Alkynes are less reactive towards electrophylic addition than alkenes. However, in case of catalytic hydrogenation, alkynes are said to be more reactive. Why? Also, if I want to add H+ as an electrophile, then will alkenes be still more reactive than alkynes?

Does the reason lie in the mechanism of both the reactions (that was obvious, I guess)? If so, please explain, I cannot seem to justify for the later one.


The first stage of electophilic addition is addition of a cation to alkene/alkyne with formation of a carbocations. Vinyl carbocations are much less stable then alkyl cations, so the path is strongly hindered for alkynes.

The first stage of catalitic hydrogenation is addtion of hydride-ion on the metal-surface to unsaturated hydrocarbon, forming a carbanion bound to metal surface. Vinyl-anion are more stable than alkyl-anyons for the same reasons vinyl-cations are less stable than vinyl-cation. Acetilnes also are much less hindered sterically than alkenes and adsorb on a metal surface much easier.

Please, note, than nearby substituents may affect these properties dramatically. Deactivation of alkenes in electrophilic addition by strong $\pi$-electron-withdrawing groups is well known.


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