Alkenes with hydrogen halides, polar

We use the term substitution with scheme (138) in the sense that it is used for aromatic compounds. Addition is reserved for processes in which a saturated intermediate is formed. To observe retention, we require only that k 2) > k(3) in (138). By analogy with the SB2 reactions at a saturated carbon (Rreevoy et al., 1967), it is probable that some demetalations with acid in a polar solvent proceed in this way. Certainly, the intermediates are wholly analogous to those proposed for the isomerization, hydration, or hydrogen halide addition to alkenes. 

The interaction of HC1 with an alkene in a non-polar medium results in a simple addition of the hydrogen halide. The centre generated from the proton and monomer is immediately deactivated by the strongly basic and mobile Cl-. In a solvating medium, e.g. nitromethane, styrene oligomerizes in the presence of HC1 [91 ]. Thus the combination of the solvated ions is relatively slow so that propagation can compete with it. 

We have already suggested two reasons why the Br radical adds to the alkene with this characteristic regioselectivity, giving a primary alkyl bromide when the polar addition of HBr to an alkene would give a tertiary alkyl bromide (1) attack at the unsubstituted end of the alkene is less sterically hindered and (2) the tertiary radical thus formed is more stable than a primary radical. In fact, of all the hydrogen halides, only HBr will add to alkenes in this fashion HCI and HI will undergo only polar addition to give the tertiary alkyl halide. Why We need to be able to answer this type of question too. 

We ve already seen several methods for preparing alkyl halides, including the reactions of HX and X2 with alkenes in electrophilic addition reactions (Sections 6.8 and 7.2). The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition. Bromine and chlorine yield trans 3,2 dihalogenated addition products. 

The hydrogen halides HCl, HBr, and HI add to alkenes to give haloalkanes (alkyl halides). These additions may be carried out either with the pure reagents or in the presence of a polar solvent such as acetic acid. The addition of HCl to ethylene gives chloroethane (ethyl chloride) ... 

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