Nitrous Acid and Nitrosation

Thiourea is an efficient catalyst for the nitrosation of morpholine and the diazotization of aniline. In the former case the efficiency of thiourea SCN Br as catalysts lies in the ratio 4200 240 1. For thiourea this reflects the large equilibrium constant for the formation of the ON—S= ion from thiourea, rather than a large rate constant for the attack of the 

Some particularly interesting reactions of nitrous acid with inorganic compounds have been studied, including that with hydrazine to give dinitrogen and dinitrogen monoxide via the formation of azide. Tracer experiments with N-enriched hydrazine with excess of nitrous acid have been interpeted in terms of a cyclic azide intermediate [equations (9) and (10)]. This accounts for the production of dinitrogen with N2 and 

The yellow species formed in the reaction between sodium nitrite and thiosulfate in acidic, aqueous solution has been identified as the 5-nitrosated thiosulfate (03SSN0) , which is suggested to be formed by parallel pathways involving the nitrosonium ion and dinitrogen trioxide. The much studied reaction between nitrite and sulfite to give hydroxylamine disulfonate has received further attention and some apparent discrepancies have been clarified. For the pH range 4.5 to 7 the rate equation is given by (11). 

The term first order in [HSOJ] is usually dominant, while the [HSO3] component only becomes important at pH values greater than 6.5 and bisulfite concentrations greater than 0.1 moldm . The latter pathway is suggested to involve direct attack of nitrite on metabisulfite. 

The rate of oxidation of formic acid by nitrous acid in concentrated perchloric acid increases with [H ] until about 7.3 mol dm when there is a sharp decrease in rate with further increase in acidity. Similar behavior observed for the reaction in nitric acid has been attributed to the conversion of HNO2 into N2O4, a suggestion that now seems incorrect in light of the current work. An explanation for the results in perchloric acid is difficult to find, and it is tentatively suggested that the reactivity of formic acid toward NO is reduced by hydrogen bonding. 

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