Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration

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.