Comments on the putative ion pair mechanism for hydrolysis of methyl halides and methyl perchlorate

Abstract

Calculations based on a linear free energy approach to the mechanism of hydrolysis of methyl halides and methyl perchlorate by Scott and Robertson suggest that rate-limiting attack of water (and other nucleophiles) on a preformed ion pair is involved. This result contrasts strongly with that of Abraham, who calculates, via a thermodynamic cycle, that the relevant ion pairs have free energies too high to allow their participation in the reactions, which are thus most likely concerted S_N2 processes, and with experimental failure to detect ion pair intermediates in systems potentially more favourable for ion pair formation. The linear free energy approach of Scott is critically examined in the light of recent work by Ritchie, and is seen to rest on false premises. The quasi-thermodynamic treatment of Robertson, Annesa, and Scott is shown to be likewise unsound. The interpretation of the secondary k_H/k_D for methyl perchlorate hydrolysis in terms of an ion-pair intermediate is shown to be unnecessary in that a looser than usual S_N2 transition state is also compatible with the results. The molecular basis of this looseness is discussed.