Elimination and addition reactions. Part XXIII. Mechanisms of elimination in nitro-compounds bearing phenoxy and phenylthio leaving groups

Abstract

Rates of elimination in 1-nitro-2-phenoxyethane and in 1-nitro-2-phenylthioethanes (PhS·CHY·CH₂·NO₂; Y = H or Ar) have been measured in ethanolic sodium ethoxide, aqueous sodium hydroxide, and aqueous triethylamine. The phenoxy-compound reacts by rate-determining loss of phenoxide from the carbanion [(E₁) anion mechanism], while for the α-arylphenylthio-compounds, ionisation is rate-determining and rate constants agree with predictions made from ionisation rates of model substrates. The simple phenylthio-compound (Y = H) undergoes elimination with a primary deuterium isotope effect of 7·4 but rate constants are larger than predicted for rate-determining ionisation and the E₂ mechanism is tentatively assigned.