Overview

Introduction

Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of alkynes produces different products such as aldehydes and ketones.       

Since the rate of acid-catalyzed hydration of alkynes is much slower than alkenes, a mercuric salt like mercuric sulfate (HgSO₄) is usually added to facilitate the reaction. Hydration of terminal alkynes follows Markovnikov's rule; however, for internal alkynes, the addition of water is non-regioselective.

Mechanism

The mechanism begins with a nucleophilic attack by the alkyne π bond on the Hg²⁺ ion resulting in the formation of a cyclic mercurinium ion intermediate. A second nucleophilic attack by water on the more substituted carbon forms an organomercuric enol that rapidly converts into a stable keto form via keto-enol tautomerism. Protonation of the keto intermediate followed by the loss of an Hg²⁺ ion yields the enol form of the product. The final step proceeds with the tautomerization of the enol to the desired ketone.

Keto-Enol Tautomerism

Unlike alkenes, acid-catalyzed hydration of alkynes is irreversible. This is because the enol intermediate formed during the hydration of alkynes is unstable and rapidly isomerizes to a more stable keto form. The chemical equilibrium that exists between the two forms is referred to as keto-enol tautomerism. Since the C=O bond is considerably stronger than the C=C bond, the equilibrium favors the keto isomer. Keto-enol tautomerism is characterized by the migration of a proton and the change in the location of a double bond.

Acid-catalyzed tautomerization is a two-step process:

Step 1: Addition of proton  across the enol double bond

Step 2: Loss of a proton to yield the keto form

Example

Acid-catalyzed hydration of 1-propyne initially forms the less stable enol isomer, propen-2-ol, which tautomerizes into a more stable keto product, propan-2-one.

Hydration of Terminal And Internal Alkynes

Acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes because they form only one final product. In contrast, unsymmetrical internal alkynes yield a mixture of products that need to be separated. This lowers the overall yield and makes the process less efficient.

Procedure

Acid-catalyzed hydration of alkenes involves the addition of water across the π bond in the presence of a strong acid, like sulfuric acid, to form alcohols.

Similarly, alkynes also undergo acid-catalyzed hydration; however, they form aldehydes and ketones instead.

For example, acetylene gives acetaldehyde, whereas other terminal alkynes form methyl ketones.

Unsymmetrical internal alkynes undergo an acid-catalyzed hydration to give a mixture of ketones; in contrast, symmetrical internal alkynes yield a single ketone.

The addition of water to terminal alkynes follows Markovnikov's rule. However, for internal alkynes the addition is non-regioselective.

Since alkynes are less reactive than alkenes towards the addition of water, a mercuric salt, like mercuric sulfate, is used as a catalyst to facilitate the reaction.

Analogous to the oxymercuration of alkenes, the mechanism of acid-catalyzed hydration of alkynes begins with the nucleophilic attack by one of the alkyne π bonds on the mercuric ion to form a bridged-mercurinium ion intermediate.

Next, water, being a nucleophile, attacks the more substituted carbon from the opposite side of the bridge and opens the ring, accounting for the observed Markovnikov's regioselectivity.

Thereafter, a proton transfer to the solvent leads to the formation of an organomercuric enol, which rapidly converts into a more stable keto tautomer via the keto-enol tautomerism.

Tautomers are constitutional isomers that interconvert and differ in the locations of a hydrogen atom and a double bond. The equilibrium favors the more stable isomer, in this case, the keto form.

Protonation of the carbonyl group of the keto tautomer forms an oxonium ion, which loses a mercuric ion to give the enol form of the final product.

The last step involves tautomerization of the enol into the desired ketone.

Lastly, acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes since they produce one final product. Unsymmetrical alkynes form a mixture of products that need to be separated, thus lowering the overall yield.