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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 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 ...
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.
Vinylogous reactions are believed to occur when orbitals of the double bonds of the vinyl group and of an attached electron-withdrawing group (EWG; the π orbitals) are aligned and so can overlap and mix (i.e., are conjugated). Electron delocalization enables the EWG to receive electron density through participation of the conjugated system.
An example of the E1cB reaction mechanism in the degradation of a hemiketal under basic conditions. The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where the hydrogen to be removed is relatively acidic, while the leaving group (such as -OH or -OR) is a relatively poor one.