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Electron Withdrawing and Electron Donating Groups

Certain atoms or groups of atoms can add or withdrawal electron density to a system. Electron withdrawinggroups (EWG) remove electron density from a system and tend to stabilize anions or electron rich structures.Conversely, EWG destabilize cations or electron poor structures. Electron donating groups (EDG) add electron

density to a system and tend to stabilize cations or electron poor systems. Conversely, EDG destabilize anions orelectron rich systems.

There two ways electron density can distribute itself through a molecule. It can move through -bonds or through -bonds. The movement of electron density through -bonds is called inductive effects. The movement ofelectron density through -bonds is called resonance effects. Whether an atom or group of atoms is ED or EW byinductive effects or resonance effects depends on certain physical features such as electronegativity, lone pairelectrons, and the presence of multiple bonds.

Groups that are EWG inductively but EDGthrough resonance . These are electronegativeatoms with lone pair electrons

Groups that are EDG only by inductiveeffects.Alkyl groups, the more branched, the moredonating

X

OH

NH2

CH 3

CH 2CH 3

CH(CH 3)2

Groups that are EWG only by inductiveeffects .Atoms with no lone pair electrons but have a

partial positive charge or a formal +1 charge.

Groups that are EWG by resonance. Theatom that is attached to the molecule isinvolved in a multiple bond.

C

F

F

F

N

H

H

H+

CH

O

C

O

R

C N

C

O

OR

Groups that are EWG inductively and EWGby resonance.

Groups that can be either EWG or EDG byresonance.

NO

O-

+= -NO 2

C CH 2H

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Inductive effects, withdrawing or donating, weaken substantially when remove by more than two s bonds. Thus, inductive effects occur close to the atom or group of atoms that cause the inductive effect(e.g. one of the groups in the table above). Resonance effects can delocalize formal charges over alarger area; therefore, resonance effects are usually are more pronounced, i.e. they can donate or removemore electron density than inductive groups.

Examples:

Which hydrogen in the molecules below is the most acidic?

CH 3CH 2CH 2C N CH 3COCH 3

O

HOCHCH 2CH 2CH 2OH

F

a) b) c)

Approach: Since strong acids have weak (or stable) conjugate bases, EWG will make an acid strongersince it helps stabilize the conjugate base. Therefore, look for EWG in the structure above and the moreacidic hydrogens should be near-by. For structure a), the nitrile functional group makes the protons onthe adjacent carbon the most acidic because the resulting anion is stabilized by resonance.

CH 3CH 2 C

H

C N

_

CH 3CH 2 C

H

C N _