Hydrogen bromide, anti-Markovnikov addition alkenes

When alkenes containing peroxides or hydroperoxides react with hydrogen bromide, anti-Markovnikov addition of HBr occurs. 

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

Exceptions to the Markovnikov rule when hydrogen bromide reacts with unsym-metric alkenes have long been known.117,118 The reaction for this anti-Markovnikov addition was explained as being a chain reaction with the involvement of bromine atoms influenced by the presence of peroxides.119-121 Both added peroxides and peroxides formed by the action of oxygen (air) on the alkene are effective. 

The regioselectivity of addition of hydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical chain reaction is readily initiated by peroxidic impurities or by light and leads to the anti Markovnikov addition product. The mechanism of this reaction is considered more fully in Chapter 11. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of a radical inhibitor. ... 

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

Before 1933, the orientation of the addition of hydrogen bromide to alkenes was the subject of much confusion. At times addition occurred in accordance with Markovnikovs rule at other times it occurred in just the opposite manner. Many instances were reported where, under what seemed to be the same experimental conditions, Markovnikov additions were obtained in one laboratory and anti-Markovnikov additions in another. At times even the same chemist would obtain different results using the same conditions but on different occasions. 

HBr reacts with an alkene to give the more substituted (and more stable) car-bocation intermediate, and the nucleophile is incorporated at that position (see Section 10.2). In experiments designed to further probe this reaction, hydrogen bromide (HBr) is added to undec-lO-enoic acid (153) in a hydrocarbon solvent, but benzoyl peroxide (149) is added to the reaction. When the product is isolated, 11-bromoundecanoic acid (154) is obtained in 70% yield. The bromine is attached to the less substituted carbon. Because Markovnikov s rule places the hydrogen atom on the less substituted carbon atom of the C=C unit and the bromine on the more substituted, formation of 154 is exactly the opposite result—an anti-Markovnikov addition. 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

This problem is even worse for alkenes more complex than ethylene. Equally simple reactions (in a formal sense ) such as the addition of hydrogen bromide or hypobromous acid are also far from being unambiguous and as a rule give rise to a mixture of isomers containing both the predominant Markovnikov (M) as well as anti-Markovnikov (aM) adducts. 

In the presence of a radical initiator, alkenes react with reactive molecules such as hydrogen bromide to give simple 1 1 adducts rather than a polymer. The initiator radical reacts rapidly with an HBr molecule to give a bromine atom (6.49), which starts the chain reaction. In the first propagation step, the bromine atom adds to the alkene 61 to give the adduct radical 62 (reaction 6.50). Since 62 abstracts a hydrogen atom from HBr by reaction (6.51) more rapidly than it would add to the alkene to form a polymer radical as in (6.43), the chain continues with reactions (6.50) and (6.51) as the propagating steps, and the product is the primary bromo compound 63. This anti-Markovniko addition is in the reverse direction to the polar addition discussed in Chapter 5. Since the radical chain reaction is faster than the polar reaction, the anti-Markovnikov product dominates if radicals are present. If the Markovnikov product is required, the reaction must be carried out in the dark, in the absence of free radical initiators, and preferably with a radical inhibitor present. 

The other reaction is the peroxide-catalysed addition of HBr to alkenes 7.19 giving the anti-Markovnikov product 7.21. The peroxide generates a bromine radical by abstracting the hydrogen atom from the HBr. The key step is the addition of the bromine atom to the double bond 7.19, which takes place to give the more-substituted radical 7.20, and this in turn abstracts a hydrogen atom from another molecule of HBr to give the primary alkyl bromide 7.21. 

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. The reaction exhibits anti-Markovnikov regioselectivity. 

Another famous appHcation of mechanistic chemistry was contributed by Morris Kharasch (1895-1957) at the University of Chicago. It was well known since the mid-19th century that addition reactions of HX (HCl, H-OH, etc) to unsymmetrical alkenes occur in a manner that places hydrogen on the olefinic carbon attached to more hydrogen atoms than the other olefinic carbon (Markovnikov s rule). However, when HX is hydrogen bromide (HBr), addition is typically anti-Markovnikov. Kharasch found that traces of peroxides (commonly present on glassware surfaces) initiate a free radical chain reaction for HBr (not for other HX). Careful removal of peroxides Ifom glassware prior to reaction removes the potential free radical mechanism and HBr adds in a normal Markovnikov mode (intermediacy of carbocation, rather than Ifee radical, intermediates). 

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. 

Several of the procedures in this chapter involve addition reactions characteristic of alkenes. Two of them, the addition of hydrogen bromide to 1-hexene (Sec. 10.5) and of borane to a-pinene (Sec. 10.8), represent examples in which both the reagent and the alkene to which it is adding are unsymmetrical. Consequently, identifying the products from these reactions provides a means of testing whether a Markovnikov or an anti-Markovnikov mode of addition has occurred. 

FIGURE 11.25 Addition of hydrogen bromide to alkenes in the presence of peroxide is anti-Markovnikov. 

As with alkenes, hydrogen bromide can add to triple bonds by a radical mechanism in an anti-Markovnikov fashion if light or other radical initiators are present. Both syn and anti additions are observed. 

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