difference in halogenation of br vs cl

[jp370]

why is it that br halogenation with synthesize a major and a minor while cl will just tag on any where and have no major and minor, it is due to the fact that br is heavier??

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[smile101]

The Br rxn is slower as opposed to Cl that reacts fast, so it is not very selective and it tends to go towards primary hydrogens (just coz there are so many of those) as well as stable carbocations, so you will get Cl at many places.
Br has to be on the most stable carbocation, so it is selective and will go towards the carbon that is highly substituted taking all the time it possible could.

Also remember that F would be most exothermic (being superfast) and I would be most endothermic (being superslow).

 

[mddang]

The more reactive, the less selective.

Cl- is more reactive than Br- due to it's smaller size (less delocalization of the - charge), which makes it less selective.