Alkyl nitrites as nitrosating agents. Kinetics and mechanism of the reactions of propyl nitrite in propan-1-ol

Abstract

Propyl nitrite in acid solution in propan-1-ol octs as an effective nitrosating (or diazotising) agent towards aniline derivatives only in the presence of halide ion or thiourea as catalysts. In the absence of such catalysts the reaction is extremely slow, which indicates that the alkyl nitrite or its protonated form is a very poor nitrosating agent towards aniline derivatives under these conditions. At low concentrations of added nucleophiles the rate constant is approximately proportional to the concentration, but at higher concentrations the rate constant levels off and tends towards being independent of the nucleophile concentration. For aniline and N-methylaniline there is no acid catalysis whilst the diazotisation of p-nitroaniline is subject to acid catalysis. All the experimental findings are consistent with a mechanism involving rapid equilibrium formation of the corresponding nitrosyl halide or nitrosothiourea adduct, which attacks the free base form of the aniline derivative in the rate-limiting stage. The N-nitrosation stage is itself reversible where the denitrosation of the primary (except for N-methylaniline)N-nitroso-intermediate competes favourably with proton loss etc. which leads to the diazonium ion formation. This accounts for the rate constant dependence upon [halide ion].