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HX Addition to Alkynes

Addition of HX to Alkynes in Comparison to Alkenes

In contrast to alkenes, no intermediate carbocation, namely a vinyl cation, is formed in the addition of HX to alkynes because the vinyl cation is even less stable than a primary carbenium ion.

HX addition to alkenes in comparison with alkynes.

The carbenium ion in the HX addition to alkenes is stabilized by hyperconjugation and the +I effects of up to three alkyl substituents. In contrast, the vinyl cation in the HX addition to alkynes is stabilized by a maximum of one alkyl substituent. The second substituent of the positively charged carbon atom is an alkylidene group (=CRH) with an sp2 carbon atom. An sp2 carbon atom shows a higher electronegativity than an sp3 carbon atom of an alkyl substituent. An alkylidene group is therefore more electron-withdrawing and as a result cannot stabilize the positive charge as well as an alkyl group. Consequently, the stability of secondary vinyl cations is more similar to that of primary carbenium ions and the stability of primary vinyl cations to that of the methyl carbenium ion.

Stability of several alkyl and vinyl cations

Relative stability of carbenium ions.

Therefore, it is unlikely that a vinyl cation is an intermediate in the HX addition to alkynes. In an alternative mechanism, a cyclic intermediate stage similar to that in the halogenation of alkenes has been proposed, .

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