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An electron-withdrawing group (EWG) is a group or atom that has the ability to draw electron density toward itself and away from other adjacent atoms. [1] This electron density transfer is often achieved by resonance or inductive effects.
Also, in aromatic carboxylic acids, electron-withdrawing groups substituted at the ortho and para positions can enhance the acid strength. Since the carboxyl group is itself an electron-withdrawing group, dicarboxylic acids are, in general, stronger acids than their monocarboxyl analogues. The Inductive effect will also help in polarization of ...
The nitro group is also strongly electron-withdrawing. Because of this property, C−H bonds alpha (adjacent) to the nitro group can be acidic. For similar reasons, the presence of nitro groups in aromatic compounds retards electrophilic aromatic substitution but facilitates nucleophilic aromatic substitution. Nitro groups are rarely found in ...
The mesomeric effect is negative (–M) when the substituent is an electron-withdrawing group, and the effect is positive (+M) when the substituent is an electron donating group. Below are two examples of the +M and –M effect. Additionally, the functional groups that contribute to each type of resonance are given below.
With respect to acidity, a common trend to note is that, inductively, an electron-withdrawing substituent in the vicinity of an acidic proton will lower the pKa (i.e. increase the acidity) and, correspondingly, an electron-donating substituent will raise the pKa. [7] The reorganization of charge due to field effects will have the same result.
When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect. Similarly, an electron-releasing group (ERG) or electron-donating group (EDG) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
The captodative effect is the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent. [2] [3] The name originates as the electron-withdrawing group (EWG) is sometimes called the "captor" group, whilst the electron-donating group (EDG) is the "dative" substituent. [3]
These substituents stabilize the negative charge on the carboxylate oxygen atom by an electron-withdrawing inductive effect (-I) and also by a negative mesomeric effect (-M). The next set of substituents are the halogens, for which the substituent effect is still positive but much more modest.