Trihaloalkane -- Chloroform

Trihaloalkanes replace three hydrogens of a hydrocarbon with three halogens, e.g. CHCl3 (chloroform) and CHI3 (iodoform). Chloroform was named by Dumas because it yields formic acid on hydrolysis. Laboratory preparation (the haloform reaction), from ethanol and bleaching powder (a source of both chlorine and Ca(OH)2), proceeds in three steps: Step 1, oxidation -- ethanol + Cl2 gives acetaldehyde + 2 HCl; Step 2, chlorination -- acetaldehyde + 3 Cl2 gives trichloroacetaldehyde (chloral) + 3 HCl; Step 3, hydrolysis -- 2 chloral + Ca(OH)2 gives 2 chloroform + calcium formate. Physical properties: chloroform is a colourless liquid with a peculiar sickly smell and a burning taste; its vapour, inhaled, depresses the central nervous system and causes unconsciousness, which is why it was used as an anaesthetic. Chemical properties: (1) Oxidation -- chloroform, in air and light, slowly oxidises to the highly poisonous gas phosgene (COCl2) + HCl, which is exactly why phosgene contamination makes old chloroform unsafe as an anaesthetic. (2) Reduction -- chloroform + Zn/HCl gives methylene chloride + HCl. (3) Nitration -- chloroform + HNO3 gives chloropicrin (trichloronitromethane), used as an insecticide and soil-sterilising agent. (4) Carbylamine reaction -- chloroform + an aliphatic or aromatic primary amine + alco …