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I got in Paula Bruice under the section Mechanism for Mercuric ion catalysed Hydration of Alkyne

Terminal alkynes are less reactive than internal alkynes towards addition of water. Hydration will take place if Hg²+ is added to acidic mixture.

Why are Terminal Alkynes less reactive than internal alkynes?

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The mechanism of hydration is an electrophilic addition and requires polarization of the carbon-carbon triple bond. This task is more difficult compared to case of alkene due to it having more %s character

In case of internal alkynes the carbocation intermediate formed ( since this mechanism is electrophilic addition the intermediate is a carbocation) is stabilized by the hyperconjugative effect and the inductive effect of the alpha carbons.

This support is taken away in case of terminal alkynes as their is no alpha-carbon. Now terminal alkynes can still be hydrated but only in presence of $\ce{Hg^2+}$ ion in acidic medium because of $\ce{Hg^2+}$'s high polarizing power(because it has a pseudo inert configuration). This polarization succeeds in catalyzing the reaction.

Ref.: Morrison and Boyd, sec 12.10 (hydration of alkynes)

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