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Question
What are important electron withdrawing groups and electron donating groups should be remembered for NEET examination point of view..?
What are important electron withdrawing groups and electron donating groups should be remembered for NEET examination point of view..?
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Solution
Electron withdrawing groups (EWG) have very much affinity towards electrons. When such a group is present in a molecule then most of the charge density will be partially shifted towards EWG. Eg. In nitromethane , EWG present is present and it is Nitro group(NO2) . In this molecule NO2 has partial negative charge since it attracts the charge density towards it's side. CH3 has partial positive charge since it suffers from partial loss of charge density.
The case of electron donating groups(EDG) is quite different. Alkyl groups , alkoxy groups belong to this category. In the case of toluene ( methyl group is attached to benzene) the methyl group present is electron donating. It donates the electron density to benzene ring.
The directing nature of EWG and EDG are different in aromatic substitution reactions. If we consider the electrophilic substitution reactions of substituted benzene, we can see that EWG like NO2 deactivates the ring and leads to meta product. For example , nitration of nitrobenzene gives mainly metadinitrobenzene .But this is not the case if the substituent present were EDG.
EDG substituent activates benzene to electrophilic aromatic substitution and ortho or para substituted product is formed.
For example nitration of toluene will give ortho nitro toluene and paranitro toluene.
In another words
An electron withdrawing group or EWG draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect.
Examples of electron withdrawing groups are
· halogens (F, Cl);
· nitriles CN;
· carbonyls RCOR';
· nitro groups NO2.
An electron releasing group or ERG (may also be called electron donating groups or EDG's) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
Examples of electron releasing groups are
· alkyl groups;
· alcohol groups;
· amino groups.
The total substituent effect is the combination of the polar effect and the combined steric effects.
The case of electron donating groups(EDG) is quite different. Alkyl groups , alkoxy groups belong to this category. In the case of toluene ( methyl group is attached to benzene) the methyl group present is electron donating. It donates the electron density to benzene ring.
The directing nature of EWG and EDG are different in aromatic substitution reactions. If we consider the electrophilic substitution reactions of substituted benzene, we can see that EWG like NO2 deactivates the ring and leads to meta product. For example , nitration of nitrobenzene gives mainly metadinitrobenzene .But this is not the case if the substituent present were EDG.
EDG substituent activates benzene to electrophilic aromatic substitution and ortho or para substituted product is formed.
For example nitration of toluene will give ortho nitro toluene and paranitro toluene.
In another words
An electron withdrawing group or EWG draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect.
Examples of electron withdrawing groups are
· halogens (F, Cl);
· nitriles CN;
· carbonyls RCOR';
· nitro groups NO2.
An electron releasing group or ERG (may also be called electron donating groups or EDG's) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
Examples of electron releasing groups are
· alkyl groups;
· alcohol groups;
· amino groups.
The total substituent effect is the combination of the polar effect and the combined steric effects.
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