Arenium ions destabilization

Substituents already bonded to an aromatic ring influence both the rate of electrophilic substitution and the position of any further substitution. The effect of a particular substituent can be predicted by a consideration of the relative stability of the first-formed arenium cation, formation of which constitutes the rate-lintiting step. In general, substituents that are electron releasing activate the ring to further substitution - they help to stabilize the arenium ion. Substituents that are electron withdrawing destabilize the arenium ion, therefore, are deactivating and hinder further substitution. 

Since charge delocalization into the 1-Py-, 4-Py-, and 9-Phe-substituents was quite effective, the presence of a-Cp3 greatly increased Ti-participation and the arenium ion character. On the other hand, diminished charge delocalization into a 2-Py-substituent, coupled with destabilization by a-Cp3, provoked ring protonation. Representative NMR data for these carbocations are shown in Pig. 5. 

The effects of other groups can be understood by similar reasoning. When the electrophile bonds ortho or para to the substituent, the positive charge is located on the carbon that is bonded to the substituent in one of the resonance structures. If the substituent is one that can stabilize the carbocation, then it accelerates the reaction and directs the incoming electrophile to the ortho and para positions. If, on the other hand, the substituent is one that destabilizes the carbocation, then it slows the reaction and directs the electrophile to the meta position so that the positive charge is never on the carbon directly attached to the substituent. The meta arenium ion is less destabilized than the ortho or para ions in this case. Some other examples will help clarify this reasoning. 

This time there is no resonance structure that has the positive charge on the carbon bonded to the nitro group. The arenium ion is still destabilized by the electron-withdrawing effect of the nitro group, but this ion is not destabilized as much as the ion produced by attack of the electrophile at the ortho position because the positive charge is never as close to the nitro group. 

Because 70 and 72 are generated much more slowly than 67, the major product is 3-bromonitrobenzene (56). Such an intermediate is very unstable, is difficult to form with a high activation energy barrier, and will react more slowly than benzene and much more slowly than derivatives bearing an electron-releasing group. Further, the arenium ion arising from attack at the ortho and para carbons is more stabilized and will form much more slowly than the less destabilized meta arenium ion, so the major product is meta. 

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