The kinetic importance of nitrous acid

Nitrous acid can have both a catalytic and an anticatalytic effect on aromatic nitration, the former being appropriate in dilute (ca. 6 M) nitric acid solutions and the latter appropriate to more concentrated solutions. 

Ingold et al. ° showed that nitrous acid retards both zeroth- and first-order 

The effect of nitrous acid on nitration in nitromethane and acetic acid is also attributed to the effect of nitrate ions even though the ionisation of the dinitrogen tetroxide is much less in these solvents. As noted above (p. 31), the anticatalytic effect of nitrous acid is not governed by k x = a+6[HN03] at nitrous acid concentrations above 0.1 M. 

In sulphuric acid, nitrous acid has a catalytic effect since dinitrogen tetroxide reacts with sulphuric acid to give nitronium ions according to equilibrium (39)102. 

Amines and phenols can be nitrated by dilute ( 5 M) solutions of nitric acid in water or acetic acid provided nitrous acid is present1 °3. A mixture of products is frequently obtained and the ortho para ratio can vary widely according to the nitrous acid concentration104 105, thereby indicating a different electrophile under these conditions. 

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