Overview

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.

Regiochemical Outcome

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.

Procedure

The relative rates of an acid-catalyzed hydration of ethene, propene, or 2-methylpropene, show that alkyl substituents at the double bond accelerate the rate significantly.

Here, the protonation of an alkene by a hydronium ion leads to the formation of a carbocation, which is the rate-determining step.

The reaction that proceeds through a tertiary carbocation is faster than a reaction  proceeding via a secondary or primary carbocation. The difference in the reaction rates is explained by comparing the stability of carbocations. The tertiary carbocation, being more stable than the secondary or primary carbocation, is formed faster.

The preference for the formation of a more stable carbocation also influences the regiochemical outcome of the reaction.

When the more stable carbocation is formed as an intermediate, the nucleophilic addition of water occurs at the more substituted carbon, followed by deprotonation of the oxonium ion yielding the Markovnikov's product.

Furthermore, the protonation of 1-butene yields a planar, achiral secondary carbocation having both faces equally accessible to the nucleophile.

The attack of water from the top face leads to (S)-2-butanol, while the attack from the bottom face produces (R)-2-butanol. Thus, when a new chiral center is generated, a racemic mixture is formed.

However, the addition of water to a chiral alkene forms a chiral carbocation with no plane of symmetry. The two faces of this intermediate are not equally accessible by a nucleophile due to different steric setups, and hence the diastereomeric products are formed in unequal amounts.

The utility of the acid-catalyzed hydration is limited due to the rearrangement of the carbocation intermediate. For instance, the protonation of 3-methyl-1-butene forms a secondary carbocation intermediate.

The less stable secondary carbocation rearranges to a more stable tertiary carbocation by the shift of hydrogen with its bonding pair of electrons through a 1,2-hydride shift. Thus, the nucleophilic attack by water at the tertiary carbocation with subsequent deprotonation yields the more substituted product.