Electrophilic aromatic substitution. Part 25. The nitration in aqueous sulphuric acid of some cinnamic acids and other styrene derivatives

Abstract

The rates of nitration of ethyl cinnamate, cinnamic acid, β-nitrostyrene, and trimethyl(styryl)ammonium ion in sulphuric acid of a range of concentrations have been determined. The nitrovinyl and the trimethyl(vinyl)-ammonium groups deactivate the aromatic nucleus by factors of ca. 330 and 60, respectively. The yields of products from ethyl cinnamate and β-nitrostyrene, nitrated over a range of sulphuric acid concentrations, are reported. With ethyl cinnamate the o : p-ratio falls with increasing acidity. The acidity dependences of the rates of reaction of ethyl 4-nitrocinnamate, 4-nitrocinnamic acid, ethyl 3-nitrocinnamate, and 3-nitrocinnamic acid with nitric acid in sulphuric acid show the primary reaction in each case to be attack by the nitronium ion (at the β-carbon atom). The reactions are completed by addition, giving nitroalcohols. A similar but much slower reaction occurs with β,4-dinitrostyrene. In every case the nitroalcohol decomposes in concentrated sulphuric acid to give the nitrobenzaldehyde. With absolute nitric acid, ethyl 4-nitrocinnamate gives the side-chain nitro-nitrate by formal syn-addition. Addition of nitromethane to the reaction solution leads to addition in both stereochemical modes. The nitro-nitrate gives the nitro-alcohol in sulphuric acid, but in aqueous media gives ethyl α-nitro-β-(4-nitrophenyl)acrylates. These apparent side-chain nitrations are thus addition–elimination reactions, as suggested by van der Lee. Similar processes have been detected in the cases of 4-nitrocinnamic acid and β,4-dinitrostyrene.