Chlorine-36, -labeled deuterium chloride

The same phenomenon, i.e. that the secondary a-deuterium KIE is determined by the changes in only the shorter reacting bond in the SN2 transition state rather than by the nucleophile-leaving group distance, has been found in a completely different reaction system. Matsson, Westaway and co-workers (Matsson et al., 1996) used ku/k14 carbon incoming nucleophile, chlorine leaving group (Hill and Fry, 1962), and ku/ku a-carbon (Fry, 1970) KIEs to model the transition states for a series of SN2 reactions between p-substituted benzyl chlorides and labelled cyanide ion (reaction (17)). 

The deuterium labels are there so that we can see that the Sn2 reaction is indeed taking place. This reaction is entirely syn even though the methoxide nucleophile must attack alongside the other chlorine atom. The reaction does not stop there since a second methoxide displaces the other chloride—also in a syn fashion. Here too there must be considerable resistance to syn attack as the second methoxide anion must approach alongside the first. 

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