Isoconjugate molecules reactions

The discussion of radical cations (see Fig. 7.7) indicates that the energy of union of an odd AH radical and an odd AH cation to form a radical cation is just half the energy of union of the two corresponding odd AH radicals to form a neutral AH. As long as the resulting AH is alternant, the rules for aromaticity for it and for the radical cation derived from it will be the same and it is easily shown that this is also true for isoconjugate molecules containing heteroatoms. The course of pericyclic reactions where the transition... 

The philosophy of perturbation theory is to consider the whole group of reactants undergoing a given reaction as variations on a central theme. We have seen how this principle is applied to heteroatomic systems by regarding them as perturbed forms of the isoconjugate hydrocarbons likewise it is convenient to treat as many such molecules as possible in terms of a fixed kernel with varying substituents attached to it. This is indeed the procedure commonly followed in chemistry, confirming the view that chemistry is in effect an exercise in perturbation theory. Our next problem then is to consider how substituents may influence reactivity. 

The reader may have noted that the reactions so far considered have all been of EOt type and may consequently have wondered if the EO/OE classification has any point to it. The reason for this bias is that the majority of odd neutral conjugated molecules are conventionally regarded as derivatives of even ones containing a — jE substituent. Thus since aniline (133) is isoconjugate with an odd AH anion (134), electrophilic substitution of (133) could be classed as OE l, the rate-determining step being conversion of (133) to an... 

G -type, the transition state (121) would be isoconjugate with the p-xylylene dianion (122), which is not antiaromatic. It is admittedly impossible to predict the MO energies in (121) on the basis of PMO theory because there are so many heteroatoms in it and because (122) is not in any case a normal molecule, being derived from an even AH [i.e., p-quinodimethane (123)] by the introduction of two electrons into the lowest antibonding MO (LUMO). It is quite possible that the HOMO and LUMO of (121) are degenerate and that the reaction is an unusual G-type process in which the BO hole arises from this accidental degeneracy. 

It is worth noting here that the Dewar approach also allows one to analyse the alternative reaction processes. The [2 + 2] cyclo-addition reaction, for example, need not be concerted but could proceed via a diradical (or zwitter-ionic) intermediate (Equation 5.4). The transition state here is isoconjugate with butadiene (a non-aromatic molecule), which has a lower energy than the anti-aromatic cyclobutadiene, and hence the activation energy should not be unreasonably high in suitably substituted systems. Tetrafluoroethylene, for example, is known to dimerize by just such a diradical mechanism. 

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