Organic Chemistry: Sn1E1 Reactions: Consequences of the Carbocation Intermediate

Stereochemical Effect

S_N2 and E2 reactions have very stereospecific properties. They owe this to their concerted mechanisms. S_N1 and E1 reactions are not concerted. They share a common carbocation intermediate. The carbocation intermediate ruins the stereospecificity of the reaction.

In the bimolecular reactions, electrons flow into the σ^* C-LG antibond. The antibond points only directly opposite the C-LG bond. Since electrons can only come in from one direction, the bimolecular reactions are stereospecific.

On the other hand, the carbocation has no σ^* C-LG antibond. Instead, the carbocation loses its original shape to become planar. In this conformation electron density can be donated from either side of the carbocation

While a S_N2 reaction at a α-carbon stereocenter would result in inversion of configuration, a S_N1 reaction on a similar stereocenter gives an equal mix of inversion and retention. This effect results in a racemic mixture.

The E1 reaction undergoes a similar effect, though it does retain Saytzeff's rule. Saytzeff's rule is founded on energetic stability, and thus is not affected by the random geometry introduced by the carbocation.

Solvent Effect

Polar, protic solvents favor S_N1 and E1 reactions. The polar and protic properties of the solvent stabilize the carbocation and solvate the leaving group.

This may seem at odds with S_N2's increased reactivity in polar, aprotic solvents. Protic solvents blunt the nucleophile. Since nucleophilicity is a key part of the reaction's rate- limiting transition state, protic solvents slow down S_N2 reactions. For the unimolecular reactions, however, the rate-limiting step does not involve the solvent's nucleophilicity. Thus polar, protic solvents accelerate unimolecular reactions.

Carbocation Rearrangement and Stability

Carbocations are most stable next to electron donating groups. Alkanes are slightly electron donating. This explains why S_N1 and E1 reactions need a secondary or tertiary α-carbon.

The carbocation-like transition state of the tertiary α-carbon is more stable than that of the secondary α-carbon, and so on. Increased stability of the rate-limiting transition state increases the rate of the reaction.

Some secondary α-carbocations take stability into their own hands and rearrange to form tertiary carbocations.