# The rate of solvolysis of  CH_3COCl  is greater than  CH_3CH_2Cl. why?

May 1, 2017

Here's my understanding of the reaction.

#### Explanation:

Solvolysis is the reaction of a substrate with the solvent.

For example, if the solvent is methanol, the solvolysis of acetyl chloride would be represented as:

The mechanism involves an in-plane ${S}_{\textrm{N}} 2$ displacement of the chloride ion.

It appears to be an ${S}_{\textrm{N}} {1}^{'}$ reaction because the concentration of the solvent is effectively a constant.

If we compare this reaction with the solvolysis of ethyl chloride (below), we see two differences.

(1) The carbonyl carbon in acetyl chloride is ${\text{sp}}^{2}$ hybridized, while the α carbon in ethyl chloride is ${\text{sp}}^{3}$ hybridized.

The carbonyl carbon has more $\text{s}$ character, so it is more electronegative. It pulls the electrons in the $\text{C-Cl}$ bond closer to itself.

This weakens the $\text{C-Cl}$ bond and makes it easier to remove. Hence, the reaction is faster.

(2) The carbonyl carbon has a resonance contributor that puts more positive charge on the carbon atom.

This makes the carbonyl more susceptible to attack by a nucleophile. It lowers the activation energy, so the reaction is faster.