Question

The final step in the hydration of an alkyne under acidic conditions is the tautomerization of an enol intermediate to give the corresponding ketone. The mechanism involves a protonation followed by a deprotonation.

Show the mechanism for each of the following tautomerizations.

Short answer

The mechanism involving the keto-enol tautomerism of the following reactions are given below.

a).

b).

c).

Step-by-step solution

Definition of tautomerization

Tautomers are structural isomers that can readily convert to each other. The process of interconversion of isomers during the reaction is referred to as tautomerization. In keto-enol tautomerism transfer of a proton takes place from the oxygen atom to the carbon atom.

Tautomerization of the given reactant

The oxygen atom donates its lone pair of electrons to carbon atom resulting in a partial positive charge on the oxygen atom. The bond attains a double bond character while the pi electrons from the C-C double bond concentrate on the adjacent carbon atom. Subsequently the adjacent carbon atom accepts the proton and the charge on the oxygen atom is neutralized to give the respective ketone.

Tautomerization of the given reactant

The oxygen atom donates its lone pair of electrons to carbon atom resulting in a partial positive charge on the oxygen atom. The C-O bond attains a double bond character while the pi electrons from the C-C double bond concentrate on the adjacent carbon atom. Subsequently the adjacent carbon atom accepts the proton and the charge on the oxygen atom is neutralized to give the respective ketone.

Tautomerization of the given reactant

The oxygen atom donates its lone pair of electrons to carbon atom resulting in a partial positive charge on the oxygen atom. The C-O bond attains a double bond character while the pi electrons from the C-C double bond concentrate on the adjacent carbon atom. Subsequently the adjacent carbon atom accepts the proton and the charge on the oxygen atom is neutralized to give the respective ketone.