Substituent effects on the spontaneous cleavage of benzyl-<i>gem</i>-dibromides in aqueous solution

Abstract

The α-bromobenzyl cations have been produced in aqueous solution by chemical initiation (solvolysis) of benzylα<i>-gem-</i>dibromides. The solvolysis reactions of benzyl<i>-gem-</i>dibromides<i> </i>in water proceed by a stepwise mechanism through α-bromobenzyl carbocation intermediates, which are captured by water to give the corresponding benzaldehydes as the sole detectable products. Rate

constant ratios <i>k</i>_Br<i>/k</i>_s<i> </i>(<i>M</i>^-1)<i> </i>for partitioning of the carbocation between reaction with bromide ion and reaction with water have been determined by analysis of bromide common ion inhibition of the solvolysis reaction. The rate constants <i>k</i>_s<i> </i>(s^-1) for the reaction of the cation with solvent water were determined from the experimental values of <i>k</i>_Br<i>/k</i>_s<i>, </i>and <i>k</i>_solv, for the solvolysis of the benzyl<i>-</i>

<i>gem-</i>dibromides,<i> </i>using <i>k</i>_Br = 5×10⁹ <i>M</i>^-1 s^-1 for diffusion-limited reaction of bromide ion with benzyl carbocations. Calculation and discussion of selectivities, <i>k</i>_Br<i>/k</i>_s , of the α-bromobenzyl cations, activation parameters for solvolysis step and for addition of water to the cation are some of the salient features not reported earlier. Effect of substituents is also discussed in terms of Hammett equation for both the steps.