Just as direct addition to C=O becomes substitution at C=O  when there is a leaving group at the carbonyl carbon, so conjugate addition becomes conjugate substitution if there is a leaving group, such as Cl, at the β carbon atom. Here is an example: substitution replaces Cl with OMe, just as it would have done in a reaction with an acyl chloride.

As with substitution at C=O, this apparently simple reaction does not involve a direct dis placement of the leaving group in a single step. The mechanism starts in exactly the same way as for conjugate addition, giving an enol intermediate.

Now the leaving group can be expelled by the enol: the double bond moves back into its original position in an elimination reaction—the sequence is often called an addition–elimination

reaction. The ‘new’ double bond has the more stable E configuration. In the next example, two consecutive conjugate substitution reactions give a 1,1-diamine.

At first sight, the product looks rather unstable—sensitive to water, or traces of acid perhaps. But, in fact, it is remarkably resistant to reaction with both. The reason is conjugation: this isn’t really an amine (or a diamine) at all because the lone pairs of the nitrogen atoms are delocalized through into the carbonyl group, very much as they are in an amide. This makes them less basic, and makes the carbonyl group less electrophilic.

مواضيع ذات صلة

How to reduce carboxylic acids to alcohols

How to reduce amides to amines

The benzyne mechanism

Other nucleophiles

The SN1 mechanism for nucleophilic aromatic substitution: diazonium compounds

The activating anion-stabilizing substituent

The leaving group and the mechanism

The intermediate in the addition–elimination mechanism

The addition–elimination mechanism

Nucleophilic aromatic substitution

Nucleophilic epoxidation

Unsaturated nitriles and nitro compounds