Simultaneous Substitutions

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Science  07 Aug 2009:
Vol. 325, Issue 5941, pp. 657
DOI: 10.1126/science.325_657c

There is a long-standing distinction in organic chemistry between unimolecular (SN1) and bimolecular (SN2) substitution mechanisms at carbon centers. The former involves departure of a substituent before arrival of its replacement, whereas the latter involves concomitant scission of the old bond and formation of the new one. Phan et al. explore a system in which both mechanisms appear to operate concurrently, depending on the identity of the attacking group, or nucleophile. Through careful kinetics measurements, they show that a series of benzhydryl bromides react with amines via an SN2 pathway, while a competitive SN1 pathway results in replacement of bromide with a dimethyl sulfoxide solvent molecule. Using a parameterized nucleophilicity/electrophilicity relation, the authors account for the results based on the lifetimes of benzhydrylium intermediates and the relative likelihood of having an amino or dimethyl sulfoxide molecule in the vicinity when bromide departs.

J. Am. Chem. Soc. 131, 10.1021/ja903207b (2009).

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