Correlation of the Rates of Solvolyses of Cinnamyl Bromide

摘要

Solvolytic rate constants at 25 oC are reported for solvolyses of cinnamyl bromide (1) in binary mixtures of water with acetone, ethanol, methanol, methanol-d, and 2,2,2-trifluoroethanol. Product selectivities are reported for solvolyses of 1 in aqueous ethanol and methanol. Rate ratios in solvents of the same YBr value and different nucleophilicity provide measures of the minimum extent of nucleophilic solvent assistance (e.g. [k40EW/k97TFE]Y = 2.88, EW = ethanol-water). With use of the extended Grunwald-Winstein equation, the l and m values are similar to the values of 0.43 and 0.88 obtained for the solvolyses of 1 using the equation (see below) which includes a parameter (I) for solvation of aromatic rings. The magnitude of l and m values associated with a change of solvent composition predicts the SN1 reaction mechanism rather than an SN2 channel. Product selectivities (S), defined by S = [ether product]/[alcohol product] ⅴ [water]/[alcohol solvent] are related to four rate constants for reactions involving one molecule of solvent as nucleophile and another molecule of solvent as general base catalyst. A linear relationship between 1/S and molar ratio of solvent is derived theoretically and validated experimentally for solvolyses of the above substrates from water up 75% 1/S = (kwa/kaw)([alcohol solvent]/[water]) + kww/kaw alcohol-water. The results are best explained by product formation from a “free” carbocation intermediate rather than from a solvent-separated ion pair.