Photochlorination

The most innovative photohalogenation technology developed in the latter twentieth century is that for purposes of photochlorination of poly(vinyl chloride) (PVC). More highly chlorinated products of improved thermal stabiUty, fire resistance, and rigidity are obtained. In production, the stepwise chlorination may be effected in Hquid chlorine which serves both as solvent for the polymer and reagent (46). A soHd-state process has also been devised in which a bed of microparticulate PVC is fluidized with CI2 gas and simultaneously irradiated (47). In both cases the reaction proceeds, counterintuitively, to introduce Cl exclusively at unchlorinated carbon atoms on the polymer backbone. 

The ultraviolet lamps used in the photochlorination process serve to dissociate the chlorine into free radicals and start the radical-chain reaction. Other radical sources, such as 2,2 -a2obisisobutyronitrile, have been used (63,64). Primary by-products of the photochlorination process include 1,1,2-trichloroethane (15—20%), tetrachloroethanes, and pentachloroethane. Selectivity to 1,1,1-trichloroethane is higher in vapor-phase chlorination. Various additives, most containing iodine or an aromatic ring in the molecule, have been used to increase the selectivity of the reaction to... 

Oxidation. Oxidation of 1,1,1,2-tetrachloroethane in the presence of ionizing radiation gives dichloroacetyl chloride, Cl CHCOCl (117). The gas-phase photochlorination of 1,1,1,2-tetrachloroethane in the absence and presence of oxygen has been studied (115). 

Photochlorination of tetrachloroethylene, observed by Faraday, yields hexachloroethane [67-72-1]. Reaction with aluminum bromide at 100°C forms a mixture of bromotrichloroethane and dibromodichloroethane [75-81-0] (6). Reaction with bromine results in an equiUbrium mixture of tetrabromoethylene [79-28-7] and tetrachloroethylene. Tetrachloroethylene reacts with a mixture of hydrogen fluoride and chlorine at 225—400°C in the presence of zirconium fluoride catalyst to yield l,2,2-trichloro-l,l,2-trifluoroethane [76-13-1] (CFG 113) (7). 

The photochlorination takes 4-6 hr. The hydrogen chloride evolved is absorbed in water. 

Bevington and Ratti [5] reported that when CCI4 was used as a solvent on the photochlorination of PS at 78°C, the reverse C atom was chlorinated. In this case, the... 

Branching. Photochlorination of 2,3-dimethylbutane in the liquid phase leads to a mixture of I and II.42... 

Side-chain photochlorination of toluene isocyanates yields important industrial intermediates for polyurethane synthesis, one of the most important classes of polymers [6]. The motivation for micro-channel processing stems mainly from enhancing the performance of the photo process. Illuminated thin liquid layers should have much higher photon efficiency (quantum yield) than given for conventional processing. In turn, this may lead to the use of low-intensity light sources and considerably decrease the energy consumption for a photolytic process [6] (see also [21]). 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

Figure 2. Kinetics of the photochlorination of a PVC film (thickness = 50 ym light intensity = 5.10-9 E s 1 cnf )...

Figure 3. Reaction scheme of the photochlorination of PVC by a chain process...

For example, vinyl chloride undergoes photochlorination to produce 1,1,2-trichloroethane<108) ... 

Both values were obtained by photochlorination of H2, HT and HD. Bigeleisen et al. also showed that k22jk23 < 0.05 at 298° by searching for HD in the photo-... 

The rate coefficient ratio k22/k23 has been determined by Klein and Wolfs-berg5 0 from the photochlorination of hydrogen in the presence of tritium-labelled HC1. For the temperature range 0-62°... 

For the anodic substitution of unactivated CH-bonds, some fairly selective reactions for tertiary CH-bonds in hydrocarbons and y—CH-bonds in esters or ketones are available [85-87]. However, in some cases, a better control of follow-up oxidations remains to be developed. Chemically, a number of selective reactions are available, such as the ozone on silica gel for tertiary CH-bonds [88], the Barton or Hoffmann-LoefHer-Freytag reaction for y-CH-bonds [89], and for remote CH-bonds, Cprop)2NCl/H [90, 91], photochlorination of fatty acids adsorbed on alumina [92] or template-directed oxidations [93]. 

