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

1 Answer
May 16, 2018

Well, #C-H# bonds ARE strong.....

Explanation:

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

#Br_2stackrel(hnu)rarr2dotBr#

#dotBr+H_2CR_2 rarr Br-H + dotCHR_2#

#Br_2 + dotCHRrarrdotBr+BrCHR_2#

You can look up #"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....