A Guidebook to Mechanism in Organic Chemistry

Nucleophilic Substitution at a Saturated Carbon Atom

obtained by its breakdown, but with aromatic primary amines some

stabilisation of this cation is conferred by delocalisation via the n

orbital system of the aromatic nucleus

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and diazonium salts may be obtained«from such amines provided the

conditions are mild. Such diazotisation must normally be carried

out under conditions of fairly high acidity (a) to provide a powerful

nitrosating agent, for primary aromatic amines are not very powerful

nucleophiles (due to interaction of the electron pair on nitrogen with

the it orbital system of the nucleus, p. 118) and (b) to reduce the ^

concentration of Ar-NHa by converting it to Ar-NH 3 (Ar-NH 2 is a
very much weaker base than R-NH 2 , p. 52) so as to avoid as yet
undiazotised amine undergoing azo-coupling with the first formed
Ar-N 2 (c/.p. 115).
Reaction also takes place with the secondary amines but cannot
proceed further than the N-nitroso compound, R 2 N—N=0, while
with"Tertiary aliphatic amines the readily decomposed nitroso-
e
trialkylammonium cation, R 3 N—NO is obtained. With aromatic
tertiary amines such as N-dialkylanilines, however, attack can take
place on the activated nucleus (cf. p. 106) to yield a C-nitroso
compound:
NR 2

N=0

OTHER NUCLEOPHILIC DISPLACEMENT REACTIONS

In the discussion of nucleophilic substitution at a saturated carbon
atom, the attack on halides by negatively charged ions, e.g. HO^9 and
EtOe, has been used almost exclusively to illustrate the mechanism
of the reactions involved. In fact this type of reaction is extremely