Full details have now appeared concerning the photoisomerization of triquinacene its bridgehead substitution via photochlorination and the S l solvolytic reactivity of these halides The three deuterated, optically active 2,3-dihydrotri-quinacenes 463 of known absolute configuration have been prepared from +)-462 (Scheme XXXVII) The dextrorotatory monodeuterated triquinacene 464 was... 

Bunce, N. J., K. U. Ingold, J. P. Landers, J. Lusztyk, and J. C. Scaiano, Kinetic Study of the Photochlorination of 2,3-Dimethyl-butane and Other Alkanes in Solution in the Presence of Benzene. First Measurements of the Absolute Rate Constants for Hydrogen Abstraction by the Free Chlorine Atom and the Chlorine Atom-Benzene tr -Complex. Identification of These Two Species as the Only Hydrogen Abstractors in These Systems, . /. Am. Chem. Soc., 107, 5464-5472 (1985). 

Preparation of 2-(2-chloroethyl)-l,3-dioxolane by the low-temperature photochlorination of 1,3-dioxolane in the presence of ethylene oxide [210]. 

All attempts to isolate 2-halogeno-l,3-dioxolanes or -1,3-dioxanes were unsuccessful. For instance, 2-chloro-l,3-dioxolane was detected only by photochlorination of 1,3-dioxolane (Ref. 118) at low temperature and 2-chloro-2-methyl-l,3-dioxolane was prepared by treatment of 2-methyl-l,3-dioxolane-2-carboxylic acid with phosphorus pentachloride at —60°, and shown119 to rearrange to 2-chloroethyl acetate on warming to 0°. For other examples of such problems, see especially, references 120 and 121. 

ZerZ-Butyl chloride [507-20-0] M 92.6, f.p. -24.6°, b 50.4 , d 0.851, n 1.38564. Purification methods commonly used for other alkyl halides lead to decomposition. Some impurities can be removed by photochlorination with a small amount of chlorine prior to use. The liquid can be washed with ice water, dried with CaCh or CaCl2 + CaO and fractionally distd. It has been further purified by repeated fractional crystn by partial freezing. 

These substituted anthracenes react with CC1 in the same manner as anthracene. On heating the exposed solutions, new peaks appear at frequency differences Av = 780 30 cm-1 from the 0 —0 vibrational peak of the long wave band, due to photochlorination. Exception is found in the case of 9-methyl anthracene. No new peaks appear but only a wavelength shift of 2-3 nm is observed. Perhaps chlorination occurs in the —CHS group and the substitution of Cl-atom is not likely to change... 

A uniquely high degree of terminal (methyl) selectivities in photochlorination of n-alkanes adsorbed on pentasil zeolites was reported.125 Monochlorination at the terminal methyl group was found to be a function of conversion, water content, Si/Al ratio of the zeolite, and loading of the alkane. 

The 2-chloro/l-chloro ratio in chlorination of 2,3-dimethylbutane, for instance, is 12.0 with sulfuryl chloride versus 4.2 in photochlorination. The chlorosulfonyl radical, however, easily decomposes under reaction conditions to yield a chlorine atom [Eq. (10.19)], which also participates in chlorination ... 

Chlorination of Alkanes. The most direct and economical method for the manufacture of chloromethanes is the thermal free-radical chlorination of methane.176 177 Whereas in the 1940s and 1950s photochlorination was practiced in some plants, thermal chlorination is the principal industrial process today. The product chloromethanes are important solvents and intermediates. Commercial operations perform thermal chlorination at about 400-450°C. Vapor-phase photochemical chlorination of methane may be accomplished at 50-60°C. Fast and effective removal of heat associated with thermally induced free-radical substitution is a crucial point. Inadequate heat control may lead to explosion attributed to the uncontrollable pyrolysis liberating free carbon and much heat ... 

C2CU 8.4 Photochlorination plus thermodynamic data Goldfinger and Martens62... 

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