Consequences of the Carbocation Intermediate
Stereochemical Effect
SN2 and E2 reactions have very stereospecific properties. They owe
this to their concerted mechanisms. SN1 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 SN2 reaction at a α-carbon stereocenter would result in inversion of
configuration, a SN1 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 SN1 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 SN2'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 SN2 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
SN1 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.
Such rearrangements greatly increase the number of products.
