Hammond postulate radical chlorination

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyI radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In Ught of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

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. 

Hammond s postulate can be applied to this series of the selectivity-determining steps of the radical chlorination. They all take place via early transition states, i.e. via transition states that are similar to the starting materials. The more stable the resulting radical, the more similar the transition state is to the starting materials. The stability differences between these radicals are therefore manifested only to a very small extent as stability differences between the transition states that lead to them. All transition states are therefore very similar in energy and thus the different reaction rates are very similar. This means that the regioselectivity of the radical chlorination of saturated hydrocarbons is generally low. 

By the Hammond postulate, the transition state for abstraction of a hydrogen by a chlorine atom resembles the reactants and has only a small amount of radical character. Therefore, the transition state leading to a tertiary radical is only slightly more stable than the transition state leading to a primary radical. 

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 ... 

Because the reaction of a chlorine radical with an alkane to form a primary, secondary, or tertiary radical is exothermic, the transition states resemble the reactants more than they resemble the products (see the Hammond postulate, Section 4.3). The... 

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. 

Abstraction of hydrogen by chlorine is exothermic, which, according to Hammond s postulate, means that the transition state for H absfracfion by Cl is reached early in the course of the reaction [Figure 8.3(a)]. Therefore, fhe strucfure of the transition state for this step resembles the reactants, namely the alkane and a chlorine atom, not the product radicals. As a result, there is relatively little radical character on carbon in this transition state, and regioselectively in radical chlorination is only slightly influenced by the relative stabilities of radical intermediates. Products are determined more by whether a chlorine atom happens to collide with a 1°, 2°, or 3° H. 

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

The discrepancy from the experimental values is due to the fact that H atoms bound to different types of C atoms are replaced by chlorine at different rates. The substitution of Cfcrt— H takes place via a tertiary radical. The substitution of Csec—H takes place via the somewhat less stable secondary radical, and the substitution of Cprjm—H takes place via even less stable primary radicals (for the stability of radicals, see Table 1.2). According to Hammond s postulate, the rate of formation of these radicals should decrease as the radical s stability decreases. Hydrogen atoms bound to Ctert should thus be substituted more rapidly than H atoms bound to Csec, and these should in turn be substituted by Cl more rapidly than H atoms bound to Cprjm. As the analysis of the regioselectivity of the monochlorination of isopentane carried out by means of Table 1.4 shows, the relative chlorination rates of C —H, C —H, and C. —H are 4.4 33 1, in agreement with this expectation. 

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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