polychlorination

Polychlorination is the introduction of multiple chlorine atoms into an organic molecule, producing polychlorinated derivatives. It occurs in both synthetic chemistry and environmental contexts and can proceed through different mechanisms depending on the substrate.

In aliphatic substrates, radical chlorination is typical. Under heat or light, Cl• radicals abstract hydrogen atoms to form carbon-centered radicals, which react with Cl• to give chlorinated products. This chain process can continue, yielding monochloro-, dichloro-, trichloro-, and higher chlorinated products. Chlorination selectivity is modest for chlorine, although more reactive C–H bonds tend to react faster, influencing the distribution of products.

In aromatic substrates, chlorination proceeds mainly by electrophilic aromatic substitution. The reaction can produce mono-, di-, tri-, and higher polychlorinated aromatics, with the degree and pattern of substitution influenced by existing substituents and ring electronics. Conditions and catalysts modulate the extent of chlorination.

Common illustrative cases include the chlorination of methane, which yields chloromethane, dichloromethane, chloroform, and carbon tetrachloride in sequence; and the chlorination of benzene, which can yield mono-, di-, tri-, up to hexachlorobenzene under appropriate conditions.

Polychlorinated compounds have widespread industrial uses but are also persistent environmental pollutants with potential toxicity and bioaccumulation. Their occurrence and fate are the focus of regulatory and risk-assessment efforts. Analytical characterization typically relies on chromatography coupled with mass spectrometry, as well as NMR and IR spectroscopy.