# Question #e5998

Jan 10, 2018

Tosylate is a better leaving group than iodide.

#### Explanation:

What makes one leaving group better than the other is how stable is the group after it has broken its bond with the parent molecule.
We can also think of it in this manner: We calculate it from the basicity of the group.
(weak base -> more stable while solvated -> better leaving the group).

Tosylate groups ( $C {H}_{3} {C}_{6} {H}_{4} S {O}_{3}^{-}$) are very good leaving groups because their conjugate bases are highly stabilized by $r e s o n a n c e$.

There is no resonance stabilization in case of iodine. However, iodide is regarded as the best leaving group amongst the halogens because iodide is a weaker base than -OH since its conjugate acid HI is a stronger acid than H2O. So, iodine is a better leaving group.

https://en.wikipedia.org/wiki/Leaving_group