Guidelines for Electron Pushing in Organic Reaction Mechanisms

The electron pushing concept is referred to usually to electron donating or electron releasing concept.  It cannot be considered as a topic by itself because it is tightly related to the concept of electron withdrawing power.   When we talk about electron donating or electron withdrawing concept we usually talk about substituents that are attached to organic molecules such as nitro group or chlorine atom. 

Substituents on organic molecules can be divided into electron donating or electron withdrawing substituents or neutral substituents such as hydrogen atom.   Substituent effect has already been investigated in previous time of last century by Hammett and coworkers.  Research has shown that resonance structures contribute to the stability of conjugated polyenes. 

Benzene is an organic molecule which its extra stability drawed the attention of scientists to investigate its energetics.  They compared its energy with that of three ethylene molecules.  They found that the energy of the benzene ring was not equal to that of the energy of three ethylene molecules although benzene also has three double bonds. 

They ascribed the extrastability of the benzene ring and the difference in energy between the benzene molecule and the energetic sum of three double bonds in ethylene to resonance structures that contribute to the stability of the benzene ring.  This remarkable stability of the benzene ring and the delocalization effect through resonance made it a very interesting molecule to investigate at the electrophilic aromatic substitution level in general and the substituent effect on the ring in particular. 

Since resonance structures in the benzene rings are mediated through the pi bonds delocalization effects substituents which have lone pairs that could be delocalized through the pi framework of the benzene ring should have a major effect on the stability of the ring and the type of substituents on the ring.

This is true for electron donating groupps which can share their electron lone pairs through the ring pi system.  Electron withdrawing groups can on the other hand receive pi electrons by delocalization of the ring pi bonds in to their empty p orbital or pi* orbital.  An example of such a group is the nitro group.

The electron donating and electron withdrawing effect has a remarkable effect on the manner of substitution on the benzene ring.  Toluene for example directs the nitration of the benzene ring to the para and ortho positions but not the meta position.  This is so due to the effect of the methyl group by increasing electron density at the ortho and para positions.  Thus facilitating the addition of the nitro group. 

Benzoic acid was investigated by Hammett and coworkers to see the effect of substituents on the degree of ionization of the acid.  In addition they wanted to see which substituent on the para position would increase the dissociation of the acid and which substituent would retard the dissociation.  They found that electron donating substituents increased the dissociation while electron withdrawing retarded the dissociation of the acid. 

There are two types of substituent effect.  These are substituent effect by resonance and substituent effect by induction.  The inductive effect is mediated through short distance according to coulomb law.  It decays quickly more that the resonance effect.  On the contrary the resonance effect or substituent effect by conjugation can be of remote effect such as that between substituents on the benzene ring in the para positions.  

An important equation which depicts substituent effect by induction is the Taft equation.  Hammett equation depicts substituent effects by resonance in the para position of the benzene ring.  Also it depicts substituent effects by induction in the meta position.  Substituents in the para position have very low inductive effect due to their remoteness of the substituent from the reactive site.