Wheland complexes charge distribution

On the left side Figure 5.9 shows the primitive model of the charge distribution in the Wheland complex. Therein the positive charges only appear ortho and para to the reacting C atom, and in each case they equal +0.33. This charge distribution is obtained by superimposing the three resonance forms of Figure 5.1. 

A more detailed analysis of the stabilizing effect of donor substituents and the destabilizing effect of acceptor substituents (both are referred to as Subst in the following) on Wheland complexes E-QH5 -Subst explains, moreover, the regioselectivity of an Ar-SE attack on a monosubstituted benzene. Isomeric donor-containing Wheland complexes and acceptor-containing Wheland complexes have different stabilities. This follows from the uneven charge distributions in the Wheland complexes. 

Fig. 5.7. Charge distribution in Wheland complexes E-C6H6+ (refined model, on the right calculation for...

The Hammett equation usually predicts substituent effects with reasonable accuracy, provided the correct choice is made between o and o. a values are based on a system in which there is little change in the degree of conjugation between the substituent and the remainder of the molecule on moving from one side of the transition state to the other. Electrophilic aromatic substitutions pass through a positively charged complex, the Wheland intermediate (I), in which the electronic distribution is different from that of either reactants or products. A generalised sequence is shown in Equation (10). The standard reaction on... 

Structures of the nine TS are shown in Scheme 1.24. At the level of theory used, TSl and TS2 appear to be very similar. Three parameters are of interest as to whether or not these structures are derived from the bromo—sigma complex. The fint is the distance between bromide ion and the bromine atom associated to the benzene molecule, while the second is the Br—C distance of the latter. The third parameter recorded on the images of the structures is the Br-C-H angle at the C atom to which Br is attached. The importance of the Br—Br distance relative to the question of whether or not the Wheland intermediate is involved relates to the separation of charge in the bromine molecule. This requires considerable energy and is not expected to take place without compensation from bond formation of the resulting Br moiety. The distribution of charge on the... 

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