Chlorine photochemical addition

In the absence of catalysts and in the dark, pure benzene does not react with bromine, as is the case also with chlorine. Photochemical addition of bromine, like that of chlorine, is a radical chain reaction.130 131 Bromine has a more powerful substituting action than chlorine, and its rate of addition is slower.132 At low temperatures light and addition of peroxides favor addition of bromine. To date, two isomeric hexabromocyclohexanes have been isolated, by very slow addition of bromine to benzene irradiated at 0° 1% sodium hydroxide solution must be placed under the benzene and frequently renewed even so, yields are poor.126b 133... 

Other methods of introducing amino substituents at C-2 by glycal additions include the use of the (saltmen)Mn(N) complex and trifluoroacetic anhydride,51 and the photochemical addition of Ar-chloro-chloroacetamide.52 In both cases tri-O-acetyl-D-glucal gives 2-amino-2-deoxy-D-glucose adducts, in the former case with the hydroxyl group at C-l, and in the latter with chlorine which can be displaced by alkoxy at that position. 

Photochemical addition of cis- or trans-dichloroethylene to the A -20-keto-steroid (62) afforded two a-face adducts (63a) and (64) (see Part II, Chapter 1, ref. 310). The chlorine configurations as shown were assigned on the basis of n.m.r. studies and an AT-ray analysis of the derivative (635)." ° The fact that both cis- and trans-dichloroethylene provide the same product composition is in agreement with the stepwise addition mechanism, forming a diradical intermediate which can undergo free rotation at the 16-17 bond before ring closure. 

Photochemical additions of alkenes to aromatic compounds are now well established and similar additions to pentafluoropyridine (86) occur,335 whereas a novel insertion into the carbon-chlorine bond occurs with 105, in a process that does not appear to involve tetrafluoropyridyl radicals.336 It has also been demonstrated recently that Diels-Alder additions occur with... 

When radical formers are added, S02C12 may replace Cl2 as source of atomic chlorine (Cl ). S02C12 alone provides only molecular chlorine (reaction e). Whilst radical chlorination by molecular chlorine (photochemical, thermal, or with addition of radical-formers) proceeds by the chain mechanism (a) and (b), the additional reactions (c) and (d) can be assumed to occur in chlorinations by S02C12 and radical-formers (peroxides, azo compounds).331-336... 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

A cross-conjugated perfluorotnene is unreactive toward bromine, however, photochemical chlorination yields the 1,4 addition product [I2 (equation 5)... 

The side-chain substitution of toluene, p-chlorotoluene, etc. is industrially practised. This reaction is carried out in a photochemical reactor. It is an exothermic reaction in which HCl is produced. The reaction is consecutive, and hence CL first reacts with toluene reacts to form the desired benzyl chloride, which is then converted to benzal chloride, and finally benzotrichloride. We may, however, well be interested in the selectivity to benzyl chloride. An additional complication arises due to nuclear chlorination, which is most undesirable. A distillation-column reactor can offer advantages (Xu and Dudukovic, 1999). 

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

The observation that l,3,5-trimethyl-2,4,6-trichloroborazine yields some dimeric (dichloromethyleneamino)dichloroborane upon photochemical chlorination 30> (c.f. Eq. (7)) prompted the synthesis of this compound by the reaction of BC13 with C1SNCC12 2I> (c.f. Eq. (7)) and also by the 1,2-addition of trichloroborane to chlorocyanide 23> as depicted in Eq. (15). 

Photodegradation of DDT by the protease-liberated flavo-protein from TX-20 resulted in the formation of TDE as the major product in addition to three other minor compounds. It has been well established that DDT conversion to TDE, anaerobically, is a reductive process involving replacement of a chlorine atom by hydrogen. On the other hand, it has been suggested that photo-lytic reactions involve a charge transfer from an amine to DDT and a subsequent pickup of a proton. Thus there is a possibility that the photochemical reaction involving flavoproteins undergoes a similar reaction scheme. Much more data are, however, needed to confirm this point. 

If handled responsibly, PFCs can be excellent choices to replace ozone-depleting compounds in many demanding, high-performance applications. Perfluorinated liquids are colorless, odorless, essentially nontoxic, and nonflammable. In addition, since they are not precursors to photochemical smog, PFCs are exempt from the U.S. EPAs volatile organic compounds (VOC) definition. Most importantly, these materials do not contain the carbon-bound chlorine or bromine, which can cause ozone depletion. 

During slow thermal chlorination, elimination of HC1 from the monochloride with the resultant formation of an alkene followed by chlorine addition may be the dominant route to yield dichloroalkanes. This mechanism, however, is negligible in rapid thermal or photochemical reactions. 

Aromatic C-H bonds are not broken in radical halogenation, because they are a little stronger than aliphatic C-H bonds. When benzene reacts photochemically with chlorine, a radical addition process takes place, and the mixture of stereoisomerir hexachloro-cydohexanes (S.78) includes one isomer which has powerful insecticidal properties but which, unlike some chlorinated insecticides, is readily biodegradable. 

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