Wheland complexes substituent effects

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

Stabilization and Destabilization of Wheland Complexes through Substituent Effects... 

Which Wheland complexes are the most stable This is determined to a small extent by steric effects and to a considerably greater extent by electronic effects As a car-bocation, a substituted Wheland complex is considerably more stable than an unsubstituted one only when it carries one or more donor substituents, and unsubstituted Wheland complexes E—C6Hj are still considerably more stable than Wheland complexes that contain one or more acceptor substituents. Therefore, donor-substituted benzenes are attacked by electrophiles more rapidly than benzene, and acceptor-substituted benzenes are attacked more slowly. 

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. 

In this example the phenyl substituent with its +M effect determines the structure of the most stable Wheland complex intermediate and thus the regioselectivity. The competing +1 effect of the alkyl substituent cannot prevail. This is understandable because it is not as effective at stabilizing an adjacent positive change as the phenyl group (cf. Table 5.3). 

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... 

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