Nucleophilic Aromatic Substitution An Addition-Elimination Reaction

Diazomethane can be used to convert a carboxylic acid into a methyl este. Propose a mechanism for this reaction. 

We have seen that aryl halides do not react with nucleophiles because the tt electron cloud repels the approach of a nucleophile (Section 9.5). 

Nucleophilic aromatic substitution takes place by a two-step mechanism. The reaction is called an S Ar reaction (substitution nucleophilic aromatic). 

In a nucleophihc aromatic substitution reaction, the incoming nucleophile must be a stronger base than the substituent that is being replaced, because the weaker of the two bases will be the one eliminated from the intermediate. 

The electron-withdrawing substituent must be ortho or para to the site of nucleophilic attack because the electrons of the attacking nucleophile can be delocalized onto the substituent only if the substituent is in one of those positions. 

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Nucleophilic Aromatic Substitution An Addition-Elimination Reaction 955... 

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