INTERPRETATION OF DIRECTING EFFECTS

We noted earlier that with the exception of the halogens ortho, para directors activate the ring toward electrophilic substitution by supplying eleetton density to the ring. But why are the ortho and para positions especially susceptible to attack To answer this question, consider the stability of the cyclohexa-dienyl carbocation that forms in the first step of the electrophilic aromatic substitution mechanism. The r ioselectivity of the reaction is controlled by the stability of the carbocation. To determine the stability of a cydohexadienyl carbocation, we must compare all the possible resonance forms. Thus, we compare the stabihty of the intermediate carbocations resulting from attack at the ortho and para positions with those resulting when an electrophile attacks at the meta position. 

we will consider the nitration of toluene at the ortho and para positions. Attack at either the ortho or the para position results in one resonance structure with a positive charge on the ring carbon atom bonded to the methyl group. This tertiary carbocation makes a major contribution to the stabihty of the resonance hybrid. 

Now consider nitration at the meta position. The resonance structures show that positive charge cannot reside on the carbon atom attached to the methyl group. Only secondary carbocations are possible, and they are less stable than tertiary carbocations. 

Substitution at any position of toluene occurs at a faster rate than substitution of benzene. However, substitution occurs faster at the ortho and para positions than at the meta position. 

What about attack at the meta position None of the resonance forms of the intermediate has a positive charge on the carbon atom bonded to the nitro group— whose nitrogen atom, we noted above, has the formal charge of +1. The resonance forms of the intermediates resulting from meta attack are therefore more stable overall than the resonance forms of the intermediates formed from ortho and para substitutions. Hence, meta substitution is favored. 

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