Ch 9: Alkynes + H3O+

Chapter 9: Alkynes

Hydration of Alkynes

Reaction type: Electrophilic Addition

Summary

Alkynes can be hydrated to form enols that immediately tautomerise to ketones
Reagents: aq. acid, most commonly H₂SO₄, typically with a mercury salt (catalyst)
The reaction without added mercury is slow
Regioselectivitypredicted by Markovnikov's rule (review)
Reaction proceeds as if via protonation to give the more stable carbocation intermediate (review).
The Hg²⁺ salt acts as a catalyst (however, for the sake of simplicity, Hg2+ is not shown in the mechanism below)

Related reactions

MECHANISM FOR REACTION OF ALKYNES WITH H₃O⁺

Step 1:
Simultaneous acid / base reaction and reaction with the nucleophile.
Protonation of the alkyne (the π electrons act pairs as a Lewis base) with Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion.

Step 2:
An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst forming an unstable enol.

Step 3:
An acid / base reaction. Reprotonation by the acid catalyst occurs on the carbon. The oxygen atom electrons help facilitate this process generating an oxonium ion.

Step 4:
Another acid / base reaction. Deprotonation of the oxonium ion creates the ketone. Steps 4 and 5 show the acid catalysed tautomerisation of the enol to the ketone.