Hydrogen halides to alkynes

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 involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

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... 

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 ... 

Important in laboratory and industrial syntheses is the addition of alcohols, carboxylic acids, and hydrogen halides to alkynes, leading to the corresponding vinyl derivatives (Scheme 2.49). These compounds may also be regarded as the derivatives of enols and in many cases they can be more conveniently obtained from carbonyl compounds (Scheme 2.50). 

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. 

---