Regioselective mononitration of chlorobenzene using caprolactam-based Brønsted acidic ionic liquids

• Green synthesis of chlorobenzene with low toxicity caprolactam Brønsted acidic ionic liquids.
• Investigation of the reaction conditions on the synthesis of chlorobenzene.
• The reasonable explanations for improving the yield and regioselectivity.

Three kinds of Brønsted acidic ionic liquids caprolactam benzenesulfonate ([CP]BSA), caprolactam hydrosulfate ([CP]HSO4) and caprolactam p-toluenesulfonate ([CP]pTSA) were synthesized. The structures of the ILs were confirmed by 1HNMR and FT-IR. Regioselective mononitration of chlorobenzene has been investigated in HNO3–Ac2Osystem with three ionic liquids. The sequence of the nitrification activity is [CP]HSO4 > [CP]pTSA > [CP]BSA. With the caprolactam hydrosulfate as the catalyst the influences of reaction time, reaction temperature and the amount of the catalyst were studied in the nitration of chlorobenzene. The results showed that the optimum reaction conditions were as follows: when the mole ratio of chlorobenzene to ionic liquid was 10:1.5, the reaction temperature was 60 °C and the reaction time was 2.5 h, the yield of mononitro-chlorobenzene could reach 71.22%. And the mass ratio of para/ortho isomer was 7.74, which was much more than 2.0 of para/ortho isomer mass ratio obtained by nitric-sulfuric mixed acids as catalyst. Besides, the ionic liquid could be used repeatedly.

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