Hydrogen halide addition to alkynes

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

The most reactive alkenes in electrophilic addition are the most thermodynamically stable. This is because they also have the lowest activation energy when forming carbocations. Hydrogen halides add to alkynes in nearly the same way they add to alkenes. 

Hydrogen halide addition to alkenes and alkynes yields organohalides ... 

FIGURE 10.62 Hydrogen halides add to alkynes as well as to alkenes. Addition is mostly anti. 

When 2 moles of a hydrogen halide add to an alkyne, the second mole usually adds with the same orientation as the first. This consistent orientation leads to a geminal dihalide. For example, a double Markovnikov addition of HBr to pent-l-yne gives 2,2-dibromopentane. 

In the absence of a catalyst, alkynes react very slowly with bromine. Scheme 3.28 particularly when compared to alkenes. When a choice exists, bromine reacts preferentially with an alkene rather than an alkyne. It is possible that radical reactions play a more important role in the addition to alkynes. When the reaction of acetylene with chlorine is catalysed by iron(lll) chloride, the reaction is fast and 1,1,2,2-tetrachloroethane is formed. The uncatalysed addition of a hydrogen halide gives a tram alkenyl halide. Further addition is restricted but can give rise to dihalides. 

Hydrohalogenation (Section 10.9) An electrophilic addition of hydrogen halide (HX) to an alkene or alkyne. 

Also mentioned above, alkynes react very quickly and to completion with hydrogen halides. Addition is anti, and follows the Markovnikov Rule. 

A. General Reaction Equation for Addition to Alkynes Alkynes add hydrogen, hydrogen halides, and halogens (chlorine... 

FIGURE 9.5 (a). Curved arrow notation and (b) transition-state representation for electrophilic addition of a hydrogen halide HX to an alkyne. 

Besides the addition of halides and hydrogen-halide adds to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels-Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures). 

Hydrogenolysis (Section 28.7) A reaction that cleaves a o bond using H2 in the presence of a metal catalyst, a Hydrogens (Section 23.1) The hydrogen atoms on the carbon bonded to the carbonyl carbon atom (the a carbon). Hydrohalogenation (Section 10.9) An electrophilic addition of hydrogen halide (HX) to an alkene or alkyne. 

We have encountered haloalkenes—alkenyl halides—as intermediates in both the preparation of alkynes by dehydrohalogenation and also the addition to alkynes of hydrogen halides. Alkenyl halides have become increasingly important as synthetic intermediates in recent years as a result of developments in organometalUc chemistry. These systems do not, however, follow the mechanisms familiar to us from our survey of the haloalkanes (Chapters 6 and 7). This section discusses their reactivity. 

---