Chloroform (Trichloromethane, \(\text{CHCl}_3\)) is a dense, colorless, volatile liquid known for its sweet, ether-like odor. It is classified as a trihalomethane, meaning three hydrogen atoms in a methane molecule have been replaced by chlorine atoms. Historically, chloroform was famously used as an inhalational anesthetic during the 19th and early 20th centuries but was abandoned due to safety and toxicity concerns. Today, it is primarily valued in industrial settings as a solvent and as a chemical intermediate in the production of refrigerants and plastics.
Intentional Synthesis in the Laboratory
The classic method for the deliberate production of chloroform is the Haloform Reaction, an organic chemistry technique historically used for industrial production. This reaction requires two main components: a compound containing a methyl ketone group (\(\text{R-C}(=\text{O})\text{CH}_3\)) or a secondary alcohol, and a halogen combined with a base. Common starting materials include acetone or ethanol, while reagents are typically chlorine (\(\text{Cl}_2\)) or sodium hypochlorite (\(\text{NaOCl}\)) mixed with sodium hydroxide (\(\text{NaOH}\)).
The formation proceeds through three stages, beginning with the base-catalyzed halogenation of the starting material’s methyl group. The base abstracts an acidic hydrogen atom, forming an enolate intermediate that reacts with the halogen. This substitution repeats three times until all three hydrogen atoms are replaced by chlorine atoms, resulting in a trihalogenated methyl ketone.
In the final stage, the highly electronegative chlorine atoms make the carbon atom attached to the oxygen susceptible to nucleophilic attack. A hydroxide ion from the base attacks this carbon, leading to the cleavage of the \(\text{C-C}\) bond and the expulsion of a stable trichloromethyl anion (\(\text{CCl}_3^-\)). This anion accepts a proton from the newly formed carboxylic acid, yielding the desired chloroform (\(\text{CHCl}_3\)) and a carboxylate salt.
Unintentional Formation During Water Disinfection
The primary public health concern is chloroform’s unintentional formation as a Disinfection Byproduct (DBP) during drinking water treatment. Water utilities use chlorine or hypochlorite to kill pathogens. The added chlorine reacts with natural organic matter in the source water, initiating chemical reactions that produce chloroform.
The precursors are Natural Organic Matter (NOM), consisting of decaying plant material, humic acids, and other organic compounds. Chlorine, typically as hypochlorous acid (\(\text{HOCl}\)) or hypochlorite ion (\(\text{OCl}^-\)), oxidizes and substitutes these precursors. This leads to the formation of trihalomethanes (THMs), with chloroform being the most abundant.
The final concentration depends on several environmental and operational factors. Higher NOM concentrations in the raw water supply increase formation potential. The process is accelerated by warmer water temperatures and longer contact times. Slightly alkaline conditions (\(\text{pH}\) 8.0) enhance chloroform formation compared to neutral or acidic conditions.
Natural Sources and Environmental Production
Chloroform is not exclusively man-made; it is also produced through natural biological and geological processes. Natural emissions account for a significant portion of the total global input of chloroform to the atmosphere and hydrologic system.
Biological production is a significant pathway, particularly in terrestrial and marine ecosystems. Certain species of fungi in soil and marine algae, such as seaweed, metabolize organic compounds and release chloroform as a byproduct. This biologically mediated chlorination often involves hypochlorous acid, resulting in a trichloroacetyl intermediate that is subsequently hydrolyzed to chloroform.
Abiotic processes also contribute to natural chloroform generation. The compound is released during biomass burning events, such as forest fires, and can also be emitted through volcanic activity. In soil, abiotic formation can occur through the decarboxylation of trichloroacetic acid or the transformation of other chlorinated compounds.