Effect of the R Group Structure on SN2 Reactions

The R group of R-LG has a very large effect on the rates of and the interplay between Sn2 and SnI reactions. One needs to focus upon several issues related to the structure of the R group to understand substitution reactions—namely, sterics, electronics, and adjacent groups. For simplicity, we split this discussion into a focus upon Sn2 reactions and then SnI reactions. However, keep in mind that there is a continuum of reactivity with varying decreases of nucleophilic attack and leaving group departure. 

Average Relative Rates of Sn2 Substitution Reactions on Various R-X Species  

In the case of adjacent groups, these effects are even more severe. For example, in the extreme, a neopentyl halide achieves a transition state where the nucleophile and leaving group are proximal to a t-butyl group. This leads to a substantial bending of the trajectory of the nucleophile away from the optimal linear approach, and therefore creates a very strained transition state. 

Transition state orbital picture with an empty k orbital that can accept electrons from a developing a C-Nuc bond 

Alternatively, the rate acceleration found for allyl and benzyl groups in Sn2 reactions can be viewed as an inductive effect. The electron withdrawing nature of the sp hybridized carbons of a vinyl or phenyl group makes the carbon more electrophilic, and therefore more reactive toward nucleophilic attack. Both effects are likely involved. 

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