Transition state, Wheland model

Wheland intermediate (see below) as its model for the transition state. In this form it is illustrated by the case mentioned above, that of nitration of the phenyltrimethylammonium ion. For this case the transition state for -nitration is represented by (v) and that for p-substitution by (vi). It is argued that electrostatic repulsions in the former are smaller than in the latter, so that m-nitration is favoured, though it is associated rvith deactivation. Similar descriptions can be given for the gross effects of other substituents upon orientation. 

M.o. theory and the transition state treatment In 1942 Wheland proposed a simple model for the transition state of electrophilic substitution in which a pair of electrons is localised at the site of substitution, and the carbon atom at that site has changed from the sp to the sp state of hybridisation. Such a structure, originally proposed as a model for the transition state is now known to describe the (T-complexes which are intermediates in electrophilic substitutions... 

The fact that the ratios of rates were much greater in chlorination than in nitration, prompted Dewar to suggest that the actual transition state was intermediate between the Wheland model and the isolated molecule model. He accommodated this variation in the relative rates within his discussion by treating yS as a variable whose value depended on the nature of the reaction. With the notation that y ) is the... 

Dewar s treatment of transition state structure, using reactivity numbers, has the logical defect that in the intermediate kinds of transition states for which it provides evidence the electron localisation is only partial. However, in obtaining the values of the reactivity numbers (which are approximate localization energies), the process of localization is considered to be complete thus, values of parameters which strictly are relevant only to the Wheland type of transition state are incorporated into a different model. ... 

What we shall be doing in the discussion that follows is comparing the effect that a particular Y would be expected to have on the rate of attack on positions o-/p- and m-, respectively, to the substituent Y. This assumes that the proportions of isomers formed are determined entirely by their relative rates of formation, i.e. that the control is wholly kinetic (cf. p. 163). Strictly we should seek to compare the effect of Y on the different transition states for o-, m- and p-attack, but this is not usually possible. Instead we shall use Wheland intermediates as models for the transition states that immediately precede them in the rate-limiting step, just as we have done already in discussing the individual electrophilic substitution reactions (cf. p. 136). It will be convenient to discuss several different types of Y in turn. 

What if we model a late transition state using a Wheland intermediate The Hiickel k energies, which are as follows ... 

Electrophilic attack occurs more readily at the a than at the )8 position in all five-membered rings. The preference for a substitution may be rationalized by comparing the energies of the transition states leading to a- and -substituted products. Taking the Wheland intermediates as models for the transition states, it is possible to see that, while for a attack, three limiting resonance structures (17-19)... 

Many reasons may be responsible for the different a )3 ratios observed in the four rings. In particular, the small ar.fi ratios for pyrrole [as well as the low sensitivity of this ring to substituent effects (Section IV, B)] may be due to the fact that in this case the Wheland intermediate is not a good model for the transition state. This hypothesis is in keeping with the Hammond postulate,183 according to which the transition state approaches closer to the unperturbed starting molecule as its reactivity increases. 

Ar-SE reactions (cf. Figure 5.1). This, in turn, means that Wheland complexes are also a good—even the best—model for the transition state of Ar-SE reactions. 

Electrophilic Substitution Reactivity Much of the electrophilic reactivity of aromatics is described in great detail in a comprehensive recent book of Taylor [10]. We shall focus attention on the electrophilic substitution reactivity of annelated benzenes and try to interpret the orientational ability of fused small rings. For this purpose we consider here Wheland transition states of the electrophilic substitution reactions. It is also convenient to take the proton as a model of the electrophilic reagent. In order to delineate rehybridization and 7r-electron localization effects, let us consider a series of angularly deformed benzenes (Fig. 21), where two vicinal CH bonds bent toward each other mimick a fused small ring. Angles c of 110° and 94° simulate five and four membered... 

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