Energetics of Chlorination vs. Bromination

[snowboarder1992]

TBR discusses how bromine has a lower bond dissociation energy than chlorine, but then goes on to say that chlorination reactions are much faster than bromination reactions. To me, these statements seem to contradict one another, but I know that I must be mistaken. Can someone please help explain why chlorination is faster than bromination even though it has a higher activation energy?

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[sakabato93]

TBR discusses how bromine has a lower bond dissociation energy than chlorine, but then goes on to say that chlorination reactions are much faster than bromination reactions. To me, these statements seem to contradict one another, but I know that I must be mistaken. Can someone please help explain why chlorination is faster than bromination even though it has a higher activation energy?

NVM, I figured it out.

During free radical halogenation, the halogen must exist as a radical until reacting with an alkane. Chlorine is much more unstable as a radical, so it reacts more quickly with the alkane substrate, which also decreases its selectivity relative to bromine. Since bromine is more stable than chlorine as a radical, it can react more slowly and selectively than chlorine.

 

[snowboarder1992]

Thanks sakabato93!