haloforms

Haloforms are a class of organohalogen compounds characterized by the presence of a trihalomethyl group. They have the general formula CHX3, where X is a halogen such as fluorine, chlorine, bromine, or iodine. The most common haloforms are chloroform CHCl3, bromoform CHBr3, and iodoform CHI3; fluoroform CHF3 is much less common. These compounds are typically volatile, with chloroform and bromoform being liquids at room temperature and iodoform a solid.

The term haloform also refers to a family of reactions that generate haloforms from methyl ketones in basic halogenation. In the haloform reaction, a methyl ketone R-CO-CH3 is treated with halogen (X2) in a basic medium (usually NaOH). The methyl group is successively halogenated to CX3, which is cleaved by hydroxide to give the carboxylate R-COO− and the haloform CX3− (which leaves as CHX3). Overall: R-CO-CH3 + 3 X2 + 4 OH− → RCOO− + CX3− + 3 X− + 3 H2O.

Applications and safety: Haloforms have roles as solvents and reagents; chloroform has a historical use as an anesthetic and extraction solvent, while iodoform is used in qualitative analysis (the iodoform test) where a yellow CHI3 precipitate indicates the presence of a methyl ketone or ethanol. Safety concerns are notable: chloroform is hepatotoxic and potentially carcinogenic, and other haloforms can be toxic or environmentally persistent. They can form as disinfection byproducts in chlorinated water, prompting cautious handling and monitoring.

See also: haloform reaction; iodoform test.