HBr, addition to alkenes

You knowr the mechanism of HBr addition to alkenes, and you know the effects of various substituent groups on aromatic substitution. Use this knowledge to predict which of the following two alkenes reacts faster with HBr. Explain your answer by drawing resonance structures of the carbocation intermediates. 

As we saw in the previous section, Markovnikov s rule tells us to place the H on the less substituted carbon, and to place the X on the more substituted carbon. The rule is named after Vladimir Markovnikov, a Russian chemist, who first showed the regiochemical preference of HBr additions to alkenes. When Markovnikov recognized this pattern in the late 19th century, he stated the rule in terms of the placement of the proton (specifically, that the proton will end up on the less substituted carbon atom). Now that we understand the reason for the regiochemical preference (carbocation stability), we can state Markovnikov s rule in a way that more accurately reflects the underlying principle The regiochemistry will be determined by the preference for the reaction to proceed via the more stable carbocation intermediate. 

The stereochemistry of HBr addition to alkenes has been thoroughly studied. The reaction of cis- or fnwu-2-butene with DBr in DO Ac gives a mixture consisting of 60% threo and 40% erythro products.116 While cyclohexene117 and alkene (l)118 undergo predominant anti addition, the alkenes (4 -(6) give... 

The reversal of orientation of HBr addition to alkenes in the presence of peroxides. A free-radical mechanism is responsible for the peroxide effect, (p. 336)... 

In this chapter, we begin with the structure and physical properties of haloalkanes. We then study radical halogenation of alkanes as a vehicle to introduce an important type of reaction mechanism, namely the mechanism of radical chain reactions. Reactions of oxygen with alkenes and a radical mechanism for HBr addition to alkenes complete the chapter. 

HBr Addition to Alkenes Under Radical Conditions Non-Markovnikov addition of... 

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... 

We can gain a general understanding of the mechanism of hydrogen halide addition to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5.12 we pointed out that carbocations are the conjugate acids of alkenes. Therefore, strong acids such as HCI, HBr, and HI can protonate the double bond of an alkene to fonn a car bocation. 

Anhydrous HBr is available in cylinders (6.8-kg and 68-kg capacity) under its own vapour pressure (24 atm at 25°C) and in lecture bottles (450-g capacity). Its main industrial use is in the manufacture of inorganic bromides and the synthesis of alkyl bromides either from alcohols or by direct addition to alkenes. HBr also catalyses numerous organic reactions. Aqueous HBr (48% and 62%) is available as a corrosive pale-yellow liquid in drums or in large tank trailers (15 0001 and 38 0001). 

Use Markovnikov s rule to predict the products of HBr addition to the alkenes shown above. 

Problem 6.31 Compare and explain the relative rates of addition to alkenes (reactivities) of HCl, HBr and HI. 

Problem 9.25 Although cyclopropanes are less reactive than alkenes, they undergo similar addition reactions, (fl) Account for this by geometry and orbital overlap, (b) How does HBr addition to 1,1-dimethylcyclopropane resemble Markovnikov addition <... 

Detailed wide-ranging studies are available on the addition of HC1 and HBr to alkenes. The most useful procedure is to react dry HC1 gas and the alkene neat or in an inert organic solvent. Water or acetic acid may also be used. Alkenes yielding tertiary or benzylic alkyl chlorides react most readily. Styrene, however, adds HC1 only at — 80°C to give a-chloroethylbenzene without polymerization.101 At more elevated (room) temperature polymerization prevails. HBr adds to alkenes in an exothermic process more rapidly than does HC1. Rearrangements may occur during addition indicating the involvement of a carbocation intermediate 102... 

In the presence of added light, heat, or peroxides, HBr addition to an alkene forms radical intermediates, and like other radical reactions, proceeds by a mechanism with three distinct parts initiation, propagation, and termination. Mechanism 15.4 is written for the reaction of CH3CH=CH2 with HBr and ROOR to form CH3CH2CH2Br. 

Both propagation steps for the addition of HBr are exothermic, so propagation is exothermic (energetically favorable) overall. For the addition of HCl or HI, however, one of the chain-propagating steps is quite endothermic, and thus too difficult to be part of a repeating chain mechanism. Thus, HBr adds to alkenes under radical conditions, but HCl and HI do not. 

In contrast to HBr, the acids HC1 and HI do not undergo free-radical addition to alkenes, even in the presence of peroxides or O2. Abstraction of H- from HC1 is too endothermic, and addition of I- to an alkene is too endothermic. However, thiols (RSH) add to alkenes by a free-radical mechanism exactly analogous to the addition of HBr. The initiator is usually AIBN or (BzO)2- The alkene may be electron-rich or electron-poor. Note that the conjugate addition of thiols to electron-poor alkenes can occur either by a free-radical mechanism or by a polar, nucleophilic mechanism. 

Hydrogen bromide in the presence of peroxides can add to an unsymmetrical alkene resulting in anti-Markovnikov products. The change in trend can be explained based on the mechanistic difference of HBr addition in the presence of peroxides. Peroxides can easily form free radicals, since the oxygen-oxygen bond in peroxides is weak. This type of addition is not seen with HCl or HI. The mechanism of HBr addition to an alkene in the presence of a peroxide is shown below. 

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