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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.
An electron donating group (EDG) or electron releasing group (ERG, Z in structural formulas) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance (mesomerism) or inductive effects (or induction)—called +M or +I effects, respectively—thus making the π system more nucleophilic.
Delocalizing the radical ion stabilizes the transition state structure. As a result, the energy of activation decreases, enhancing the rate of the overall reaction. According to the captodative effect, the rate of a reaction is the greatest when both the EDG and EWG are able to delocalize the radical ion in the transition state structure. [7]
The polarization of a single σ covalent bond due to the electronegativity difference. Transfer of shared π-bond electron pairs to one atom under the influence of a strong external field. Permanent effect. Temporary effect. Always observed. Only observed in the presence of an electrophilic reagent. Induced charges are partial charges (δ + or ...
For example, in a normal-demand scenario, a diene bearing an electron-donating group (EDG) at C1 has its largest HOMO coefficient at C4, while the dienophile with an electron withdrawing group (EWG) at C1 has the largest LUMO coefficient at C2. Pairing these two coefficients gives the "ortho" product as seen in case 1 in the figure below.
An electric effect influences the structure, reactivity, or properties of a molecule but is neither a traditional bond nor a steric effect. [1] In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry (stereochemistry) of a molecule.
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For example, the substituent may determine the mechanism to be an SN1 type reaction over a SN2 type reaction, in which case the resulting Hammett plot will indicate a rate acceleration due to an EDG, thus elucidating the mechanism of the reaction. Another deviation from the regular Hammett equation is explained by the charge of nucleophile.