Resonance chloronitrobenzenes

Nucleophilic aromatic substitution occurs only if the aromatic ring has an electron-withdrawing substituent in a position ortho or para to the leaving group. The more such substituents there are, the faster the reaction. As shown in Figure 16.18, only ortho and para electron-withdrawing substituents stabilize the anion intermediate through resonance a meta substituent offers no such resonance stabilization. Thus, p-ch oronitrobenzene and o-chloronitrobenzene react with hydroxide ion at 130 °C to yield substitution products, but m-chloronitrobenzene is inert to OH-. 

That pyridine species with decreased electron densities at positions adjacent to the nitrogen atom make significant contributions to the resonance hybrid even in the ground state has been shown by dipole measurements (12, 14). Interaction with a nucleophilic agent is consequently expected to follow a mechanism analogous to that involving o-chloronitrobenzene (Equation 4). 

Scheme VI is not consistent with our findings, as it requires the formation of alkoxy-substituted products in significant amount. Scheme VII is more attractive. Evidence has been reported that nitroarenes undergo aromatic proton abstraction in basic solutions (46b). Moreover, preliminary experiments performed with an ion cyclotron resonance (ICR) mass spectrometer indicate that proton abstraction is a major path in the reaction of t-BuO with 4-chloronitrobenzene in the gas phase at low pressures (C.

Draw resonance contributors for the carbanion that would be formed if iwera-chloronitrobenzene were to react with hydroxide ion. Why doesn t the reaction occur ... 

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