A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems "

With most primary amines this is virtually the sole product, with

N-alkylated anilingg some coupling may also take place on the

benzene nucleus while with tertiary amines (N-dialkylanilines) only

the product coupled^n carbon is obtained:

NR 2

N=N-Ph

This difference in position of attack with primary and secondary

aromatic amines, compared with phenols;probably reflects the relative

electron-density of the various positions in the former compounds

exerting the controlling influence for, in contrast to a number of other

aromatic electrophilic substitution reactions, diazo coupling is sen­

sitive to relatively small differences in electron density (reflecting the

rather low ability as an electrophile of PhN 2 e), Similar differences in

electron-density do of course occur in phenols buttiere control over

the position of attack is exerted more by the relative strengths of the

bonds formed in the two products: in the two alternative coupled

Aromatic amines are in general somewhat less readily attacked than

phenols and coupling is often carried out in slightly acid solution,

thus ensuring a high [PhN 2 ffiJ without markedly converting the amine,

©

ArNHa, into the unreactive, protonated cation, ArNH 3 —such

aromatic amines are very weak bases (cf. p. 52). The initial diazo tisation

of aromatic primary amines is carried out in strongly acid media to

ensure that as yet unreacted amine is converted to the cation and so

prevented from coupling with the diazonium salt as it is formed.

With aromatic amines there is the possibility of attack on either

nitrogen or carbon, and, by contrast with phenols, attack takes place

largely on nitrogen in primary and secondary amines (i.e. N-

alkylanilines) to yield diazo-amino compounds: