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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]
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.
The inductive effect can be used to determine the stability of a molecule depending on the charge present on the atom and the groups bonded to the atom. For example, if an atom has a positive charge and is attached to a -I group its charge becomes 'amplified' and the molecule
The main discussion of these abbreviations in the context of drug prescriptions and other medical prescriptions is at List of abbreviations used in medical prescriptions. Some of these abbreviations are best not used, as marked and explained here.
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. The term polar effect is sometimes used to refer to electronic effects, but also may have the more narrow definition of effects resulting from non-conjugated ...
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