Unexpected Products and Reaction Mechanisms of the Aqueous Chlorination of Cimetidine JEFFREY M. BUTH, † WILLIAM A. ARNOLD,* ,‡ AND KRISTOPHER MCNEILL* ,† Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, and Department of Civil Engineering, University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, Minnesota 55455 Many pharmaceuticals and personal care products (PPCPs) resist degradation in wastewater treatment plants. Thus, they may be transformed by chemical disinfectants in the final treatment stage, generating products that may possess enhanced toxicity/biological activity relative to the parent compounds. For this reason, the reaction of cimetidine, an over-the-counter antacid, with the frequently used disinfectant, free chlorine, was investigated. Cimetidine degraded rapidly in the presence of excess free chlorine, indicating that it will likely undergo significant transformation during wastewater disinfection. Four major products were isolated and extensively characterized by comparison of liquid chromatographic retention times to known standards, mass spectrometry, 1 H- and 2D-nuclear magnetic resonance spectroscopy, and infrared spec- troscopy. An expected sulfur oxidation product, cimetidine sulfoxide, was identified along with three unexpected products: 4-hydroxymethyl-5-methyl-1H-imidazole, 4-chloro- 5-methyl-1H-imidazole, and a product proposed to be either a - or δ-sultam. The last three products are formed by transformations not frequently observed in free chlorine reactions of PPCPs such as C-C bond cleavage and intramolecular nucleophilic substitution. The unex- pected transformations yielded compounds with more substantial structural changes than would be observed in common free chlorine reactions such as N-chlorination or electrophilic halogenation. The reaction pathway was elucidated by kinetic analysis and by independently treating isolated intermediates with free chlorine, and reaction mechanisms were proposed. Finally, the predicted no- effect concentration (PNEC) of the chlorination products was estimated, and the products 4-hydroxymethyl-5-methyl-1H- imidazole and 4-chloro-5-methyl-1H-imidazole were estimated to have lower PNECs than cimetidine. Introduction Recently, concern has grown regarding the presence of pharmaceuticals and personal care products (PPCPs) in aquatic systems (1, 2). Because these pollutants are specif- ically designed to elicit a biological response, it has been suggested that some can inflict biological harm even at trace concentrations, with possible synergistic effects (1, 2). It has been demonstrated that many PPCPs are not effectively removed by conventional wastewater treatment processes (3-5), thus passing the compounds to surface waters that receive wastewater effluent. Consequently, a wide range of PPCPs have been detected in rivers throughout the world (6-8). Their increasing production and use present a continuous source of these compounds to surface waters. Because PPCPs often persist through the wastewater treatment processes, they may be available to react with free chlorine or other oxidants used for disinfection. Free chlorine (HOCl/OCl - ) is a nonspecific oxidant used in wastewater disinfection that has been shown to degrade various PPCPs to chlorinated, oxidized, and fragmented byproducts (9- 16). Concern exists that the reaction of free chlorine with PPCPs will generate products with increased toxicity or bioactivity relative to the parent compounds. For example, the toxicants, 1,4-benzoquinone and N-acetyl-p-benzo- quinone imine, were generated from the chlorination of acetaminophen (13), and a chlorinated solution of bisphenol A exhibited a 24-fold increase in estrogenic binding affinity compared to the nonchlorinated solution (17). Because chlorination products may pose a greater threat than parent PPCPs, elucidation of product structures, formation kinetics, and toxicity are crucial for a complete assessment of environmental risk. Cimetidine (structure shown in Scheme 1), a nonpre- scription antacid sold as Tagamet, has an annual usage of 160 000 kg/year in the U.S. (18). Accounting for a 52% loss by human metabolism and an estimated removal efficiency of 70% for primary and secondary wastewater treatment yields an annual cimetidine load of 23 000 kg in U.S. wastewater effluent (18). Cimetidine, however, has only been detected in 10% of surveyed waterways throughout the U.S. (6). Previously, the chlorination of cimetidine was studied along with other PPCPs and was reported to degrade completely by reaction with free chlorine to form a single unidentified product (12). Thus, the disinfection of waste- water with free chlorine is an additional treatment step that may lead to reduction of the cimetidine load to the environ- ment. Further questions regarding oxidation kinetics, product identification, reaction mechanisms, and pH dependence were beyond the scope of the prior survey (12). The goal of the present study is to thoroughly investigate the reaction of cimetidine with free chlorine. The kinetics of cimetidine degradation and product evolution over a broad pH range were monitored by high-pressure liquid chroma- tography (HPLC). Significant structural alterations in products required extensive product characterization including high- resolution mass spectrometry (HRMS), 1 H- and 2D-NMR spectroscopy, infrared (IR) spectroscopy, and HPLC retention time matching with authentic standards. Examining the evolution of cimetidine chlorination products and the reactivity of individual products with free chlorine enabled the determination of the predominant reaction pathways. Additionally, the structure-activity model, ECOSAR (19), was used to estimate the toxicity of the reaction products. Experimental Section General. Cimetidine was purchased from Sigma. Sodium hypochlorite (NaOCl) was obtained from Aldrich (g4%) and Fisher (4-6%) and diluted to make aqueous stock solutions which were standardized by iodometric titration (20). Am- monium chloride (NH4Cl) and sodium sulfite (Na2SO3) were * Address correspondence to either author. Phone: 612-625-8582 (W.A.A.); 612-625-0781 (K.M.). Fax: 612-626-7750 (W.A.A.); 612-626- 7541 ( K.M.); E-mail: arnol032@umn.edu (W.A.A.); mcneill@ chem.umn.edu (K.M.). † Department of Chemistry. ‡ Department of Civil Engineering. Environ. Sci. Technol. 2007, 41, 6228-6233 6228 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 17, 2007 10.1021/es070606o CCC: $37.00  2007 American Chemical Society Published on Web 07/25/2007