Specific enol equivalents for carboxylic acid derivatives

Specific enol equivalents for carboxylic acid derivatives 

We established in Chapter 12 a hierarchy for the electrophilic reactivity of acid derivatives that should by now be very familiar to you—acyl chlorides at the top to amides at the bottom. But what about the reactivity of these same derivatives towards enolization at the a position, that is, the CH2 group between R and the carbonyl group in the various structures You might by now be able to work this out. The principle is based on the mechanisms for the two processes, mechanism of nucleophilic attack mechanism of enolate formation 

See how similar these two mechanisms are. In particular, they are the same at the carbonyl group itself. Electrons move into the C=0 n orbital the C=0 bond becomes a C-O single bond as a negative charge develops on the oxygen atom. It should come as no surprise that the order of reactivity for enolization is the same as the order of reactivity towards nucleophilic attack. 

Enolate formation and electrophilic reaotlvlty of acid derivatives  

Electrophilic reactivity Derivative Structure Reactivity towards enolate formation 

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Among the enolates of carboxylic acid derivatives, esters are the most widely used. Ester enolates cannot be used in crossed aldols with aldehydes because the aldehyde is both more enolizable and more electrophilic than the ester. It will just condense with itself and ignore the ester. The same is true for ketones. A specific enol equivalent for the ester will therefore be needed for a successful ester aldol reaction. 

For the remainder of this section we shall look at how to make specific enol equivalents of the remaining carboxylic acid derivatives. 

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See also in sourсe #XX -- [ ]

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