Radical Addition of HBr to Alkenes

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

Free radical addition of HBr to alkenes peroxide effect Preparation of alkyl halides... 

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... 

This type of reaction is important industrially since it is one of the few that allows compounds containing functional groups to be made from alkanes. As you might guess, since it needs light for initiation, the process is another example of a radical chain reaction. As with the radical addition of HBr to alkenes, we can identify initiation, propagation, and termination steps in the mechanism. 

The addition of HI to propene, 1-bromopropene, allyl chloride, or allyl bromide gave only the normal addition products, and antioxidants did not inhibit the reaction. Moreover, HI inhibited the radical addition of HBr to alkenes. One possible route for this inhibition would be the reaction of peroxides with HI to produce I2. Kharasch, M. S. Norton, J. A. Mayo, F. R. /. Am. Chem. Soc. 1940, 62, 81. 

Physical Properties of Haloalkanes Preparation of Haloalkanes by Halogenation of Alkanes Mechanism of Halogenation of Alkanes Allylic Halogenation Radical Autoxidation 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... 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

Radical addition of HBr to an alkene depends upon the bromine atom adding in the first step so that the more stable radical is formed. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, we can see that the preferred secondary radical will be produced if halogenation occurs on the terminal carbon atom. However, this new radical is also an allylic radical, and an alternative resonance form may be written. 

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. 

A well-known example of the application of mechanistic understanding to help to control product yields is also of commercial significance - the addition of HBr to alkenes which may occur via cationic or radical mechanisms, Scheme 2.1 [2a]. Very pure alk-l-enes (1), in the absence of peroxides, react to give the 2-bromo-products (2) by Markovnikov addition. In the presence of peroxides or other radical sources, anti-Markovnikov addition gives the 1-bromo-products (3). 

Radical Addition of HBr to Unsymmetrical Alkenes Now we must explain the anti-Markovnikov orientation found in the products of the peroxide-catalyzed reaction. With an unsymmetrical alkene like 2-methylbut-2-ene, adding the bromine radical to the secondary end of the double bond forms a tertiary radical. 

The reversal of orientation in the presence of peroxides is called the peroxide effect. It occurs only with the addition of HBr to alkenes. The peroxide effect is not seen with HC1 because the reaction of an alkyl radical with HC1 is strongly endothermic. 

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 polymers described in Section 15.14A are prepared by polymerization of alkene monomers by adding a radical to a 7t bond. The mechanism resembles the radical addition of HBr to an alkene, except that a carbon radical rather than a bromine atom is added to the double bond. Mechanism 15.5 is written with the general monomer CH2=CHZ, and again has three parts initiation, propagation, and termination. 

---