Only one alkyne forms an aldehyde when it undergoes the...

Key Concepts

Acetylene (Ethyne)

Acetylene, or ethyne, is the simplest alkyne featuring two hydrogen atoms bonded to a triple-bonded carbon framework. Its symmetric structure means that when it undergoes mercuric-ion catalyzed hydration, the resulting enol (vinyl alcohol) tautomerizes to form acetaldehyde—making acetylene unique in yielding an aldehyde rather than a ketone under these reaction conditions.

Alkyne Hydration

Alkyne hydration is a reaction where water is added across a carbon–carbon triple bond to form an enol, which subsequently undergoes tautomerization to yield a carbonyl compound. This reaction is a key method for transforming alkynes into useful functional groups, such as aldehydes or ketones, under acidic or mercuric-ion catalyzed conditions.

Mercuric-Ion Catalysis (Oxymercuration)

Mercuric-ion catalysis, also known as oxymercuration, is a reaction process that avoids the complications of carbocation rearrangements seen in acid-catalyzed hydration. In the context of alkyne hydration, the mercuric ion forms a mercurinium ion intermediate that directs the addition of water in a controlled, Markovnikov manner, ultimately leading to the formation of an enol that can tautomerize to a carbonyl compound.

Markovnikov's Rule

Markovnikov's rule predicts the regioselectivity in the addition of reagents to unsymmetrical alkenes or alkynes, with the more electropositive part of the reagent adding to the carbon atom with the most hydrogen substituents. In the hydration of alkynes, this rule normally guides water addition to the more substituted carbon, although for certain substrates like acetylene, the symmetry forces a unique register leading to an aldehyde.

Enol-Keto Tautomerism

Enol-keto tautomerism is the process by which an unstable enol form rearranges to a more stable keto (carbonyl) form. After the hydration of an alkyne produces an enol intermediate, the system spontaneously shifts to the keto form, which defines whether an aldehyde or a ketone is formed based on the structure of the starting alkyne.

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