Electrophilic addition reactions of alkynes

The alkynes behave in many ways as if they were doubly unsaturated alkenes. For example, bromine adds to ethyne in two stages —first to give trans-1,2-dibromoethene by antarafacial addition, and finally to give 1,1,2,2,-tetra-bromoethane  

Likewise, anhydrous hydrogen fluoride adds first to give fluoroethene and ultimately to give 1,1-diflu oroethane  

Exercise 10-19 The ethenyl carbocation, CH2=CH , apparently is formed much more easily by addition of a proton from HCI to ethyne than it is by SN1 reactions of ethenyl chloride. Deduce from bond energies why this should be so. 

Alkynes, unlike alkenes, are not hydrated readily in aqueous acid unless a mercuric salt is present as a catalyst. Also, the products that are isolated are either aldehydes or ketones instead of alcohols. Even though the addition of one molecule of water to ethyne probably gives ethenol (vinyl alcohol) initially, this compound is unstable relative to its structural isomer (ethanal) and rapidly rearranges  

Similarly, addition of water to propyne leads to 2-propanone by way of its unstable isomer, 2-propenol  

As a center of high electron density, the triple bond is readily attacked by electrophiles. This section describes the resnlts of three such processes addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov rule is followed in transformations of terminal alkynes The electrophile adds to the terminal (less snbstituted) carbon atom. 

Addition of hydrogen halides forms haloalkenes and geminal dihaloalkanes 

The addition of hydrogen bromide to 2-bntyne yields (Z)-2-bromobntene. The mechanism is analogons to that of hydrogen halide addition to an alkene (Section 12-3). 

The stereochemistry of this type of addition is typically anti, particularly when excess halide ion is used. A second molecule of hydrogen bromide may also add, with regioselec-tivity that follows Markovnikov s rule, giving the product with both bromine atoms bound to the same carbon, a geminal dihaloalkane. 

The addition of hydrogen halides to terminal afkynes also proceeds in accord with the Markovnikov rule. 

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