Markovnikov additions hydrohalogenation

Electrophilic (and Markovnikov) Additions Hydrohalogenation, Halogenation, and Hydration (Section 13-7)... 

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. 

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. 

In this reaction, a hydrogen atom and a halogen are added across an alkene. The halogen (Cl) is ultimately positioned at the more substituted carbon, which verifies that this reaction takes place via an ionic mechanism (Markovnikov addition). The ionic mechanism for hydrohalogenation has two steps (1) protonation of the alkene to form the more stable carbocation and (2) nucleophilic attack. Each step must be drawn precisely. 

The proposed mechanism is consistent with the experimental observations discussed earlier in this section. The reaction proceeds via a Markovnikov addition, just as we saw for hydrohalogenation, because there is a strong preference for the reaction to proceed via the more stable carbocation intermediate. Similarly, reaction rates for substituted alkenes can be justified by comparing the carbocation intermediates in each case. Reactions that proceed via tertiary carbocations will generally occur more rapidly than reactions that proceed via secondary carbocations. 

For unsymmetrical alkenes, the placement of the halogen represents an issue of regiochemistry. Hydrohalogenation reactions are regioselective, because the halogen is generally placed at the more substituted position, called Markovnikov addition. 

HX Hydrohalogenation When treated with HX, an alkyne undergoes Markovnikov addition (excess HX gives two addition reactions to afford a geminal dihalide). 

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