Evidence for non-borderline behaviour in the reaction between 1-methylheptyl methanesulphonate and sodium azide in aqueous dioxan

Abstract

Rate constants and product proportions for the substitution reactions of 1-methylheptyl methanesulphonate (ROMs) in 25% dioxan–water containing varying amounts of sodium azide at constant ionic strength are reported. The first-order rate constants are linear with respect to [NaN₃], as is the product ratio [RN₃] : [ROH]. If it is assumed that the rates of the competing azide-forming and solvolysis reactions are insensitive to changes in [NaN₃] under conditions of ionic strength ‘buffering’, the rate–product correlation found is suggestive of identity of the rate-determining and product-determining steps. This is contrary to predictions based on the borderline variant of the Sneen–Larsen ‘unified’ mechanism, which involves an intimate ion-pair as a common precursor of RN₃ and ROH. A suggestion that the concerted S_N2 mechanism may be mythical is thus seen to rest on unsure ground. A totally different situation prevails when diphenylmethyl chloride in 80% acetone–water is the substrate. The first-order rate constants are insensitive to changes in [NaN₃] at constant ionic strength whilst the productforming steps clearly involve competition between azide ion and water for an intermediate.