Does rate increase or decrease in SN1 and SN2 if Chlorine is replaced with Bromine?

[Ash366]

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This is from TBR:

53: if bromine were used as leaving group from the cyclohexane in lieu of chlorine, what effect would you expect on rate? (note: C-Cl bond is stronger than C-Br bond)?

A: Both SN1 and SN2 rates would increase
or
D. Both the SN1 and SN2 rates would decrease.

I chose D because I thought of their reactivity: Fluorine> Cl>Br>Iodine. Bromine is less reactive thus will take a longer time thus their rates will decrease.

Answer is A.

 

[Ash366]

So using your view, Bromine would cause the rates to proceed faster just because it is more stable on its own?
I thought that if Bromine is less reactive than Chlorine, then the rate will be slower.

 

[amy_k]

Yes be careful about this one Ash366 - you are stating the reactivity of halide ions acting as nucleophiles or bases to form a bond with another molecule or ion (this order is based on ion instability which increases reactivity in such reactions), whereas in this question we are looking at the ability of the halides to come off a molecule as a leaving group, so the reverse order would be true, since the ions are now being ranked on their relative stability which increases the speed of this kind of reaction.