Hydrogen bromide, addition to alkene

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. 

The order of reactivity of the hydrogen halides is HI > HBr > HCl, and reactions of simple alkenes with HCl are quite slow. The studies that have been applied to determining mechanistic details of hydrogen halide addition to alkenes have focused on the kinetics and stereochemistry of the reaction and on the effect of added nucleophiles. The kinetic studies often reveal complex rate expressions which demonstrate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or Itydrogen... 

In the absence of peroxides, hydrogen bromide ladds to alkenes in agreement with Markovnikov s rule in the presence of peroxides, the direction of addition is exactly reversed (see Sec. 6.7). 

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. 

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). 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

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. 

Redical Addition to Alkenes the Anti-Markovnikov Addition of Hydrogen Bromide... 

Alkenes are converted to alkyl halides by the addition of HX (HCl, HBr or HI). Addition of HX to unsymmetrical alkenes follows Markovnikov s rule. The reaction is regioselective, and occurs via the most stable carbocation intermediate. For example, addition of hydrogen bromide (HBr) to propene yields 2-bromopropane as the major product. 

The electrophilic addition of hydrogen-bearing boranes to alkenes (hydroboration), now recognized as a vital component of organic synthesis, is covered in Chapter 1.7 of this volume. The boron compounds that do fall under the realm of this chapter are boron bromides. Suzuki and coworkers have demonstrated... 

Electrophilic addition to alkenes, such as the addition of hydrogen bromide under ionic conditions, follows the Markownikoff rule which states that in the addition of HX to an alkene, the hydrogen atom (the electrophile) becomes attached to the less-substituted carbon atom. The addition of halogens such as bromine proceeds via a halonium ion, and takes place with an overall trans stereochemistry. 

RADICAL ADDITION TO ALKENES THE ANTI-MARKOVNIKOV ADDITION OF HYDROGEN BROMIDE... 

For the material presented in Table 6.1, it is generally true that, at least over some of the concentration ranges examined, the reactions appear to be first order in alkene and first order in addend. Thus, in the addition of hydrogen bromide (HBr) to 1-methylcyclohexene (in acetic acid solvent), products can be accounted for as shown in Scheme 6.30. 

Scheme 6.30. A representation of the addition of hydrogen bromide (HBr) to 1-methylcyclohexene in acetic acid (CH3CO2H), that is, under acidic conditions, showing that the solvent intrudes into the electrophihc addition reaction. The intrusion of solvent will complicate the kinetic arguments. Initially, the formation of 1-bromo-l-methylcyclohexane is first order in alkene and first order in hydrogen bromide (HBr) only at low (HBr).The kinetics become much more complicated as the reaction progresses.

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