Reactions photochlorination

Many types of reactions have mechanisms in this category thermal cracking, some polymerizations, many liquid phase oxidations and combustion reactions, photochlorinations, and others. 

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... 

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). 

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

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]. 

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 ... 

Cylindrical photochemical reactors placed in coaxial radiation fields are often used for carrying out photochlorination reactions, but their most important application remains in the area of water sterilization [52-54],... 

We use the same reactor geometry for photochlorinations because, given the high quantum yields of these chain reactions, irradiance must not be optimized to achieve good productivity. These reactors may then be equipped with fluorescent tubes for which no external cooling and no water filters are needed. 

An interesting 1,3-chlorine shift reaction is reported to occur during the photochlorination of Ar-benzylperfluoroalkanimidoyl chlorides 39.27 Photochlorination of 39 gives a mixture of 40 and its isomer 41. which was interpreted as being caused by a 1,3-chlorotropic isomerization of 40 to 41. In the presence of triethylamine, the mixtures of 40 and 41 isomerize completely to 1,3-dichloride 42. which was explained as having resulted from an equilibrium between 40 and 41. via a reversible 1,3-chlorotropic shift, with 40 transformed completely by a base-catalyzed 1.3-prototropic rearrangement to isomer 42. 

The purpose of photochlorination of polymers is to improve their stability with respect to fire hazards. Polymers made of C and H atoms only are quite dangerous in this respect, since they undergo combustion reactions in the presence of oxygen to form C02 and H20. Replacement of H by Cl reduces the flammability of such polymers. 

The fact that photochlorination can be a chain reaction reduces the cost of light energy to acceptable levels. This would not be the case if one photon would be needed for each Cl2 molecule. 

The synthesis illustrates several important types of reactions that we have discussed in this and previous chapters. First, the alkyl group R usually is a C12 group derived from the straight-chain hydrocarbon, dodecane, which on photochlorination gives a mixture of chlorododecanes ... 

For addition of Cl atoms, the dissociation energy of the radicals AX has been estimated at about 20-22 kcal./mole. At room temperature k ifki(A) should be well below 10-3 and mechanism (C) should be obeyed and has indeed been frequently observed. At higher temperatures (about 225°C.) k 2/k2(A) 10-3 and a change to mechanism (B) should occur. This has been confirmed experimentally by Adam et al. (1) in a study of the photochlorination of tetrachlorethylene. They observed a maximum in the rate and a change in mechanism at about 180°C., as a result of the increased importance of the radical decomposition reaction (—2). From their data they were able to deduce the Arrhenius parameters for this reaction. In extensions of this work Goldfinger and his collaborators have carried out competitive experiments with a number of hydrocarbons and chlorinated hydrocarbons. 

Over a period of years Dainton and his co-workers have studied intensively photochlorination processes. Their work also indicated (31) that at temperatures below 150°C. the kinetics of photochlorination of trichloroethylene and tetrachloroethylene are explainable in terms of reactions (l)-(6). Above a characteristic limiting concentration the observed rates are in agreement with eq. (C). However, in a more recent work Ayscough et al. (6) have compared the rates of geometrical isomerization (Ri) of pure cis- and trans-1,2-dichloroethylene with the rates of the simultaneously occurring photochlorination. This work is of... 

Under normal conditions in photochlorination experiments the lifetime of AC1 radicals before reacting with Cl2 is about 10 -5 sec. which allows ample time for many free rotations about the CC bond. The dichloroethylene formed in the thermal decomposition of the C2H2C13 radicals [reaction (-2) ] will consist of a fraction x of the trans isomer and (1 — x) of the cis isomer. It is readily seen that... 

The photochlorination rate expressions may be expected to be somewhat modified as a result of the inclusion of the hot radical reactions. At temperatures below 150°C. the unimolecular decomposition of the (thermalized) AC1 radical, i.e., reaction ( — 2), may be expected to be negligible and the rate laws are... 

Asycough et al. (8) studied the competitive photochlorination of mixtures of cts-1,2-dichloroethylene and either vinyl chloride or trichloroethylene. An expression for the ratio of the rates of formation of the photochlorination products from the two olefins competing for Cl atoms was derived. This was based on the mechanism which included reactions of the hot radicals and reactions (1), (2 ), and (3)... 

There can be little doubt that the summarized work of Dainton and and his collaborators has pointed to a new important feature of the reactions of chlorine atoms with olefins. The incorporation of the reactions of hot radicals into the general olefin photochlorination mechanism brings these reactions into closer analogy with the other atomic addition reactions discussed in this article. It may be anticipated that further work on such effects will be forthcoming. It would be in particular desirable to obtain further verification of the postulated collisional deactivation of the excited AClf radicals by carrying out... 

A different experimental approach to the study of chlorine atom reactions with olefins will be mentioned briefly. Wijnen(l06) has studied the photolysis of phosgene as a source of chlorine atoms in the presence of ethylene, and Guercione and Wijnen (49) have carried out similar experiments with propylene. The features of these processes are quite different from those encountered in photochlorination in the presence of molecular chlorine, since the chain propagating reaction (3) cannot occur. Although in the photolysis of phosgene Cl and COC1 are initially formed, it appears that all COC1 radicals further decompose into CO and Cl. 

The reactions are generally considered within the framework of the chain mechanism analogous to that for photochlorination. However, much less information seems to be available on the gas phase addition of Br atoms to olefins. 

Figure 9.10. Magnetic effect on the solid-state photochlorination of methylcyclopropane at 77 K. The lower panel shows the variation of quantum yield of chain reaction initiated by Cl2 photodissociation. Data in the upper panel represent the intensity of recombinational phosphorescence. (From Tague and Wight [1992].)...

Brown, H. C., and G. A. Russell The photochlorination of 2-methyl-propane-2-d and a-d1-toluene the question of free radical rearrangement or exchange in substitution reactions. J. Amer. chem. Soc. 74,3995 (1952). 

K. U. Ingold, J. Lusztyk, K. D. Raner, The unusual and the unexpected in an old reaction. The photochlorination of alkanes with molecular chlorine in solution, Acc. Chem. Res 1990, 23,... 

The variation in selectivity of radicals in different solvents has been interpreted as being due to radical-solvent interaction which changes the reactivity of the radical. Thus the selectivity of chlorination of 2,3-dimethylbutane which may react at either a tertiary or a primary position increases in aromatic solvents (Table 26 Russell, 1958, 1960). Since the effect appears to be proportional to the basicity of the aromatic substrate, it was concluded that aromatic solvents yield a complexed chlorine atom which is consequently less reactive and therefore more selective in its reactions. Confirmation of this came from the finding that the increased selectivity of the photochlorination of 2,5-dimethylhexane in aromatic solvents was due to an increase in the activation energy of the reaction (Russell,... 

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