes the C–N–E angle (E = C or H) below 109.5°; in trimethylamine it is 108°.

Classes: replacing one H of NH₃ gives a primary (1°) amine, RNH₂ or ArNH₂; two gives a secondary (2°) amine, R–NHR′; three gives a tertiary (3°) amine. An amine is 'simple' when all its groups are the same and 'mixed' when they differ.

Common names put the alkyl group in front of 'amine' as one word (methylamine), with di- or tri- for repeated groups. IUPAC names primary amines as alkanamines by replacing the '-e' of the alkane with 'amine': CH₃NH₂ is methanamine.

With two or more –NH₂ groups, locants and di-, tri- are used and the 'e' of the hydrocarbon stays: H₂N–CH₂–CH₂–NH₂ is ethane-1,2-diamine. Groups on nitrogen get the locant N: CH₃NHCH₂CH₃ is N-methylethanamine and (CH₃CH₂)₃N is N,N-diethylethanamine.

In arylamines –NH₂ sits directly on the ring. C₆H₅NH₂ is aniline, also an accepted IUPAC name, or benzenamine.

IUPAC names for common ones: propan-1-amine (n-propylamine), propan-2-amine (isopropylamine), N-methylethanamine (ethylmethylamine), N,N-dimethylmethanamine (trimethylamine), prop-2-en-1-amine (allylamine), hexane-1,6-diamine (hexamethylenediamine), 2-methylaniline (o-toluidine), 4-bromobenzenamine (p-bromoaniline).