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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.
Electron-withdrawing groups exert an "inductive" or "electron-pulling" effect on covalent bonds. The strength of the electron-withdrawing group is inversely proportional to the pKa of the carboxylic acid. [2] The inductive effect is cumulative: trichloroacetic acid is 1000x stronger than chloroacetic acid.
The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter M. [2] The mesomeric effect is negative ( –M ) when the substituent is an electron-withdrawing group , and the effect is positive ( +M ) when the ...
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).
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 ...
The more positive is the value of pEDA the more pi-electron donating is a substituent. The more negative pEDA, the more pi-electron withdrawing is the substituent (see the table below). The pEDA parameter for a given substituent is calculated by means of quantum chemistry methods. The model molecule is the monosubstituted benzene.
For meta-directing groups (electron withdrawing group or EWG), σ meta and σ para are more positive than σ’. (The superscript, c, in table denotes data from Hammett, 1940. [11] [page needed]) For ortho-para directing groups (electron donating group or EDG), σ’ more positive than σ meta and σ para.
In this usage, a leaving group is a less formal but more commonly used synonym of the term nucleofuge. In this context, leaving groups are generally anions or neutral species, departing from neutral or cationic substrates, respectively, though in rare cases, cations leaving from a dicationic substrate are also known. [3]