Hydrogen-halogen exchange reaction

The relative difficulty in alkylating normal paraffins is presumably due to the fact that the exchange reactions of step 1 and step 3 do not take place as easily with hydrogen atoms attached to secondary carbon atoms as with those attached to tertiary carbon atoms (cf. Bartlett, Condon, and Schneider, 15). Under the conditions required to obtain the hydrogen-halogen exchange reaction with normal paraffins, polymerization of the olefin (which may be considered to occur via a reaction similar to that of step 2) takes place very readily and becomes the predominant reaction. 

Alkyl halides can be reduced conveniently by trialkylsilanes in the presence of catalytic amounts of BF3 or AlCl3 . Hydrogen-halogen exchange reaction of primary, secondary and tertiary chlorides or bromides is quite efficient under these conditions (equation 40). 

Pentafluorophenyllithium can be readily prepared by direct reaction of penta-fluorophenyl halides with lithium amalgam [193,194] or lithium-hydrogen and lithium-halogen exchange reactions of pentafluorophenyl halides with alkyl-lithiums [195-205] (Scheme 70). 

Halogen exchange reactions, too, are valuable sources of fluorinated pyrimidines, especially 2-, 4-, and 6-fluoro derivatives. Sources of fluoride ion have included hydrogen, sodium, potassium, cesium, antimony, silver fluorides, and sulfur tetrafluoride. All of the reactions required heating. Prepared from nucleophilic fluorination processes have been... 

To qualify as R X, it is thus necessary that the R part occurs more naturally as a resonance and/or inductively stabilized carbanion, rather than as a destabilized carbocation82. Hydrogen is more electropositive than any of the halogens83. Accordingly, it is thus expected that when R is a standard alkyl group, the halogen exchange reaction of equation 48 is endothermic ... 

Boron Trichloride. Boron trichloride is prepared commercially by the chlorination of boron carbide (equation 15). Direct chlorination of boric acid or a sodium borate in the presence of carbon is an alternative method. Most of the boron trichloride produced is converted to filaments of elemental boron by chemical vapor deposition (CVD) on tungsten wire in a hydrogen atmosphere. Numerous laboratory preparations of boron trichloride have been reported. One of the most convenient is the halogen exchange reaction of aluminum chloride with boron trifluoride or a metal fluoroborate. 

Finally, hydrogen-halogen exchange may occur, leading to side reactions, and unexpected products (compare 5.8.4.2.2). 

Scheme 5.12 Preparation of organolithium compounds (a) halogen-lithium exchange reaction (b) hydrogen-lithium exchange reaction...

Tribromo-l,3,5-triazine (cyanuric bromide) has no industrial importance. It is prepared in 60% yield by the addition of aluminum tribromide to a solution of cyanogen bromide in nitromethane at 40 - 60"C.220 Cyanuric iodide, which cannot be prepared by the trimeriza-tion of cyanogen iodide, can be obtained by the halogen-exchange reaction of cyanuric chloride and hydrogen iodide (cf. Section 2.3.1.5.5.l.l.l.)221... 

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