The Ion-pair Sn2 Mechanism

0-0-3 M NaN3, McLennan (1974) found a rate-product correlation in agreement with the classical SN2 mechanism. Similar rate/product correlations were obtained when thiourea was used as nucleophile instead of azide thiourea has the advantage that salt effects can be ignored because, by control experiments, it was established that the non-nucleophilic urea had a negligible effect on the reaction rate (McLennan, 1975). 

Raaen et al. (1974) have interpreted 12C/14C kinetic isotope effects as evidence against the ion pair mechanism they also dispute an alternative interpretation by Graczyk and Taylor (1974), who used 3 5 Cl/3 7 Cl isotope effects in a study of the hydrolysis of p-methoxybenzyl chloride. 

The solvent acts as a kinetically significant nucleophile in the overall solvolysis process for many simple secondary substrates, and this appears to be the major cause of the variation in relative rates with changes in solvent (Table 2, p. 11). This conclusion is supported by the quantitative correlations discussed in Section 6. The stereochemical evidence further suggests that, even when the magnitude of nucleophilic solvent assistance is less than a rate factor of 10 at 25°, solvolyses (e.g. of cylcohexyl tosylate in formic acid) can proceed with essentially complete inversion of configuration. These results are consistent with an SN 2 mechanism and the evidence for ion pair intermediates can then be considered in one of two ways. 

The latter mechanism has been termed Sn2 (intermediate) (Bentley and Schleyer, 1976), and the intermediate [34] has been referred to as an ion-sandwich (Bordwell et al., 1975 see also, Doering and Zeiss, 1953). The former term emphasizes the Sn2 character of the reaction, whereas the latter emphasizes the ionic 

A change of mechanism from SN1 to Sn2 can be envisaged (Fig. 9) to involve an increase in the magnitude of nucleophilic solvent assistance from zero (SN 1). We suggest that solvolyses for which nucleophilic solvent assistance is greater than a rate factor of 10 (Table 2) should be classified as Sn2.s This includes many solvolyses 

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Several mechanisms satisfy the theoretical requirements of the Sn2 reaction (Fig. 3). We shall use the term classical Sn2 to refer to the reaction in which nucleophilic attack on covalent substrate leads directly to product. Two simple ion pair mechanisms will also be considered the Sn 2 (intermediate) is similar to the classical Sn2 mechanism except that the pentavalent species [5] is an intermediate rather than a transition state the ion-pair SN2 mechanism involves... 

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