Catalysis in aromatic nucleophilic substitution. Note II. Piperidino substitution reactions of some 2‐l‐3‐nitrothiophenes and 2‐l‐5‐nitrothiophenes in methanol and benzene

Abstract

AbstractThe rates of piperidino substitution of some 2‐L‐3‐nitrothiophenes (I) and 2‐L‐5‐nitrothiophenes (II) (L = Cl, Br, I, OC6H4NO2‐p, and SO2Ph) have been measured in methanol and in benzene at various piperidine concentrations. The reactivity of compounds (I) is not affected by the piperidine concentration in both methanol and benzene, except for the case of L = I (Ic). Probably due to association effects, the reactivity of Ic in benzene decreases as the piperidine concentration is increased. The reactions of compounds II follow overall second order kinetics in methanol while in benzene a different behaviour is observed as a function of the nature of the leaving group. In fact, the piperidino substitutions of IIa‐c (L = Cl, Br, I) are mildly accelerated at high piperidine concentrations (a moderate solvent effect); on the contrary the reactivity of IId and e shows a strong dependence on the piperidine concentration, pointing out a genuine base catalysis.