Br vs I in terms of leaving groups

[dl9006]

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I'm confused about what defines a good leaving group.


I thought that if the leaving group conjugate aicd has a low pKa, the better the leaving group.

How come in a SN1 reaction, t-butyl bromide reacts slower than t-butyl iodide?

isn't HBR a stronger acid than HI? so shouldn't Br leave before I does?

 

[nze82]

A good leaving group is defined as species that remains quite stable after leaving.
Br and I both become Br- and I- anions after they leave.
Species with a smaller charge density (Charge/Size) tend to be more stable.
I is a larger atom compared to Br, therefore; it has a smaller charge density, which will in turn make I- a more stable anion that serves as a better leaving group!