Transhalogenation

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. 

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Nickel fluoride is used in marking ink compositions (see Inks), for fluorescent lamps (4) as a catalyst in transhalogenation of fluoroolefins (5), in the manufacture of varistors (6), as a catalyst for hydrofluorination (7), in the synthesis of XeF (8), and in the preparation of high purity elemental fluorine for research (9) and for chemical lasers (qv) (10). 

The tnhahdes of phosphoms usually are obtained by direct halogenation under controlled conditions, eg, in carbon disulfide solution in the case of the triiodide. Phosphoms trifluoride [7647-19-0] is best made by transhalogenation of PCl using AsF or Cap2. AH of the phosphoms tnhahdes are both Lewis bases and acids. The phosphoms tnhahdes rapidly hydroly2e in water and are volatile. Examination by electron diffraction has confirmed pyramidal stmctures for the gaseous tnhahde molecules (36). Physical properties and heat of formation of some phosphoms hahdes are hsted in Table 7. 

Phosphoms pentafluoride is synthesized easily by transhalogenation of phosphoms pentachloride using arsenic triduoride at low temperature,... 

Transhalogenation, or the replacement of one halogeno substituent by another, is of little interest as a reaction but of great preparative importance. Accordingly, it is discussed in Section 2.13.3.3.9/. 

In view of the high price and other difficulties associated with the use of phosphoryl bromide, the transhalogenation of 2-chloro- and 2-chloro-4,6-dimethyl-pyrimidine (919) by treatment with inexpensive phosphorus tribromide to give their bromo analogues in >50% yield, assumes potential importance as a general method (67JCS(C)1204). 

Refluxing perfluorotoluene neat for 4 h with aluminum tnbromide produce p-bromoperfluorotoluene in 54% yield [<8/] Fluoropyndines can be transhalogen-ated at position 2 andfor 4 by heating in the presence of calcium halide in a nonhydroxylic solvent [82] (equation 62)... 

Boron tnfluonde also reacts with hydroxy acndones to form difluoro boron complexes [111] (Table 28) Another route to fluoroboraties involves transhalogen-ation by fluonnation of the correspondmg chloro- and bromoboranes with lithium or potassium fluonde under mild conditions [112] (Table 29)... 

This type of transhalogenation reaction, which is common amongst the halides of main group elements, always proceeds in the direction which pairs the most electropositive element with the most electronegative, since the greatest amount of energy is evolved with this combination. 

N-methyl-4-piperidyl benzilate methobromide To a suspension of 0.15 mol of freshly prepared silver bromide in 300 ml of anhydrous methanol is added a solution of 0.1 mol of quaternary iodide obtained as above. The mixture is stirred and refluxed for several hours after which time transhalogenation is complete. The mixture is cooled, the insoluble silver... 

To elucidate the mechanism of the reaction transhalogenation of 1,2,4,5-tetra-bromobenzene (Br4-Bz) was investigated in detail. Subsequent halogen exchange yields tetrachlorobenzene (CI4-BZ) via the three intermediates B, C and D. 

Bartnicki EW, CE Castro (1969) Biodehalogenation. The pathway for transhalogenation and the stereochemistry of epoxide formation from halohydrins. Biochemistry 8 4677-4680. 

Castro CE, EW Bartnicki (1968) Biodehalogenation. Epoxidation of halohydrins, epoxide opening, and transhalogenation by a Elavobacterium sp. Biochemistry 1 3213-3218. 

Harper DB, Kalin RM, Larkin MJ, Hamilton JTG, Coulter C (2000) Microbial transhalogenation A complicating factor in determination of atmospheric chloro- and bromomethane budgets. Environ Sci... 

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