Structure-Function Relationship for the R Group

As stated, Sn2 reactions are common for electron rich metals and R groups susceptible to nucleophilic attack. The general order of reactivity for R groups is similar to that found for organic mechanisms CH3 1° 2° 3°. Benzyl and allyl are particularly good. 

However, what kind of mechanism can explain mixed stereochemistry such as that given in Eq. 12.22 Two interesting examples that shed light on the mechanism are given in Eqs. 12.23 and 12.24. The reaction displayed in Eq. 12.23 yields products from both inversion and retention. Importantly, the reaction is very slow imless a radical initiator, such as benzoyl peroxide, is added, and the rate slows if a radical trap is added. Further support for radicals in some oxidative addition mechanisms can be seen in Eq. 12.24. Here, the common radical clock, cyclopropycarbinyl (see Section 8.8.8), was used to probe the possible involvement of radical intermediates. Indeed, ring opening occurs, supporting the intermediacy of radicals. 

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