Chapter 9: Alkynes

Summary

• Alkynes can be hydrated to form enols that immediately tautomerise to ketones
• Reagents:  aq. acid, most commonly H₂SO₄, with a mercury salt
• Regioselectivity predicted by Markovnikov's rule (review)
• Reaction proceeds via protonation to give the more stable carbocation intermediate (review).
• Not stereoselective since reactions proceeds via planar carbocation.

• Hydration of alkenes

MECHANISM FOR REACTION OF ALKYNES WITH H3O+
Step 1: An acid / base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
Step 2: Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion.
Step 3: An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst forming an unstable enol.
Step 4: 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 5: 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.