Gas-Phase Nucleophilic and Elimination Reactions in Simple Alkyl Nitrates

Abstract

There has been increasing interest in the gas-phase reactivity of alkyl nitrates because of their well-known applications as explosives and because of their role in atmospheric and in marine processes. This manuscript describes an experimental study by FT-ICR techniques of the gas-phase reactions of OH(-) and F(-) with methyl and ethyl nitrate. For methyl nitrate, the main reaction channel is found to be an elimination process promoted by abstraction of an α proton from the methyl group. Nucleophilic displacement of nitrate anion through an S(N)2 process at the carbon center is also found to be an important reaction channel with methyl nitrate. In ethyl nitrate, formation of NO(3)(-) is greatly enhanced and this is attributed to the ease of an E2-type elimination process promoted by proton abstraction at the β position of the ethyl group. Theoretical calculations at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d) level of theory are consistent with the relative importance of the reaction channels and suggest that these reactions proceed through a double well potential. The calculations also predict that nucleophilic attack by OH(-) at the nitrogen center (Sn2@N) is energetically the preferred pathway but experiments with (18)OH(-) showed no evidence for this channel. Single-point calculations reveal a strong preference for approach to the carbon center and may explain the lack of reactivity at the nitrogen center. Calculations were also carried out for NH(2)(-) and SH(-) to establish the reactivity pattern to provide a better understanding of environmentally relevant nitrate esters.