SnI reactions in cyclic systems

We noted above that the inversion of configuration that accompanied Sn2 reactions was particularly apparent in cyclic systems, and that cis derivatives would be converted into trans products in disubsti-tuted rings, and vice versa (see Section 6.1.5). Should 

The net result is that the product mixture consists of two diastereoisomers. 

As we have just seen, SnI reactions are highly favoured at tertiary carbon, and very much disfavoured at primary carbon. This is in marked contrast to Sn2 reactions, which are highly favoured at primary carbon and not at tertiary carbon. With Sn2 reactions, consideration of steric hindrance rationalized the results observed. This leads to the generalizations for nucleophilic substitutions shown in Table 6.8, with secondary substrates being able to participate in either type of process. 

The most distinguishing feature of the SnI mechanism is the intermediate carbocation. Formation of the carbocation is the rate-determining step, and this is more favourable in polar solvents that are able to assist in facilitating the charge separa-tion/ionization. A useful, though not always exact, 

RCH2CI very slow RCH2OH hydroxide is a much better 

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