Question

Question #60fd2

Answer 1

Hydroxide ion is an exceptionally POOR leaving group.

Explanation:

To a first approximation, the facility of a leaving group may be correlated to the acidity of the leaving group's conjugate acid.

`X^-, X=Cl, Br, I`, are all good leaving groups, as these are all poor bases, as the conjugate bases of strong acid. On the other hand, `F` is a poor leaving group, and `HF` is a relatively poor acid. By this same reasoning, the conjugate acid of hydroxide, water, as a relatively poor acid, suggests that hydroxide would be a poor leaving group.

Most of the time, if we want to remove an hydroxide group, we react the alcohol with `"p-toluenesulfonyl chloride"` to make the sulfonyl ester, i.e.:

`ROH + p-H_3C-C_6H_4SO_3Cl + "base" rarr`

`p-H_3C-C_6H_4SO_3OR + "base"*HCl`

The resonance stabilized sulfonate ester is a good leaving group.