A closer look at
substituent effects in aromatic rings

Electrophilic aromatic substitution is a two step process with a positively charged intermediate. The transition state of the first, rate determining step resembles this intermediate. So we want to test the hypothesis that substituents which stabilise the positive charge increase the rate of the reaction. This stabilisation depends on the position of the substituent with respect to the positive charge, which is the cause of the directive effect these substituents have.

In the first part of this exercise one can introduce substituents in two compounds that act as a model for the transition state.
Both are structures with a positive charge that is stabilized or destabilized by substituents at the various positions. In this way one can compare the effects a substituent has in either the meta or the para position. (The ortho position is often not included in these studies because it introduces a new steric effect on top of the electronic effect we want to measure.)

1. The benzenonium ion, upper structure, a 'protonated benzene' single structure, for which the 'Heat of Formation' can be determined, with a substituent first meta, than para, to establish the more stable one. If the para isomer is more stable, the substituent will prefer the reaction to take place at the para position, and it is classified as para-directing. 2. The benzyl cation, lower structure, a structure which looks less like the transition state, but is easily accessible for practical experiments. In fact the dissociation constant of benzyl chlorides is used to define the Hammett σ+ scale.

Another approach is the computational simulation of the first step of the substitution reaction. This gives for each position the value of the calculated activation energy. Using this figures not only the meta and para positions can be compared, but also the difference with the unsubstituted compound. In other words: whether the substituent is activating (causes a lower activation energy) or deactivating with respect to benzene.

3. The 'nitration of benzene'

inludes a reaction path calculation for the reaction between benzene and a nitronium ion. From the energy plot the activation energy can be determined.
From these crude calculations one can derive a qualitative scale of activating and deactivating substituents, and their directive effects. A table of real Hammett sigma constants can be found in your text book, or by a Google search.

It is more difficult to see why or how a substituent stabilizes the positive charge in the benzenonium intermediate or the benzyl cation.
Normally in calculations (of alkanes) C-H bonds are polarized, with charges like C -0.22, H +0.22.
In the benzenonium ion the ortho C's have a charge of more or less zero, the para is 0.14, indicating the donation of charge. So they become less negative, more positive.
Another method of comparison is by adding the hydrogen charge to the charge of the carbon it is connected to.
This information can be read from the output files, the mopac.arc or mopac.out files that are listed on the display page after a calculation.

Carbon charges in benzenonium ion:

  C      -0.2328  protonated C
  C       0.0491  ortho to C1, more positive
  C      -0.2115  meta, no effect
  C       0.1399  para, more positive
  C      -0.23  meta
  C       0.0469  ortho, more positive

In Chime, one can display a surface coloured by electrostatic potential. However, the information listed above hardly shows up in the electrostatic potential map, because Chime detects that the whole species is positive, and makes the whole structure dark blue. 'Minor' differences between -0.2 and +0.2 become invisible. When using the so-called Rasmol colouring scheme (lower model), at least the difference between meta and ortho/para positions can be seen. You can click on the mopac.arc files, after a calculation, in order to read the charges calculated for a structure. Check this for meta-OH and para-OH (carbons and oxygen!).

Can you think of another way to check for the difference between ortho/para and meta positions in the stabilization of charge, by e.g. chlorine? (hint: any geometrical aspects?)