Which of the following on hydration will give an aldehyde? (a) HC ≡C-CH3 (b) CH3 CH2-C ≡C-CH3 (c

step 1 As we all know that C2 is the only alkyne which can give an aldehyde an aldehyde on hydration in

Which of the following on hydration will give an aldehyde?...

Which of the following on hydration will give an aldehyde? (a) $\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH}_{3}$ (b) $\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}$ (c) $\mathrm{HC} \equiv \mathrm{CH}$ (d) $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \equiv \mathrm{CH}$

Which of the following on hydration will give an aldehyde?
(a) $\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH}_{3}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}$
(c) $\mathrm{HC} \equiv \mathrm{CH}$
(d) $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \equiv \mathrm{CH}$

Key Concepts

Alkyne Hydration Reaction

This reaction involves the addition of water across the carbon–carbon triple bond of an alkyne, typically in the presence of a catalyst such as mercuric sulfate. The process initially produces an enol intermediate, which then undergoes rapid rearrangement (tautomerization) to form a carbonyl compound.

Markovnikov Addition

Markovnikov’s rule in hydration reactions states that when an unsymmetrical alkyne reacts with water, the hydroxyl group attaches to the more substituted carbon atom. This rule is key in determining the regiochemistry of the product, affecting whether a ketone or an aldehyde is ultimately formed.

Enol-Keto Tautomerization

After the Markovnikov addition of water to an alkyne, the initial enol intermediate is not usually isolated. Instead, it rapidly shifts through tautomerization to its more stable keto form. This process is essential because it finalizes the transformation from an alkyne intermediate to a carbonyl compound such as an aldehyde or ketone.

Regiochemistry in Hydration Reactions (Terminal vs Internal Alkynes)

The nature of the substituents attached to the alkyne’s carbons plays a crucial role in determining the final product. In terminal alkynes, if the carbon atom with fewer substituents (bearing a hydrogen) is involved, the hydration can yield an aldehyde. In contrast, more substituted (internal) alkynes typically give rise to ketones. This difference is pivotal when predicting the outcome of the hydration reaction.

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