Trihalomethanes halogen compounds

Carbene precursors are compounds that have or acquire good leaving groups (e.g., halide ions). Thus, halogen compounds frequently are carbene sources. Trihalomethanes are the oldest known sources of dihalocarbenes but there are other methods for generating carbenes, and some of these are listed for reference in Table 14-2 (see also Section 14-10C). There is a question as to whether a free carbene actually is formed in some of these reactions, particularly those involving metals, but for our purposes we will classify them as routes to carbenes or carbenelike species. 

Trihalomethanes (THM) Compounds formed by the reaction between the halogen elements of chlorine and bromine and methane forming chloroform, bromoform, dibromochloromethane and bromodi-chloromethane. The methane (CH4) molecule has three of its hydrogen atoms replaced by either bromine or chlorine. 

Trihalomethanes (TTHMs) are organic chemicals that contain one carbon atom, one hydrogen atom, and three halogen atoms. The most common trihalomethanes found in water are trichloromethane (chloroform), bromodichloromethane, dibromochlo-romethane, and tribromoethane (bromoform). Chloroform is found in the highest concentrations. It is formed by the reaction of free chlorine with certain natural organic compounds in the water. 

Chlorine has been the most widely used disinfectant in the United States however, it produces trihalomethanes (THMs) and other halogenated organic compounds in drinking water. Because of this, water suppliers are beginning to utilize other disinfectants, such as ozone, chlorine dioxide, and monochioramine, or combinations of disinfectants, such as ozone followed by chlorine. 

Low-level chlorination of 0.5-1.5 mg/1, resulting in a chlorine residual of 0.1 -0.2 mg/1, is used to reduce the degree of biofouling. In sea-water, chlorine oxidizes bromide, present at about 65 mg/1, to bromine, which also contributes to the generation of halogenated by-products [176]. The by-products include hypobromous acid, hypobromite, chloramines, bromamines, trihalomethanes, haloacetonitriles,haloacetic acids, and small amounts of halophenols. However, some haloforms and bromophenols as well as other organobromine compounds are also produced naturally in coastal waters [ 181,182]. Empirical equations for the disappearance of chlorine/bromine derived oxidants from brackish water have been published [183]. 

Boyce, S. D. and J. F. Hornig. 1983. Reaction pathways of trihalomethane formation from the halogenation of dihydroxyaromatic model compounds for humic acid. Environ. Sci. Technol. 17 202-211. 

Chloramines resulting from the reaction of chlorine with nitrogen-containing compounds (ammonia, urea) are the main chemicals responsible for chlorine smell and eye/skin irritation in swimming pools. Saltwater chlorination lowers the chloramine level by a mechanism similar to breakpoint chlorination. The excess concentration of chlorine relative to ammonia close to the electrode surface results in chloramine decomposition prior to injection of treated water into the pool [7]. In the presence of organics, saltwater chlorination, as all chlorine-based disinfections, can generate unhealthy halogenated by-products, in particular trihalomethanes and haloacetic acids. 

Although halogenated carbenes such as dichlorocarbene ( CCl2) are common and can be generated (among other ways) by base treatment of the corresponding trihalomethane, for example, chloroform (trichloromethane, HCCI3), with base, the discussion of that and related processes and those compounds is reserved until... 

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