This lesson provides an in-depth discussion of the stereochemical outcomes in an S_N1 reaction.

In the first step of an S_N1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.

In the formed carbocation, the positively charged carbon is sp² hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the carbocation is formed, making it achiral. Thus, a nucleophile can approach this symmetrical carbocation from either side with equal likelihood and rate.

The frontside attack leads to the retention of configuration, while a backside attack yields to an inversion of configuration in the product. However, in an achiral substrate, with either mode of attack, no difference in the product’s configuration is observed; in a chiral substrate, an optically inactive racemic mixture is expected.

Nevertheless, an enantiomeric excess with a predominantly inverted product is often observed because complete racemization cannot be achieved due to the ionization step. Upon ionization, the ions remain loosely associated, forming an intimate ion pair. During this period, the anion shields the carbocation from the frontside attack until they diffuse apart. Thus, the nucleophile is more prone to attack the unhindered backside resulting in products with an inversion of configuration. Once fully dissociated, both sides of the carbocation are available for substitution, and a racemic mixture of products is obtained. Thus, overall a net excess of inverted product is observed in an S_N1 reaction.