DICHLOROBENZENE

any of three isomeric substances produced by the chlorination of benzene or chlorobenzene in the presence of iron(III) chloride. All three are colourless, denser than water, and insoluble in it. They belong to the family of organic halogen compounds. The reaction of benzene with chlorine in the presence of iron(III) chloride replaces the hydrogen atoms by chlorine atoms. The first product is chlorobenzene (C6H5Cl); and continued treatment with chlorine leads to ortho- and para-dichlorobenzene (C6H4Cl2), but very little of the meta isomer is formed. The ortho and para isomers can be separated by fractional freezing: upon cooling the mixture, the para isomer crystallizes while the ortho isomer remains liquid and drains away; meta-dichlorobenzene may be prepared from the other isomers by heating them under pressure with aluminum chloride. When the chlorination is conducted under strong illumination and without iron(III) chloride or a similar catalyst, the reaction yields benzene hexachloride (q.v.) instead of chlorobenzene or polychlorobenzenes. Ortho- or 1,2-dichlorobenzene is a mobile liquid that is used as a solvent, as an insecticide, and in the manufacture of other chemicals, particularly dyestuff intermediates. Meta- or 1,3-dichlorobenzene also is a liquid. Para- or 1,4-dichlorobenzene is a crystalline solid that has a strong camphorlike odour and is widely used as a moth repellent. The para isomer is the most easily isolated of the dichlorobenzenes and was first described in 1864; the ortho and meta isomers were not recognized until 1875.