Radical additions of HBr

Bromination. 1-Bromoalkanes are produced commercially by the anti-Markovnikow free-radical addition of HBr to a-olefins. These are further reacted with dimethyl amine to produce alkyldimethyl amines, which ultimately are converted to amine products for household cleaning and personal care. 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

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. 

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

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

The authors assumed that dibromide 10 formed as major product may arise from the dehydrofluorination of monoadduct 9 followed by a radical addition of HBr to the intermediate olefin, according to the following scheme ... 

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... 

B Free-Radical Addition of HBr Anti-Markovnikov Addition... 

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. 

Draw a reaction-energy diagram for the propagation steps of the free-radical addition of HBr to isobutylene. Draw curves representing the reactions leading to both the Markovnikov and the anti-Markovnikov products. Compare the values of AG° and Ea for the rate-limiting steps, and explain why only one of these products is observed. 

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. 

In dealing with processes where stereoisomer formation is subject to some kind of control, two terms have been used—stereoselective and stereospecific. These terms have caused some problems, and to some extent have been used synonymously. The meanings attached to these terms by chemists are unambiguous [52]. A stereospecific process is one in which a particular stereoisomer reacts to give one specific stereoisomer (or a racemate) of a product. Thus, two starting materials, differing only in stereoisomerism, must be converted into stereoisomerically different products. A classical example of chemical stereospecificity is the free radical addition of HBr to 2-bromo-2-butene. At — 78°C, the (E)-olefin, 24, forms meso-2,3-dibromobutane, 25, while the (Z)-isomer, 26, yields ( )-tAreo-2,3-dibromobutane, 27 plus 28. (Note in addition that this addition is regiospecific—2,2-dibromobutane is not formed.)... 

The energy changes during propagation in the radical addition of HBr to CH2=CH2 can be calculated from bond dissociation energies, as shown in Figure 15.9. 

Problem 15.31 Draw an energy diagram for the two propagation steps in the radical addition of HBr to propene. Draw the transition state for each step. 

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. 

The radical addition of HBr to 3,3,3-trichloropropene, involves migration of a chlorine atom in an intermediate step. 

The first propagation step in the addition chain is a radical addition. The second propagation step in the addition chain is a radical abstraction. Radical addition of HBr is a typical reaction. Bromine radical adds to the multiple bond to form the most stable of the possible radicals. The radical addition of HBr gives the opposite regiochemistry of addition as the polar addition of HBr. 

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