8.6 : 酸催化水合的区域选择性和立体化学

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.