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

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Answer 1 · 2017-05-01T17:29:32

Here's my understanding of the reaction.

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:

Acyl chloride

The mechanism involves an in-plane #S_text(N)2# displacement of the chloride ion.

It appears to be an #S_text(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.

alkyl chloride

(1) The carbonyl carbon in acetyl chloride is #"sp"^2# hybridized, while the α carbon in ethyl chloride is #"sp"^3# hybridized.

The carbonyl carbon has more #"s"# character, so it is more electronegative. It pulls the electrons in the #"C-Cl"# bond closer to itself.

This weakens the #"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.

Resonance

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