Substituted benzenes groups that donate electrons

Figure 16.12 Electrostatic potential maps of benzene and several substituted benzenes show that an electron-withdrawing group (-CHO or —Cl) makes the ring more electron-poor (yellow-green), while an electron-donating group (-OH) makes the ring more electron-rich (red).

Certain groups attached to an aromatic ring can donate electrons into its delocalized molecular orbitals. Examples of these electron-donating substituents include —NH2 and —OH. Electrophilic substitution of benzene is much faster when an electron-donating substituent is present. For example, the nitration of phenol, C6H5OH, proceeds so quickly that it requires no catalyst. Moreover, when the products are analyzed, the only products are found to be 2-nitrophenol (ortho-nitrophenol, 37) and 4-nitrophenol (pnra-mtrophcnol, 38 . 

The Fricdel-Crafts type polyalkylation of alkyl-substituted benzenes with 3 becomes easier and faster as the number of electron-donating methyl groups on the phenyl group increases. This is consistent with the fact that the alkylation occurs in the fashion of electrophilic substitution. The tendency of starting incthylben-zenes to form reoriented products also increases in the same order from toluene to mesitylene. 

A pAa of 4.45 indicates that p-cyclopropylbenzoic acid is a weaker acid than benzoic acid. This, in turn, indicates that a cyclopropyl group must be electron-donating. Since electron-donating groups increase reactivity in electrophilic substitution reactions, p-cyclopropylbenzene should be more reactive than benzene toward electrophilic bromination. 

The alkenes most reactive to cationic polymerization contain electron-donating functional groups that can stabilize the carbocation intermediate. The reactivity order of substituents in cationic polymerization is similar to the reactivity order of substituted benzenes in electrophilic aromatic substitution reactions. 

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