Addition of Hydrogen Halides to Alkynes

Alkynes react with many of the same electrophilic reagents that add to the carbon-carbon double bond of alkenes. Hydrogen halides, for example, add to alkynes to form alkenyl halides. 

The regioselectivity of addition follows Markovnikov s rule. A proton adds to the carbon that has the greater number of hydrogens, and halide adds to the carbon with the fewer hydrogens. 

When formulating a mechanism for the reaction of alkynes with hydrogen halides, we could propose a process analogous to that of electrophilic addition to alkenes in which the first step is formation of a carbocation and is rate-determining. The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion. 

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their involvement in these additions has been questioned. For example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the corresponding additions to alkenes, the difference is not nearly as great as the difference in carbocation stabilities would suggest. 

Furthermore, kinetic studies reveal that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third-order overall and second-order in hydrogen halide. 

It is usually possible to prepare a dihaloaUcene by simply adding one molar equivalent of the halogen  

Addition of bromine to acetylenedicarboxylic acid, for example, gives the trans isomer in 70% yield  

Alkenes are more reactive than alkynes toward addition of electrophilic reagents (i.e., Br2, CI2, or HCI). Yet when alkynes are treated with one molar equivalent of these same electrophilic reagents, it is easy to stop the addition at the alkene stage. This appears to be a paradox and yet it is not. Explain. 

The hydrogen atom of the hydrogen halide becomes attached to the carbon atom that has the greater number of hydrogen atoms. 1 -Hexyne, for example, reacts slowly with one molar equivalent of hydrogen bromide to yield 2-bromo-l-hexene and with two molar equivalents to yield 2,2-dibromohexane  

The addition of HBr to an alkyne can be facilitated by using acetyl bromide (CHsCOBr) and alumina instead of aqueous HBr. Acetyl bromide acts as an HBr precursor by reacting with the alumina to generate HBr. For example, 1-heptyne can be converted to 2-bromo-1-heptene in good yield using this method  

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Addition of hydrogen halides to alkynes preparation of alkyl dihalides and tetrahalides... 

Hydrogen halides may add to acetylenes in a similar way to afford alkenyl halides.565 The use of silica and alumina, in this case, provides a simple means for facilitating addition of hydrogen halides to alkynes that does not occur readily in solution. Arylalkylacetylenes yield the corresponding syn-addition products (45) which undergo isomerization on extended treatment ... 

The generalized scheme for the addition of hydrogen halides to alkynes in solution, as shown for the specific case of 2-butyne in Scheme 6.66, depicts a bimolecular process. Therefore, it might be expected to be, experimentally, first order each in alkyne and hydrogen halide (and thus second order overall) as shown in Equation 6.67. 

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