Hydrogen bromide addition

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

The addition of S—H compounds to alkenes by a radical-chain mechanism is a quite general and efficient reaction. The mechanism is analogous to that for hydrogen bromide addition. The energetics of both the hydrogen abstraction and addition steps are favorable. Entries 16 and 17 in Scheme 12.5 are examples. 

The preferred stereochemistry of addition to cyclic alkenes is anti The additions are not as highly stereoselective as hydrogen bromide addition, however. 

To understand the reason for the pronounced selectivity in the orientation of addition of electrophiles, it will help to consider one example, hydrogen bromide addition to 2-methylpropene. Two different carbocation intermediates could be formed by attachment of a proton to one or the other of the doublebond carbons ... 

Detailed studies of systems involving aluminium-based Lewis acids and hydrogen halides are scarce. Fontana and Kidder investigated the polymerisation of propene initiated by the pair aluminium bromideTiydrt n bromide. The cocatalytic role of the latter acid was clearly proved since no polymerisation could be detected in its absence. The dependence of the rate of polymerisation upon the cocatalyst concentration and the induction periods observed make this system similar to those in which stannic chloride induces the polymerisation of olefins in the presence of variable quantities of water (see Sect. IV-C-3-b). With relatively large quantities of added hydrogen bromide, addition of this acid to the mcmomer to give fso-propyl bromide must have constituted an important side reaction. 

Besides the ionic mechanism for hydrogen bromide addition, there is an efficient free-radical mechanism. As emphasized in Part A, Section 12.4, this mechanism results in regioselectivity which is opposite to that of the ionic mechanism. 

FIGURE 10.9 A first-guess mechanism for the reaction of an alkene and bromine. For analogy, it draws on the mechanism of hydrogen bromide addition to an alkene. 

FIGURE 11.24 Both hydrogen chloride and hydrogen iodide addition always follow the Markovnikov rule, but hydrogen bromide addition is strangely capricious. 

Hydrogen bromide addition to alkenes is only one example of a vast number of radical chain reactions. Another example involves carbon tetrahalides, such as CCI4 and CBr4, which add to alkenes in the presence of initiators. The steps in the peroxide-initiated reaction are shown in Figure 11.32. 

PROBLEM 11.42 Predict the products of hydrogen bromide addition to 1-butyne. Consider both ionic (a) and radical-initiated (b) reactions. Rationahze your predictions with arrow formalism descriptions. Remember that a second molecule of hydrogen bromide can add to the initial product(s) in both cases. 

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