Dual reaction channels for solvolyses of acyl chlorides in alcohol–water mixtures

Abstract

Rate constants are reported for solvolyses at 0 °C of trimethylacetyl chloride (3) in 90–30% v/v acetonitrile–, acetone–, ethanol– and methanol–water mixtures, of adamantane-1-carbonyl chloride (4) in 90–60% acetone–, ethanol– and methanol–water mixtures, and of cyclopropanecarbonyl chloride (5) in 90–40% acetone–water. Quantitative product data (acid and ester) are also reported for solvolyses of trimethylacetyl chloride in 98–20% ethanol– and methanol–water mixtures. Product selectivities (S) show maxima in 90–95% alcohol–water mixtures, similar to those, reported previously for solvolyses of p-chlorobenzoyl chloride (1a), benzoyl chloride (1b), 2,4,6-trimethylbenzenesulfonyl chloride (2a) and 4-methoxy-2,6-dimethylbenzenesulfonyl chloride. (2b). Using rate–rate profiles, logarithms of rate constants are dissected into the competing reaction channels. Given the sharp maxima in S that occur at different alcohol compositions for different substrates, and the link to ‘breaks’ in rate–rate profiles, the results are consistent with competing mechanisms having different rate-limiting steps. The competing mechanisms involve two broad reaction channels: (1) nucleophilic attack by one molecule of solvent assisted by a second molecule of solvent acting as a general base catalyst—consequently, in aqueous alcohols there are four mechanistic combinations operating simultaneously within this one reaction channel; (2) nucleophilic attack by solvent occurs via a carbocationic reaction within the S_N2-S_N1 mechanistic spectrum, involving for example, a solvent-separated ion pair intermediate (S_N1 ) or a concerted nucleophilic attack (S_N2).