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HX Addition to Alkynes

Reactivity and Reactions of Alkynes

Because of the similar electronic structure of alkenes and alkynes, they show, as expected, similar chemical reactivity. Alkynes participate in the same addition reactions as alkenes. However, some differences can be observed.

Alkynes are electron-rich nucleophiles with a cylindrical π electron cloud formed of two π bonds around a carbon-carbon σ bond. An electrophilic reagent can therefore easily react with the relatively weak alkyne π bond. In the addition of hydrogen chloride to an alkyne, for example, one of the π bonds in a nucleophilic reaction attacks the partially positively charged, electrophilic hydrogen atom of hydrogen chloride. The intermediate vinyl cation rapidly reacts with the chloride anion.

Fig.1
Fig.2

Primary vinyl cations are less stable than primary carbenium ions. Therefore, the mechanism shown is unlikely, particularly for acetylene. Alternatively, one could propose an intermediate π complex analogous to the cyclic bromonium ion in the addition reaction of bromine to a double bond. A back-side attack of the nucleophilic chloride anion on this π complex would result in the expected addition product.

Fig.3
Addition of HCl to alkynes: Mechanism with an intermediate π complex

The intermediate formation of a π complex in the addition reaction of electrophilic reagents like HCl to alkynes is substantiated by the fact that rearrangement products are hardly observed and the reaction proceeds stereoselectively. In the addition of HCl to 2-butyne, for example, (Z)-2-chloro-2-butyne mainly is formed.

Fig.4
Stereoselective addition of HCl to 2-butyne

Electrophiles can be added to alkynes in the same way as to alkenes. The second π bond of alkynes causes some complications but it also enables new synthesis pathways not possible with alkenes.

Exercise: reactivity and reactions of alkynes

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