8.6: Regioselectividad y estereoquímica de la hidratación catalizada por ácido
The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
The reaction proceeds with the slow protonation of an alkene by a hydronium ion to form a carbocation, which is the rate-determining step.
The reaction involving a tertiary carbocation intermediate is faster than a reaction proceeding through a secondary or primary carbocation. This can be justified by comparing their relative stabilities and the delocalization of the positive charge. Tertiary carbocations are the most stable and thus formed faster.
The formation of a stable carbocation intermediate determines the regiochemical outcome as it directs the nucleophilic addition of water to the more substituted carbon following Markovnikov's orientation.
Stereochemical Outcome of Achiral Alkenes
For an achiral alkene such as 1-butene, protonation results in a secondary carbocation.
The trivalent carbon is sp2-hybridized with a plane of symmetry. It can react with water either from the top or the bottom face with equal probability.
The reaction from the top face leads to (S)-2-butanol, while the reaction from the bottom face leads to (R)-2-butanol. Thus, the formation of a new chiral center leads to a racemic mixture of enantiomeric products.
Stereochemical Outcome of a Chiral Alkene
The protonation of a chiral alkene forms a chiral carbocation with no plane of symmetry. The carbocation does not react equally from the top and bottom faces because one of the faces is more accessible than the other due to different steric setups, leading to a mixture of R and S products. Thus, two diastereomeric products are produced in unequal amounts, and the mixture is optically active.
Rearrangement of a Carbocation
In some cases, the carbocation formed in the first step can rearrange to a more stable carbocation. For example, the protonation of 3-methyl-1-butene forms a 2° carbocation intermediate, which rearranges to a more stable 3° carbocation via a 1,2-hydride shift.