Hydrohalogenation, of alkynes

In the previous chapter, we saw that alkenes will react with HX via a Markovnikov addition, thereby installing a halogen at the more substituted position  

A similar Markovnikov addition is observed when alkynes are treated with HX  

Once again, the halogen is installed at the more substituted position. 

When the starting alkyne is treated with excess HX, two successive addition reactions occur, producing a geminal dihalide  

In Section 9.3, we proposed the following two-step mechanism for HX addition to alkenes  

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With a terminal alkyne, both H atoms bond to the terminal carbon that is, the hydrohalogenation of alkynes follows Markovnikov s rule. 

One possible mechanism for the hydrohalogenation of alkynes involves a vinylic carbocation, while another possible mechanism is termolecular. 

Hydrohalogenation of alkenes and alkynes. In the presence of A1203 or Si02, hydrogen halides or their precursors add to alkenes or alkynes at useful rates and often in quantitative yield.1... 

The methods for the synthesis of alkynes have been extensively reviewed in the past 20 years two books, which deal particularly with the preparative aspects of alkyne chemistry, have been published. Except for the syntheses of acetylene and propyne, which are prepared in technical processes from carbides, from methane by oxidation, or by electric arc processes, all carbon-carbon triple bonds must be generated by an elimination reaction. Again, as in the synthesis of alkenes, the most important is the de-hydrohalogenation. 

Hydrogen chloride does not add easily to alkenes at a preparatively useful rate and reacts even slower with alkynes. It appeared that the presence of silicagel or alumina appreciably facilitates hydrohalogenation of alkenes and alkynes when the reaction is performed in methylene chloride. Initially, a product of suprafacial addition is formed, which equilibrates to the thermodynamic E/Z-equilibrium. For instance. 

According to this mechanism, the catalytic and selective formation of 1-halo-1,3-dienes could be performed through a hydrohalogenative dimerization of alkynes in one step with the same catalytic system. Dienylchlorides could be obtained in 1,2-dichloroethane (DCE) by direct addition of HCl or with a hydrohalogenative system consisting of separate proton and halide sources [5] [Eqs. (2) and (3)]. 

Alkynes undergo hydrohalogenation, the addition of hydrogen halides, HX (X = Cl, Br. 1). 

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

Whereas alkanes undergo substitution reactions, alkenes and alkynes undergo addition reactions. The principal addition reactions of the unsaturated hydrocarbons are halogenation, hydration, hydrohalogenation, and hydrogenation. Polymers can be made from alkenes or substituted alkenes. 

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. 

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