Chlorine abstraction activation energy

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. 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 Hammond s postulate (Section 4.4.2), the transition state would be expected to be more 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 ... 

Such a process would have an intensity exponent of unity as observed. A further fact in support of the disproportionation mechanism is that the yield of CF2CI2 is largely independent of the ketone pressure at room temperature an abstraction mechanism would require a first-order dependence upon ketone pressure. While the dimer of CF2, tetrafluoro-ethylene, has never been observed in the reaction mixture, a white solid collected in the cell which was probably a polymer of CF2. While the experimental conditions are not strictly comparable, it is significant that the absorption spectrum of CF2 has been observed in the flash photolysis of 1,3-dichlorotetrafluoroacetone.39 When the temperature is raised, however, the yield of CF2C12 in normal photolysis, increases rapidly suggesting an energy of activation and this process can only be chlorine abstraction. The rate function ... 

There are a few trends in the rate constants that become apparent from an examination of Table 2. Successive halogen atom substitution decreases the rate constant for hydrogen atom abstraction by Cl atoms. The decrease is more pronounced at the j8 than at the a position and it is more pronounced for fluorine than for chlorine substitution. A comparison of C—H bond energies to activation energies shows a clear correlation between the two quantities [40]. For the successive fluorination of methane, the C—H bond energy decreases after the substitution of one fluorine atom,... 

These reaction coordinate diagrams for the formation of primary and tertiary alkyl radicals by halogen atom abstraction from 2-methylpropane illustrate a larger difference in the activation energies for the reaction with a bromine atom (b) than with a chlorine atom (a). This difference is consistent with the higher selectivity of bromination. 

Fluorine, being much more reactive than chlorine, is even less selective than chlorine. Because the energy of activation for the abstraction of any type of hydrogen by a fluorine atom is low, there is very little difference in the rate at which a 1°, 2°, or 3° hydrogen reacts with fluorine. Reactions of alkanes with fluorine give (almost) the distribution of products that we would expect if all of the hydrogens of the alkane were equally reactive. 

Alko l radicals and chlorine atoms are highly reactive in hydrogen abstraction leading to the formation of macroalkyl radicals (P), which in turn react with almost zero activation energy with ground state oxygen which is itself a diradical (reaction 2) ... 

In Section 8.4A, we were given the fact that the selectivity for abstraction of a 3° H compared to a 1° H in chlorination is 5 1 this ratio directly reflects the relative reaction rates of these hydrogens with chlorine atoms. Using this ratio of reaction rates and the relationship between and rate constants, we can calculate that the difference in activation energies, AAG, for the abstraction of a 3° H versus a 1° H is about 4 kJ (1 kcal)/mol. However, we can calculate from the primary and tertiary... 

Table 11.11 provides further insight into the origin of the selectivity in free radical halo-genations. The is essentially zero for all hydrogen atom abstractions by fluorine atom, and the free energy barrier arises solely from the log A term of the Arrhenius equation, which is near 13 for all the halogenations given in Table 11.11. The activation energies for abstraction by chlorine atoms are also exceedingly small but in the direction of the trends discussed. Lastly, the activation energies for abstraction by bromine atoms are substantial, and they clearly produce the differential reactivities of 3°, 2°, and 1° C-H bonds. Because of these differences the relative selectivities of Table 11.10 are temperature dependent.

It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55], Chemiluminescence from alkanes can be greatly enhanced by addition of HC1. The proposed explanation is that energy transfer from active nitrogen dissociates HC1 to produce chlorine atoms, which have rapid hydrogen-atom abstraction reactions with alkanes,... 

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