Nitrosation by positive nitrosating agents

For nitrosation by aqueous nitrous acid, one of the more important kinetic terms has the form of equation (19) where S can stand for a wide range of substrates. This term with S as the free amine, was first found in the 

Unfortunately, in spite of the importance of this kinetic form, the identity of the electrophile is not yet clearly established. The kinetic form (19) is consistent with nitrosation by either the nitrous acidium ion [eqn (20)] or the nitrosonium ion [eqn (21)]. 

The nitrosonium ion is a well-known chemical species and the equilibrium constant for its formation has been determined spectrophotometrically as 3 x 10-7 mol-1 dm3 (Bayliss et al., 1963).10 There is no spectroscopic evidence for the existence of the nitrous acidium ion. 

8 A number of the conjugate acids of aromatic amines also undergo diazotization with the kinetic form of equation (19) but these species are much less reactive than the free amines and are therefore excluded from the present discussion. In these reactions, the interaction of NO+ with the protonated amine appears to facilitate the proton loss. 

9 Since O nitrosation is not the rate-determining step in these reactions, the observed order with respect to nitrous acid is not necessarily the order with respect to nitrous acid in the nitrosation stage. 

---