Question

1. a) Use the Arrhenius equation to calculate the ratio of the rate constants k for the hydrogen abstraction in methane with a chlorine atom and with a bromine atom at 25oC. Assume the A values for the two reactions are the same, and use Ea = 4 kcal/mol for chlorination and Ea = 19 kcal/mol for bromination. (b) The high selectivity of radical bromination towards secondary and tertiary C-H bonds disappears if equimolar mixture of Br2 and Cl2 are used. The selectivity of the bromination between alkanes and the mixture of Br2 and Cl2 is very similar to the selectivity of chlorination. Explain this phenomenon

Answer 1

b. Slow reaction​ rate for bromination is because of low bond erngy of HBr than HCl.

If equimoleculer Br₂ and Cl₂ is used, then chain initiation reaction will occurs with Cl₂ as high bond energy of HCl makes this step enthalpytically more favourable (exothermic reaction) and because of low bond energy for Br₂, chain propagation reaction will occurs by Br₂. And as a whole activation energy for the reaction will be decreased so rate will be increased.