Free-Radical Addition of HBr to Alkenes

Propagation A radical reacts to generate another radical. 

Step 1 A bromine radical adds to the double bond to generate an alkyl radical on the more substituted carbon atom. 

Step 2 The alkyl radical abstracts a hydrogen atom from HBr to generate the product and a bromine radical. 

The bromine radical generated in Step 2 goes on to react in Step 1, continuing the chain. EXAMPLE Free-radical addition of HBr to propene. 

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THE PEROXIDE EFFECT. FREE RADICAL ADDITION OF HBr TO ALKENES... 

Mechanism 8-2 Ionic Addition of HX to an Alkene 332 Mechanism 8-3 Free-Radical Addition of HBr to Alkenes 334 8-4 Addition of Water Hydration of Alkenes 337... 

The peroxide effect. Free radical addition of HBr to alkenes... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

Anti-Markovnikov free-radical-induced addition of HBr to alkenes can be prevented by carrying out the reaction in the presence of small amounts of antioxidants that inhibit the reaction of oxygen with the alkene to form peroxides. 

In Section 8-3B, we saw the effect of peroxides on the addition of HBr to alkenes. Peroxides catalyze a free-radical chain reaction that adds HBr across the double bond of an alkene in the anti-Markovnikov sense. A similar reaction occurs with alkynes, with HBr adding with anti-Markovnikov orientation. 

The anti-Markovnikov addition of HBr to alkenes was probably the first free-radical addition reaction to be discovered. The discovery was inadvertent around the turn of the twentieth century, scientists studying the regiochemistry of addition of HBr to alkenes found that the proportion of Markovnikov to anti-Markovnikov addition products varied inexplicably from run to run. Eventually, it was discovered that impurities such as O2 and peroxides greatly increased the amount of anti-Markovnikov addition product. The results were later explained by a free-radical addition mechanism. The anti-Markovnikov regiochemistry derives from the addi-ton of the Br- radical to the less substituted C of the alkene (steric reasons) to give the lower energy, more substituted radical (electronic reasons). In a polar reaction, Br- would add to the more substituted C of the alkene. 

The reaction with HBr is also significant in terms of regiochemistry. The reaction results in the anti-Markovnikov orientation, with the bromine adding to the less-substituted carbon of the double bond. The anti-Markovnikov addition of HBr to alkenes was one of the earliest free radical reactions to be put on a firm mechanistic basis. In the presence of a suitable initiator, such as a peroxide, a radical chain mechanism becomes competitive with the ionic mechanism for addition of HBr. 

In a second example, addition of hydrogen bromide converts 2-butene, which is achiral, to 2-bromobutane, which is chiral. But, as before, the product is racemic because both enantiomers are formed at equal rates. This is true regardless of whether the starting alkene is cis- or trans-2-hvXtnt or whether the mechanism is electrophilic addition or free-radical addition of HBr. 

The reversal of orientation in the presence of p)eroxides is called the peroxide effect. It occurs only with the addition of HBr to alkenes. The reaction of an alkyl radical with HCl is strongly endothermic, so the free-radical chain reaction is not effective for the addition of HCl. 

Addition of HBr to alkenes in the presence of peroxides can lead to non-Markovnikov addition because under these conditions, a free-radical chain mechanism operates. 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

Note that both mechanisms for the addition of HBr to an alkene (with and without peroxides) follow our extended statement of Markovnikov s rule In both cases, the electrophile adds to the less substituted end of the double bond to give the more stable intermediate, either a carbocation or a free radical. In the ionic reaction, the electrophile is H+. In the peroxide-catalyzed free-radical reaction, Br is the electrophile. 

Similarly, the reaction of an iodine atom with an alkene is strongly endothermic, and the free-radical addition of HI is not observed. Only HBr hasjust the right reactivity for each step of the free-radical chain reaction to take place. 

Hydrogen halides also add to alkynes. The addition of HBr to alkjmes can be difficult to interpret because (as with alkenes) both ionic and free radical mechanisms may occur, and the free radical process can be difficult to suppress. Reaction of HBr with propjme (63) in the liquid phase at —78°C led to the formation of (Z)-l-bromopropene (64, equation 9.63), indicating stereoselective anti addition. When the reaction was carried out at room temperature, however, a mixture of Z (64) and (65) isomers was obtained (equation 9.64). The results suggested that addition of a bromine atom to propyne produces the vinyl radical 66, which abstracts a hydrogen from HBr to produce 64 at -78°C but which can isomerize (with Eg > 17kcal/mol) to the radical 67 at room temperature. 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

The recognition of anti-Markownikoff orientation when HBr was added to alkenes in the presence of traces of peroxides or air lead to the discovery of the large and important class of free radical addition reactions to unsaturated systems89). The ant Markownikoff orientation of these reactions i.e., the preference of initial radical at-... 

Problem 6.39 Suggest a chain-propagating free-radical mechanism for addition of HBr in which Br- attacks the alkene to form the more stable carbon radical. M... 

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

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