Question

Predict the initial and final products for the following reaction involving 2-pentyne. Hint: the products are isomers and in equilibrium with each other.

Answer 1

Concepts and reason

Hydration is the process of adding water to unsaturated compounds like alkenes or alkynes to form alcohol. Addition of water to an alkyne can be done by using a strong acid in a mercury catalyst, which then forms enols that undergo tautomerism to form ketone. Mercury is used to avoid the formation of less stable carbocation from an alkyne.

Fundamentals

The mechanism of mercury-catalyzed addition of water to an alkyne:-

Step1: Formation of intermediate called mercurinium ion.

Step2: Ring-opening reaction by the nucleophilic attack of water to form an oxonium ion.

Step3: Formation of enol through deprotonation and demercuration.

Step4: Enol to keto conversion - enol acts as a nucleophile; it reacts with an electrophile at α-carbon, which has an acidic oxygen atom $\left( {{\rm{O - H}}} \right)$ . The hydrogen present at the acidic oxygen atom acts as a bond donor and acceptor.

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