Why do alkanes need UV light to undergo a reaction with Bromine?

1 Answer
May 16, 2018

Well, C-HCH bonds ARE strong.....

Explanation:

...and in fact it is the bromine molecule, with a WEAKER Br-BrBrBr bond, that undergoes photolysis to give TWO neutral bromine radicals...

Br_2stackrel(hnu)rarr2dotBrBr2hν2.Br

dotBr+H_2CR_2 rarr Br-H + dotCHR_2.Br+H2CR2BrH+.CHR2

Br_2 + dotCHRrarrdotBr+BrCHR_2Br2+.CHR.Br+BrCHR2

You can look up "initiation, propagation, termination steps"initiation, propagation, termination steps of radical reactions in your text; this is worth doing. It is the propagation step that is the important step. The formation of a radical, and its subsequent reaction, creates ANOTHER radical, which can propagate the radical chain of reaction....