Alkene hydrohalogenation

Select the Radical alkene hydrohalogenation reaction. The animation shows initiation steps and propagation steps for the overall process. Write out the steps of the reaction and indicate which are initiation steps and which are propagation steps. 

Alkene hydrohalogenation (Section 9.2) Addition of HX to a It bond, giving an alkyl halide. 

In each mechanism above, the first step involves protonation of the alkene to form a carbocation. Then, in both cases, a nucleophile (either X or H2O) attacks the car-bocation to give a product. The difference between these two reactions is in the nature of the product. The first reaction above (hydrohalogenation) gives a product that is neutral (no charge). However, the second reaction above (hydration) produced a charged species. Therefore, one more step is necessary at the end of the hydration reaction— we must get rid of the positive charge. To do this, we simply deprotonate ... 

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. 

Like the hydrohalogenation reaction, the hydration of unsymmetrlcal alkenes, such as propene, leads to two products. As well as propan-2-ol, propan-l-ol is also formed. 

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. 

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. 

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... 

Alkenes also undergo a variety of other addition reactions in which a reagent is added across the double bond. Hydration and hydrohalogenation are classic examples. 

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) ... 

Problem 6.23 Arrange the following alkenes in order of increasing reactivity on addition of hydrohalogen acids a) H2C=CH2, (b) (CH3)2C=CH2, (r) CH3CH=CHCH,. ... 

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. 

Many hydrohalogenation reactions begin with an achiral reactant and form an achiral product. For example, the addition of HBr to cyclohexene, an achiral alkene. forms bromocyclohexane. an achiral alkyl halide. 

Because addition converts sp hybridized carbons to s[ hybridized carbons, however, sometimes new stereogenic centers are formed from hydrohalogenation. For example, Markovnikov addition of HCl to 2-ethyl-1-pentene, an achiral alkene, forms one constitutional isomer, 3-chloro-3-methylhexane. Because this product now has a stereogenic center at one of the newly formed sp hybridized carbons, an equal amount of two enantiomers—a racemic mixture—must form. 

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

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. 

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