Trihalogenation

Trihalogenation is the introduction of three halogen atoms into a molecule. In organic chemistry, this term most often refers to replacing three hydrogen atoms in a hydrocarbon with halogen atoms (chlorine, bromine, iodine, or fluorine), producing trihalogenated derivatives such as CHCl3, CHBr3, or CCl3H. The concept also covers sequences of halogenation on saturated substrates or addition of halogen across multiple bonds when halogen is used in excess, yielding polyhalogenated products.

For alkanes, tri-halogenation commonly occurs via radical chain reactions in the presence of UV light or heat. Halogen molecules generate radicals that abstract hydrogen from the substrate, creating carbon-centered radicals that react with another molecule of halogen to form C–X bonds. Chlorination is rapid but less selective, giving mixtures of mono-, di-, and tri-halogenated products, whereas bromination is more selective for certain C–H sites. A familiar example is the stepwise chlorination of methane, which can produce chloromethane (CH3Cl), dichloromethane (CH2Cl2), and culminate in chloroform (CHCl3) under appropriate conditions.

Common tri-halogenated compounds include chloroform (CHCl3), tribromomethane (CHBr3), and iodoform (CHI3), as well as mixed-halogen products like CHBrCl2 or CCl2HBr. In reactions on alkenes or alkynes, excess halogen can lead to further addition and formation of polyhalogenated products, though outcomes depend on the substrate and halogen used.

Trihalogenation is of interest for synthesis and industrial applications, but many trihalogenated compounds raise environmental and safety concerns due to toxicity and persistence.