Activation Energy for Halogenation

We know that the enthalpy change for the first propagation step in halogenation is always less exothermic than the enthalpy change for the second step. This step is rate-determining. The second step, in which the methyl radical reacts with the halogen, is quite exothermic and has a very low activation energy (less than 1 kj mole ). 

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The temperature dependence of the rate constants of radical addition (k ) is described by the Arrhenius equation (Section 10.2). At a given temperature, rate variations due to the effects of radical and substrate substituents are due to differences in the Arrhenius parameters, the frequency factor, A , and activation energy for addition, . For polyatomic radicals, A values span a narrow range of one to two orders of magnitude [6.5 < log (A /dm3 mol-1 s-1) < 8.5] [2], which implies that large variations in fcj are mainly due to variations in the activation energies, E. This is illustrated by the rate constants and Arrhenius parameters for the addition to ethene of methyl and halogen-substituted methyl radicals shown in Table 10.1. 

Calculations of the activation energies for the 1,3-shifts of substituents in a-imidoylketenes, which interconvert with o -oxoketenimines via a four-membered cyclic transition state, show that barriers are lowest for substituents with accessible lone pairs (halogens, OR, NR2) (Scheme 31).54... 

As a final example of this section, we cite Benson and Haugen s (1965) electrostatic model for the prediction of activation energies for four-center gas phase reactions. Of interest here are the additions of hydrogen, halogens, interhalogens and hydrogen halides to alkenes. To obtain the calculated value of the activation energy for the addition reaction (196),... 

TABLE 2. Activation energy for homogeneous halogen exchange by astatine in halobenzenes. Reproduced with permission from Reference 24... 

XIII.5 are collected some of the relevant thermal data on halogen reactions, while in Table XIII.6 are listed the experimental activation energies for those reactions. 

Table XIII.6. Some Experimental Activation Energies for the Reactions OP Halogen Molecules and Atoms in the Gas Phase...

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