Resonance effects, activating-deactivating

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

Inductive and resonance effects account for the directing effects of substituents as well as for their activating or deactivating effects. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 16.13. 

This is so because the methyl substituent can affect the rate and the position of further substitution. A substituent can either activate or deactivate the aromatic ring towards electrophilic substitution and does so through inductive or resonance effects. A substituent can also direct the next substitution so that it goes mainly ortho/para or mainly meta. 

The effect of fluorine, chlorine, or bromine as a substituent is unique in that the ring is deactivated, but the entering electrophile is directed to the ortho and para positions. This can be explained by an unusual competition between resonance and inductive effects. In the starting material, halogen-substituted benzenes are deactivated more strongly by the inductive effect than they are activated by the resonance effect. However, in the intermediate carbocation, halogens stabilize the positive charge by resonance more than they destabilize it by the inductive effect. 

It is evident that the manifestations of indirect deactivation and indirect activation are due to the substituent resonance effect and virtually point to increased transfer of the resonance effect to the reaction site. 

Substituents on the benzene ring affect both the reactivity of the rii toward further substitution and the orientation of that substitution. Grouj can be classified as ortho- and para-directing activators, ortho- an para-directing deactivators, or meta-directing deactivators. Sul stituents influence aromatic rings by a combination of resonance and indu five effects. Resonance effects are transmitted through -ir bonds indu tive effects are transmitted throu [Pg.654]

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