Chlorine-free radicals, source from

Chlorine free radicals used for the substitutioa reactioa are obtaiaed by either thermal, photochemical, or chemical means. The thermal method requites temperatures of at least 250°C to iaitiate decomposition of the diatomic chlorine molecules iato chlorine radicals. The large reaction exotherm demands close temperature control by cooling or dilution, although adiabatic reactors with an appropriate diluent are commonly used ia iadustrial processes. Thermal chlorination is iaexpeasive and less sensitive to inhibition than the photochemical process. Mercury arc lamps are the usual source of ultraviolet light for photochemical processes furnishing wavelengths from 300—500 nm. 

In a hood, through a rapidly stirred suspension of 76 gm (0.396 mole) of cyclohexanoneazine in 300 ml of petroleum ether (b.p. 60°-90°C) cooled to —60°C is passed a slow stream of gaseous chlorine until a slight excess of the gas is noted. The excess of chlorine is removed by ventilation at the water aspirator. Then the solution is concentrated to half-volume by gentle evaporation at reduced pressure. The reaction system is filtered free from tarry impurities and the filtrate is allowed to stand for 24 hr at room temperature. The product gradually separates out and is isolated by filtration. Evaporation of the mother liquor may afford another crop of product. The total yield is 81.5 gm (78 %). The product, after recrystallization from petroleum ether, has a melting point of 66°C. (NOTE Since aliphatic azo compounds are inherently unstable and may serve as free radical sources, the stability of the product should always be checked with due precautions, and excessive exposure to heat should always be avoided.)... 

Increasing atmospheric amounts of chlorine atoms or free radicals probably result in the destruction of ozone the source of the chlorine atoms is thought to be synthetic substances known as chlorofluorocarbons (CFCs). Some CFCs may be released from air-conditioning equipment or aerosol spray cans, and some may result from the production of plastic foams. Several international agreements, including the Montreal Protocol of 1987 and the Copenhagen amendment of 1992, have been established to limit the production of CFCs. 

Chlorine atoms may be generated from molecular chlorine under mild conditions using a catalytic amount of an initiator, In-ln. Thus, homolysis of a molecule of initiator occurs upon irradiation or gentle heat to give free radicals, In (Eq. 9.3). These free radicals may then react with molecular chlorine to produce In-Cl and a chlorine atom (Eq. 9.4) to initiate the free-radical chain reaction. For safety and convenience, sulfuryl chloride, SO2CI2, rather than molecular chlorine is used in this experiment as the source of chlorine radicals. 

The polymerization of 2-chloro-l,3-butadiene was one of the reactions considered by U.S. industry to replace rubber made from natural sources located in areas of the world that could be cut off in a crisis such as war. This diene structurally resembles isoprene, with a chlorine atom replacing the methyl group of isoprene. Free radical polymerization gives a mixture of cis and trans double bonds as well as a mixture of 1,2 and 1,4-addition products. Polymerization of 2-chloro-l, 3-butadiene using a Ziegler-Natta catalyst yields neoprene, a compound with trans double bonds. 

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