Polyhalogenated Alkanes and Alkenes

Polychlorination of methane yields the di-, tri-, and tetrachloromethanes cheaply and efficiently  

These substances have excellent solvent properties for nonpolar and slightly polar substances. Chloroform once was used widely as an inhalation anesthetic. However, it has a deleterious effect on the heart and is oxidized slowly by atmospheric oxygen to highly toxic carbonyl dichloride (phosgene, COCl2)-Commercial chloroform contains about 1% ethanol, which destroys any COCl2 formed by oxidation. 

Carbon tetrachloride commonly was employed as a cleaning solvent, although its considerable toxicity entails considerable hazard when used indiscriminately. It has been used as a fire-extinguishing fluid for petroleum fires, but its toxicity and tendency to form still more toxic carbonyl dichloride makes it undesirable for confined areas. The common laboratory practice of removing traces of water from solvents with metallic sodium should not be applied to halogenated compounds carbon tetrachloride-sodium mixtures are shock sensitive and can detonate. 

Trichloroethene ( Tri-Clene , bp 87°) is a widely used dry-cleaning solvent. It can be prepared from either ethene or ethyne  

Compared with monohaloalkanes, polyhalogen compounds have quite different reactivities and behavior toward nucleophiles and bases. Thus di-chloromethane reacts with hydroxide ion by an SN2 mechanism much less readily than methyl chloride. The chloromethanol formed then undergoes a rapid E2 elimination to give methanal (formaldehyde), a substance that exists in water largely as dihydroxymethane  

---