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Diagram showing the ortho, meta and para positions relative to a substituent X on a benzene ring. Electron donating groups are typically divided into three levels of activating ability (The "extreme" category can be seen as "strong".) Electron withdrawing groups are assigned to similar groupings.
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.
It is generally considered an inductively withdrawing group (-I), because of the higher electronegativity of sp 2 carbon atoms, and a resonance donating group (+M), due to the ability of its π system to donate electron density when conjugation is possible. [5] The phenyl group is hydrophobic. Phenyl groups tend to resist oxidation and reduction.
Therefore, these electron-withdrawing groups are meta directing because this is the position that does not have as much destabilization. The reaction is also much slower (a relative reaction rate of 6×10 −8 compared to benzene) because the ring is less nucleophilic.
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 ...
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]
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.
Also, in aromatic carboxylic acids, electron-withdrawing groups substituted at the ortho and para positions can enhance the acid strength. Since the carboxyl group is itself an electron-withdrawing group, dicarboxylic acids are, in general, stronger acids than their monocarboxyl analogues. The Inductive effect will also help in polarization of ...