Hammond postulate radical brominations

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

To explain the difference between chlorination and bromination, we return to the Hammond postulate (Section 7.15) to estimate the relative energy of the transition states of the rate-determining steps of these reactions. The rate-determining step is the abstraction of a hydrogen atom by the halogen radical, so we must compare these steps for bromination and chlorination. Keep in mind ... 

According to the Hammond postulate, the transition state of an endothermic reaction resembles the produets, so the energy of aetivation to form the more stable 2° radical is lower and it is formed faster, as shown in the energy diagram in Figure 15.5. Because the 2° radical [(CH3)2CH-] is converted to 2-bromopropane [(CH3)2CHBr] in the seeond propagation step, this 2° alkyl halide is the major product of bromination. 

The first step in the mechanism is endothermic and rate determining. The 3° radical produced in anti-Markovnikov attack (A) of bromine radical is several kJ/mole more stable than the 1° radical generated by Markovnikov attack (B). The Hammond Postulate tells us that it is reasonable to assume that the activation energy for anti-Markovnikov addition is lower than for Markovnikov addition. This defines the first half of the energy diagram. 

There are important differences in the reactions of other halogens relative to bromination. In the case of chlorination, although the same chain mechanism as for bromination is operative, there is a key difference in the greatly diminished selectivity of the chlorination. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of the Hammond postulate, the transition state would be expected to be very reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical. ... 

Unless there is a significant rate difference between different types of hydrogen atoms, radical chlorination is not very useful. The rationale for the greater selectivity of the bromine radical relative to the chorine radical is based on the Hammond postulate and a later transition state for the bromination. If the bromination transition state (see Chapter 7, Section 7.3) is later, it is closer in energy to the products. This means that the stability of the radical product (3° > 2° > 1°) is more important in determining the product distribution. For an earlier transition state (chlorination) there is less difference in energy between the transition states for the three types of hydrogen atoms and less selectivity. 

Use of the Hammond postulate to compare the transition states in the first propagation step of radical chlorination and radical bromination. 

The alkyl radical-forming step is exothermic for chlorination, endothermic for bromination. Applying Hammond s postulate to these elementary steps, we conclude that alkyl radical character is more highly developed in the transition state for abstraction of hydrogen by a bromine atom than by a chlorine atom. Thus, bromination is more sensitive to the stability of the free-radical intermediate than chlorination and more selective. 

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