Chlorine migration

Here the electrophile is tert butyl cation formed by a hydride migration that accompa nies ionization of the carbon-chlorine bond... 

At the cathode, water molecules are discharged yielding gas and hydroxide ions, OH. Some of the caustic generated in the cathode compartment back-migrates to the anode compartment and reacts with dissolved chlorine (Cl2, ) to form chlorate as follows... 

Trityl Ethers. Treatment of sucrose with four molar equivalents of chlorotriphenylmethyl chloride (trityl chloride) in pyridine gives, after acetylation and chromatography, 6,1, 6 -tri-O-tritylsucrose [35674-14-7] and 6,6 -di-O-tritylsucrose [35674-15-8] in 50 and 30% yield, respectively (16). Conventional acetylation of 6,1, 6 -tri-O-tritylsucrose, followed by detritylation and concomitant C-4 to C-6 acetyl migration using aqueous acetic acid, yields a pentaacetate, which on chlorination using thionyl chloride in pyridine and deacetylation produces 4,l, 6 -trichloro-4,l, 6 -trideoxygalactosucrose [56038-13-2] (sucralose), alow calorie sweetener (17). 

In chlorination, loss of a proton can be a competitive reaction of the cationic intermediate. This process leads to formation of products resulting from net substitution with double-bond migration ... 

An alkyl group can also migrate from oxygen to nitrogen or phosphorus [I, 72] (Michaelis-Arbuzov rearrangement) With this methodology, tetrafluoro-pyndine phosphonates and phosphmates can be obtained [75, 74], Chlorine fluoride... 

To establish the nature of the chlorine migration (intramolecular or intermolec-ular), rearrangement of 54 was carried out in the presence of 4-(phenylethynyl)-1,3-dimethylpyrazole 55 (molar ratio of 54 55 = 2). About 40% of the chlorine migrates to the 5 position of the acceptor molecule 55 (Scheme 107). 

The halogen migration is completely suppressed by halogen-metal exchange when the chloroethynyl group is in position 5 of the pyrazole ring. The concentrations of 3-pyrazolyl and 4-pyrazolyl anions are probably small, and they cannot compete with NH2 anions for chlorine bonded to the acetylenic carbon. 

Among the halogens, fluorine does not undergo rearrangement, and what evidence there is suggests that the rearrangement of chlorine is an intramolecular process (1,2 shift) whereas that of bromine appears to take place by both inter-and intramolecular routes. Less is known about iodine migration. 

In an early work Kandrorand Freidlina473 suggested the occurrence of 1,3 migration of the chlorine atom from sulfur to carbon, viz.,... 

We recognize redox reactions by noting whether electrons have migrated from one species to another. The loss or gain of electrons is easy to identify for monatomic ions, because we can monitor the charges of the species. Thus, when Br ions are converted into bromine atoms (which go on to form Br2 molecules), we know that each Br ion must have lost an electron and hence that it has been oxidized. When 02 forms oxide ions, 02-, we know that each oxygen atom must have gained two electrons and therefore that it has been reduced. The difficulty arises when the transfer of electrons is accompanied by the transfer of atoms. For example, is chlorine gas, Cl2, oxidized or reduced when it is converted into hypochlorite ions, CIO" ... 

A preliminary migration of the chlorine from C-2 to C-6 was ruled out by the fact that recovered 58 had the same isotopic distribution as the starting 58. 

In addition the role played by the sorbent on which the chromatography is carried out must not be neglected. For instance, it is only on aluminium oxide layers and not on silica gel that it is possible to detect caffeine and codeine by exposure to chlorine gas and treatment with potassium iodide — ben2idine [37]. The detection limits can also depend on the sorbent used. The detection limit is also a function of the h/ f value. The concentration of substance per chromatogram zone is greater when the migration distance is short than it is for components with high h/ f values. Hence, compounds with low h/ f values are more sensitively detected. 

Solvent wiping. Rubbers tend to swell by application of solvents and the mechanical interlocking of the adhesive is favored. Although chlorinated hydrocarbon solvents are the most effective, they are toxic and cannot be used toluene and ketones are currently the most common solvents. The treatment with solvents is effective in the removal of processing oils and plasticizers in vulcanized mbbers, but zinc stearate is not completely removed and antiozonant wax gradually migrates to the mbber/polyurethane adhesive interface. Table 27.1 shows the moderate increase in adhesion produced in SBR by MEK wiping. 

The treatment of SBR with fumaric acid solutions avoids the migration of antiadherent moieties to the surface and the treatment with TCI solutions is effective to enhance the adhesion of several mbbers to polyurethane adhesive. Therefore, the combined use of mixtures of TCI and FA solutions should be more effective in improving the adhesion of difficult to bond SBR. The wettability of SBR is improved by treatment with 3 wt% TCI/EA followed by treatment with 0.5 wt% FA/EtOH (3 wt% TCI-0.5 wt% FA), with 0.5 wt% FA/EtOH followed by treatment with 3 wt% TCI/EA (0.5 wt% FA-3 wt% TCI), or with TCI + FA mixtures.However, the extent of the surface modifications produced and the adhesive strength of adhesive joints are mainly due to chlorination with TCI/EA. 

Miller RE, Guengerich FP. 1982. Oxidation of trichloroethylene by liver microsomal cytochrome P-450 Evidence for chlorine migration in a transition state not involving trichloroethylene oxide. Biochemistry 21 1090-1097. 

Adriaens P (1994) Evidence for chlorine migration dnring oxidation of 2-chlorobiphenyl by a type 11 metha-notroph. Appl Environ Microbiol 60 1658-1662. 

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