Stereochemistry of the SN1 Reactions

Stereochemistry of the S_N1 Reaction

– The S_N2 reaction is stereospecific: the nucleophile attacks from the back side of the electrophilic carbon atom, giving inversion of configuration.

– In contrast, the S_N1 reaction is not stereospecific.

– In the S_N1 mechanism, the carbo-cation intermediate is sp² hybridized and planar.

– A nucleophile can attack the carbocation from either face.

– The following Figure shows the S_N1 solvolysis of a chiral compound, (S)-3-bromo-2,3-dimethylpentane, in ethanol.

– The carbocation is planar and achiral; attack from both faces gives both enantiomers of the product.

– Such a process, giving both enantiomers of the product (whether or not the two enantiomers are produced in equal amounts), is called racemization.

– The product is either racemic or at least less optically pure than the starting material.

– If a nucleophile attacks the carbocation in the previous Figure from the front side (the side the leaving group left), the product molecule shows retention of configuration.

– Attack from the back side gives a product molecule showing inversion of configuration.

– Racemization is simply a combination of retention and inversion.

– When racemization occurs, the product is rarely completely racemic, however; there is often more inversion than retention of configuration.

– As the leaving group leaves, it partially blocks the front side of the carbocation.

– The back side is unhindered, so attack is more likely there.

– The following Figure  shows a cyclic case where one of the faces of a cyclopentane ring has been (labeled) by a deuterium atom.

– Deuterium has the same size and shape as hydrogen and it undergoes the same reactions.

– It distinguishes between the two faces of the ring: the bromine atom is cis to the deuterium in the reactant, so the nucleophile is cis to the deuterium in the retention product.

– The nucleophile is trans to the deuterium in the inversion product.

– The product mixture contains both cis and trans isomers, with the trans isomer slightly favored because the leaving group hinders approach of the nucleophilic solvent from the front side.

MECHANISM: Racemization in the S_N1 Reaction

The S_N1 reaction involves ionization to a flat carbocation, which can be attacked from either side.

Step 1: Ionization of a tetrahedral carbon gives a flat carbocation

Step 2: A nucleophile may attack either side of the carbocation.

These two products may be different if the carbon atom is stereogenic.