Solvent effects on the rates of miscellaneous nucleophile-promoted reactions in aqueous-alcoholic solvent mixtures

Abstract

The rates of E2 elimination from DDT and S_N2 substitution on allyl bromide by alkoxide and hydroxide ions have been measured using aqueous-alcoholic solvent mixtures. The results are anomalous in terms of previous theories. The E2 hydrogen nucleophilicity order of OEt- > OMe- > OH- towards 2-phenethyl bromide in dioxan has been obtained by extrapolating rates of reaction in alkaline dioxan-alcohol or dioxan-water mixtures to 100% dioxan. The nucleophilicity order contrasts with the basicity order: OEt- > OH- > OMe- but is the same as the carbon nucleophilicity order. The validity is questioned of using similar carbon- nucleophilicity and elimination-reactivity orders as positive evidence for the E2C mechanism of eliminations promoted by weak bases which are also strong carbon nucleophiles.