The use of ionic liquids of the type 1-butyl-3-methylimidazolium aluminium chloride ([bmim]Cl/AlCl3) as catalyst for the synthesis of monododecyl diphenyl oxides has been demonstrated. By using acetonitrile as an IR spectroscopic probe, the Lewis acidity-activity correlation for the ionic liquid catalysts in the alkylation reaction has been studied. A comparative study of the ionic liquids catalyzed system and the traditional AlCl3 system has been carried out and the effects of varying reaction time, temperature, the relative amount and acid strength of the ionic liquids, as well as the molar ratio of diphenyl oxide to a-dodecene have been explored. The yield of the target product with [bmim]Cl/AlCl3 ionic liquids as catalyst for the alkylation of the diphenyl oxide with a-dodecene is much higher than that observed using AlO3 as catalyst. Furthermore, use of ionic liquids leads to a significant simplification in product isolation, and the system can be regarded as environmentally benign. The maximum yield of the monododecyl diphenyl oxides is about 90% when [bmim]Cl/AlCl3 ionic liquids are used as catalyst under mild reaction conditions. The highest yield is obtained at a reaction temperature of 80 degreesC with a molar ratio of diphenyl oxide to a-dodecene of 7 and a-dodecene to ionic liquid molar ratio of 2. The potential for reuse of the acidic ionic liquids has been investigated. The main cause of deactivation of the acidic ionic liquid was found to be the leaching of the active catalytic species Al2Cl7- as a result of the interaction between the acidic ionic liquid and diphenyl oxide. (C) 2004 Elsevier B.V. All rights reserved.