A Guidebook to Mechanism in Organic Chemistry

Nitrosation of Amines

The leaving group in an Swl reaction determines the reaction rate;
the lower the energy of the C-leaving group bond and the greater the
tendency of the leaving group to form an anion, the more readily the
reaction will proceed via the SN\ mechanism.

NITROSATION OF AMINES
In the examples considered to date it is carbon that has been under­
going nucleophilic attack but similar attack may also take place on
nitrogen as, for example, in the flitrosation of amines where the amine
acts as the nucleophile:

H

I A

R*rN=0

H >X

In the familiar reaction of primary amines with nitrites and acid, the

species that is acting as the effective nitrosating agent has been shown

to depend on the conditions though it is apparently never HNOjtVself.

Thus at low acidity N 203 (X=ONO) obtained by

2HN0 2 ^ ONO—NO + HaO

is thought to be the effective nitrosating agent while as the acidity

9

increases it is first protonated nitrous acid, H 2 0—NO(X=H 2 0 )

and finally the nitrosonium ion ®NO (cf. p. 106), though nitrosyl

halides, e.g. NOCI, also play a part in the presence of halogen acids.

Though the latter are more powerful nitrosating agents than N 203 ,

the reaction with aliphatic amines is nevertheless inhibited by increas­

ing acidity as the nucleophilic R • NH 2 is a relatively strong base which

is progressively converted into the unreactive cation, R- NH 3.

With aliphatic primary amines the carbonium ion obtained by

breakdown of the highly unstable R-N 2 ® can lead to the formation of

a wide range of ultimate products (cf. p. 85). The instability of the

diazonium cation is due to the very great stability of the N 2 that may be

H

I®

->- RN—N=0

H Xe

— H®

H

R-N—N=0

H®

-H.O

R'N=N—OH

V