Question

How can hydroxide and alkoxide ions (not good leaving groups) be activated by means of Lewis or Brønsted acids?

Answer 1

Lewis and Brønsted acids convert the leaving groups into their conjugate acids.

Good leaving groups tend to be weak bases.

Converting them into their conjugate acids makes them weaker bases.

Hydroxide and alkoxide ions are not good leaving groups. For example,

We can add a strong acid to convert the leaving group to its conjugate acid, an oxonium ion.

• The conjugate acid of OH⁻ is H₂O (`"p"K_"a"` = 16).
• The conjugate acid of H₂O is H₃O⁺ (`"p"K_"a"` = 1.7).

So water is a much better leaving group than OH⁻.

The same principle applies to the cleavage of ethers.

We can take advantage of the same principle to make halogens better leaving groups.

When the Lewis acid Ag⁺ combines with a halogen such as Cl, the resulting species [R-Cl-Ag]⁺ has a much weaker C-Cl bond. The AgCl is an excellent leaving group.

Whether we use a Brønsted acid or a Lewis acid, the conjugate acid is always a better leaving group.