CHLOROHYDROCARBON
solvents are finding even greater use in time of war than before. Production capacities for scarce raw materials as well as for the solvents themselves have had to be expanded repeatedly. Supplies of solvents for less essential civilian uses have been greatly curtailed or elimhated. Some peacetime uses, such as carbon tetrachloride for fire extinguisher fluid, have been directly transposed to war uses. Trichlorethylene is speeding production of tanks, airplanes, guns, and ammunition through its previously established use for solvent degreasing of metals. On the other hand, curtailment of civilian suppIy is often caused by diversion t o purely war uses. Perchlorethylene is going into the manufacture of the smoke screen ingredient hexachlorethane, and for long periods little or none has *
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e
The photograph shows equipment for solvent distillation of crude trichlorethylene.
been available for dry cleaning. Methyl chloride and ethylene dichloride are used in the synthetic rubber program. Methylene cholride i s serving as a nonflammabIe paint remover in ship reconditioning. Likewise, tetrachlorethane is contributing in an important way to the war program. Many other peacetime and wartime applications of the chlorohydrocarbons depend to a great extent on their near or absolute nonflammability, coupled with their excellent solvent power for oils, fats, and other organic materials. This discussion is confined to the commercially important groups of chlorinated aliphatic hydrocarbons-namely, the chlorine substitution products of methane, ethylene, ethane, and propane. Chlorinated derivatives, such as chloroalcohols and chloroethers, and the chlorinated aromatics are beyond the scope of the paper. The most commonly used of the chlorinated aliphatics are methylene chloride (dlchloromethane)7 chloroform, and carbon
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INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1943
tetrachloride of the methane group; trichlorethylene and perchlorethylene (tetrachloroethylene) of the ethylene group; ethylene dichloride (1,2-dichloroethane) of the ethane group; and propylene dichloride (1,2-dichloropropane) of the propane group. Of lesser importance as solvents are methyl chloride, the trans and cis isomers of sum-dichlorethylene, ethyl chloride, p-trichlorethane (1,1,2-trichloroethane), symtetrachlorethane, and pentachlorethane. Methods for commercial production of these chlorinated aliphatics show considerable variety. I n general, the chloromethanes are derived from either methane or methanol. However, chloroform is also commonly made from acetone, and carbon tetrachloride from carbon disulfide. The chloroethylenes are most commonly obtained from acetylene. The chloroethanes are made variously from acetylene, ethylene, or ethanol. Propylene dichloride is manufactured by direct addition of chlorine t o propylene. FLAMMABILITY
The chief value of the chloro compounds over unsubstituted hydrocarbons is low flammability or nonflammability. Except for methyl chloride, ethyl chloride, dichlorethylene, ethylene dichloride, and propylene dichloride, all have flash points above 80” F. and therefore are considered nonflammable under Bureau of Explosives regulations. Methylene chloride is rated by the Underwriters’ Laboratories as practically nonflammable a t ordinary temperatures; chloroform and trichlorethylene are rated nonflammable and nonexplosive. Carbon tetrachloride, perchlorethylene, 0-trichlorethane, tetrachlorethane, and pentachlorethane are also nonflammable. Of the nonflammables, methylene chloride, chloroform, and trichlorethylene, and some others with hydrogen in the molecule, form weakly combustible mixtures at higher temperatures with high concentrations of solvent in air. Methyl chloride, ethyl chloride, and dichlorethylene are rated by the Underwriters’ Laboratories as moderately flammable. Ethylene dichloride is likewise moderately flammable and is given a fire hazard rating of 60-70 by the Underwriters, which places it in a class with ethanol. Propylene dichloride is moderately flammable. Flash points of these flammable chlorohydrocarbons are shown in Table I. STABILITY
Each of these chlorinated aliphatic hydrocarbons is sufficiently stable for at least a narrow range of conditions of commercial application. Some conditions of use, however, may bring about a breakdown with formation of acidic decomposition products. It
TABLE I. FLAMMABILITY OF CHLOROHYDROCARBON SOLVENTS Chlorohydrocarbon
Flammability”
Flash Point,
c.
Methyl chloride Methylene chloride Chloroform Carbon tetrachloride
Moderate NonHammable Nonflammable Nonflammable (0)
