Stereochemistry of SN1 Reactions Ion Pairs

The contact ion pair R R2HC LBr , in contrast to a free carbenium ion R R2HC , is chiral. Starting from enantiomerically pure (R)-2-bromooctane, the contact ion pair first produced is also a pure enantiomer. In this ion pair, the bromide ion adjacent to the carbenium ion center partially protects one side of the carbenium ion from the reaction by the nucleophile. Consequently, the nucleophile preferentially reacts from the side that hes opposite the bromide ion. Thus, the solvolysis product in which the configuration at the reacting C atom has been inverted is the major product. To a minor extent the solvolysis product with retention of configuration at the reacting C atom is formed. 

It was actually found that 83% of the solvolysis product was formed with inversion of configuration and 17% with retention. This result is equivalent to the occurrence of 66% inver- 

The reaction proceeds with 66% inversion of configuration and 34% racemization. 

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