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For example, fluorine is a stronger electron-withdrawing substituent than methyl, resulting in an increased Lewis acidity of boron trifluoride relative to trimethylborane. Electron-withdrawing groups also tend to reduce Lewis basicity. [3]
The captodative effect is the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent. [2] [3] The name originates as the electron-withdrawing group (EWG) is sometimes called the "captor" group, whilst the electron-donating group (EDG) is the "dative" substituent. [3]
If the electronegative atom (missing an electron, thus having a positive charge) is then joined to a chain of atoms, typically carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect. In short, alkyl groups tend to donate electrons, leading to the +I ...
Electron donating groups are generally ortho/para directors for electrophilic aromatic substitutions, while electron withdrawing groups (except the halogens) are generally meta directors. The selectivities observed with EDGs and EWGs were first described in 1892 and have been known as the Crum Brown–Gibson rule.
When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect. Similarly, an electron-releasing group (ERG) or electron-donating group (EDG) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
The mesomeric effect is negative (–M) when the substituent is an electron-withdrawing group, and the effect is positive (+M) when the substituent is an electron donating group. Below are two examples of the +M and –M effect. Additionally, the functional groups that contribute to each type of resonance are given below.
Groups that are electron-withdrawing by resonance decrease the electron density especially at positions 2, 4 and 6, leaving positions 3 and 5 as the ones with comparably higher reactivity, so these types of groups are meta directors (see below).
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.