Chemistry, Third edition

AMINES 341

Amines

The formulae for primary amines are of the form R–NH 2 , where –NH 2 is the amino

group. Examples of primary amines are

CH 3 NH 2 methylamine

CH 3 CH 2 NH 2 ethylamine

CH 3 CH 2 CH 2 NH 2 propylamine

H 2 NCH 2 CH 2 CH 2 NH 2 propan-1,3-diamine

Secondary amines have the general formula

R

\

CH 3

\

NH e.g. dimethylamine NH

R

/

CH 3

/

Tertiary amines have the structure

R

\

CH 3

\

R—N e.g. trimethylamineCH 3 —N

R

/

CH 3

/

Physical properties of amines

1.Amines have very offensive fishy smells (trimethylamine smells like rotting

salmon!). Amines occur in decomposing protein material. The amines

NH 2 (CH 2 ) 4 NH 2 and NH 2 (CH 2 ) 5 NH 2 , putrescine and cadaverine, are found in

decaying animal flesh.

2.Primary and secondary amines have N–H bonds and therefore undergo hydro-

gen bonding with each other and with water. Tertiary amines cannot hydrogen

bond with each other, but the lone pair of electrons present on the nitrogen atom

enables them to hydrogen bond with water. Amines with low molecular mass are

therefore miscible with water:

Basic properties of amines

An unshared pair of electrons on the nitrogen of an amine can accept a proton from

an acid. The product is an amine salt:

CH 3 NH 2 HCl\==\CH 3 NH 3 , Cl

methyl ammonium

chloride

In this reversible reaction, the amine is acting as a weak base. This reaction can be

likened to the reaction of ammonia with hydrogen chloride. However, amines tend

18.5