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(2-methyl-l, 3-butadiene), an unsaturated aliphatic hydrocarbon, CH2=C(CH3)—CH=CH2. It is a colorless, mobile, highly volatile inflammable liquid with a characteristic odor. Its melting point is — 145.95°C; boiling point, 34.067°C; flash point, -48°C; density, 0.681 g/cm3 (20°C); refractive index nD20, 1.42194; and heat of polymerization, —74.9 kJ/mole (—17.9 kcal/mole). Mixtures of isoprene and air are explosive at volume concentrations of 1.66–11.5 percent. Isoprene is insoluble in water but readily soluble in most hydrocarbon solvents. It forms binary azeotropes with such compounds as methanol, ethanol, acetone, diethyl ether, and carbon disulfide, as well as ternary azeotropes, for example, with acetone and water. Addition occurs readily between isoprene and hydrogen, halogens, hydrogen halides, and primary and secondary amines across the double bonds. An important property of isoprene is the capacity to easily polymerize and copolymerize, for example, with butadiene, styrene, acrylonitrile, and propylene.
The basic industrial methods of preparing isoprene are (1) the reaction of isobutylene with formaldehyde to give 4, 4-dimethyl-1, 3-dioxane, followed by catalytic decomposition of the latter to isoprene and formaldehyde (the so-called dioxane method); (2) the catalytic dehydrogenation of isopentane and isoamylenes; and (3) dimerization of propylene to 2-methyl-l-pentene, followed by isomerization to 2-methyl-2-pentene, followed by pyrolysis (650°-800°C) to isoprene. In addition, isoprene can be separated from the gases resulting during the thermal decomposition of petroleum products (from a C5 fraction of the byproducts of ethylene production).
Isoprene is stored in the presence of inhibitors, such as hydroquinone, to prevent spontaneous polymerization. In high concentrations isoprene acts as a narcotic; in low concentrations it stimulates the mucous membrane. The maximum permissible concentration of isoprene in air is 40 mg/m3. Isoprene is used to make isoprene rubbers and butyl rubber.