Hydrohalogenation of alkenes

The mechanism for the acid-catalysed hydration reaction is very similar to that for the hydrohalogenation of alkenes and also proceeds via a carbocatlon intermediate. It is outlined below using water and propene. 

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

Special reagents introduced more recently allow significant improvements in hydrohalogenation of alkenes. HC1, HBr, and HI in aqueous-organic two-phase systems under phase-transfer conditions readily add to alkenes affording halides in nearly quantitative yields.123 Appropriately prepared silica and alumina have been found to mediate the addition of HC1, HBr, and HI to alkenes.124 The method is very convenient since it uses hydrogen halides prepared in situ 124 125... 

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... 

Hydrohalogenation of alkenes is taking place in reaction with HC186 or SOCl287 supported on silica (equation 3) ... 

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. 

Epoxides can be synthesized from internal nucleophilic substitution of halo-hydrins, which can be derived from hydrohalogenation of alkenes. 

Decomposes upon irradiation with UV, initiate free radical polymerisations and other radical reactions. A classic example of a radical reaction that can be initiated by AIBN is the an/i-Markovnikov hydrohalogenation of alkenes. 

Anti-MarkovnikofT hydrohalogenation of alkenes can be achieved by reaction of the organopentafluorosilicate adducts (62) with cupric halides, molecular halogens, or iV-bromosuccinimide, to produce chlorides or bromides (Scheme... 

The most direct approach to the four-membered S-ring is cycloaddition of sulfenes to alkenes. The sulfenes are generated in situ by base-induced de-hydrohalogenation of sulfonyl chlorides. Because of their special susceptibility to cycloaddition reactions, research into the chemistry of sulfenes is expanding. 

Hydrohalogenation of an alkene to form an alkyl halide is the reverse of the dehydrohalogenation of an alkyl halide to form an alkene, a reaction discussed in detail in Sections 8.4 and 8.5. 

Alkenes are prepared from alcohols either by direct dehydration or by de-hydrohalogenation of intermediate alkyl halides to avoid rearrangement we often select dehydrohalogenation of halides even though this route involves an extra step. (Or, sometimes better, we use elimination from alkyl sulfonates.)... 

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. 

Write equations predicting the products of addition reactions of alkenes hydrogenation, haiogenation, hydration, and hydrohalogenation. 

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