Bromination of alkanes is more selective

Bromine will also halogenate alkanes, and it does so much more selectively than chlorine. For example, the following reaction yields tert-butyl bromide with less than 1% of the primary isomer.

In this case, the fi rst step of the radical chain reaction, the abstraction of H by Br•, is endo thermic for both the primary and tertiary hydrogen atoms, but more so for primary radical formation, so the tertiary radical is preferred.

Of course, in both brominations and chlorinations the overall reaction is favourable because the second step—the halogenation of the alkyl radical—is significantly exothermic, by about 106 kJ mol⁻¹ for chlorination and about 83 kJ mol−1 for bromination. The same is true for fluorination, but with fluorine this step is so exothermic that fluorination becomes danger ously explosive. Conversely, radical iodination is impossible because the final step is insufficiently exothermic to make up for the endothermic formation of an alkyl radical. Why is bromination so much more selective than the chlorination of alkanes? This is a good example of the Hammond postulate, applied to real chemistry. Because the products of the fi rst step of the bromination (R• plus HBr) are higher in energy than the starting materials, the transition state must be similar in structure and energy to the radical being formed; the difference in energies of the primary and tertiary product radicals should therefore be markedly reflected in the different energies of the transition states TS1 and TS3, and ΔG1‡ will be significantly larger than ΔG3‡. For the chlorination reaction, the products were just slightly lower in energy than the starting materials, so the transition states for the two possible reactions both resembled the starting materials rather more and the products rather less. These are the same for both tertiary and primary hydrogen abstractions, of course, so the difference in energy of the product radicals exerts a less pronounced effect on the difference in energy of the transition states.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct