Fluorobenzene, bromination

For the organic contaminants, the required bromine product quality wilt also be site specific. If the catalytic oxidation unit is dedicated to a single bromination process, phase separation and drying may be the only purification required. Contaminants in the recovered bromine which are either the starting materials or products of the original bromination reaction should not present a problem if present in bromine recycled to the bromination reactor. In this case, the catalytic reactor would be operated to minimize the formation of undesirable brominated byproducts. For example, if phenol is present in the waste HBr from a tribromo-phenol manufacturing process, minor tribromophenol contamination of the bromine recycled to the reactor should not be a problem. Similarly, fluorobenzene in bromine recycled to a fluorobenzene bromination process should not present a problem. 

A third approach is to apply gas phase bromination. Thus fluorobenzene has been brominated with high p-selectivity over MgY (ref. 22) or with HBr/02 over CuY (ref. 23). 

The order in reactivity in the Y-zeolite catalyzed bromination found is toluene > benzene > fluorobenzene > chlorobenzene > bromobenzene... 

Fluorine acidifies an ortho proton rather more than chlorine or bromine, whose weak directing effects are more or less comparable . Deprotonation of fluorobenzene is feasible in THF, and at temperatures below — 50°C the lithiated species 162 is stable and does not collapse to a benzyne (Scheme 81) ... 

Eighty-eight milliliters (256 g., 1.58 moles) of bromine is added over a 4-hour period to a mixture of 8 g. of iron turnings and 165 g. (1.72 moles) of fluorobenzene [Org. Syntheses Coll. Vol. 2, 295 (1943) ]. The reaction mixture is allowed to stand overnight and then is heated on a steam bath until no more hydrogen bromide is evolved (about 3 hours). The material is steam-distilled, and the organic portion of the distillate is separated, washed with sodium carbonate solution, and dried over calcium chloride. Two distillations give... 

Octafluorotelluranthrene 5.0 g (39.2 mmol) of tellurium powder and 10.0 g (24.9 mmol) of 1,2-diiodotetra-fluorobenzene are placed in a 20 cm long test tube, and the tube is evacuated to 0.2 torr and sealed. The sealed tube is placed in an oven at 300° for 1 day, cooled, and carefully opened. The solid and liquid in the tube are extracted with dichloromethane, the mixture is filtered, and the filtrate is washed first with 50 m/ of concentrated aqueous sodium thiosulfate solution and then with 50 ml of distilled water. The solvent is removed under vacuum, the residual dark brown oil is dissolved in chloroform, and bromine is added until the color of bromine persists. The precipitate is filtered and washed with chloroform to give 5,5,10,10-tetrabromoperjluorotelluranthrene yield 3.5 g (29%) m.p. 281°. 

It is well known that in halobenzenes, the para-position relative to the halogen is the more basic site and the mefa-position the least basic °. The higher activity for thepara-position in fluorobenzene results from the need to add a proton to a position that is not disfavoured by the a-electron withdrawal by fluorine atom, due to its strong inductive effect. The effect is smaller for chlorine, bromine and iodine. Thus when there is competition between the hydroxy group and a halogen atom in directing the ring protonation. 

With aii this in mind, how would you expect fluorobenzene to react Most electron density is removed first from the ortho positions by induction, then from the meta positions, and then from the para position. Any conjugation of the lone pairs on fluorine with the it system would increase the eiectron density in the ortho and para positions. Both effects favour the para position and this is where most substitution occurs. But is the ring more or less reactive than benzene This is hard to say and the honest answer is that sometimes fluorobenzene is more reactive in the para position than benzene (for example, in proton exchange and in acetyiation—see later) and sometimes it is less reactive than benzene (for example, in nitration, as shown by the table above). In all cases, fluorobenzene is significantly more reactive than the other halobenzenes. We appreciate that this is a rather surprising conclusion, but the evidence supports it. For example, fluorobenzene reacts with bromine and an iron catalyst (it does need a cataiyst it is not as reactive as phenol) at only -20 °C to give the para-biomo derivative. 

The rates of the reactions fall into two pairs and follow a U-shaped sequence fluorobenzene nitrates most quickly, followed closely by iodobenzene chloro-, and bromobenzene nitrate at around half these rates. Chlorine and bromine suffer because both are quite electronegative and neither has good lone pair overlap in fluorine, overlap is good in iodine, electronegativity is much less. 

One liquid for which the value of seems suspect is 1-bromonaphthalene. It contains a polar carbon-bromine bond. I have not been able to find the dipole moment of 1-bromonaphthalene, but the values for bromobenzene, chlorobenzene and fluorobenzene which are chemically similar to 1-bromonaphthalene are 1.70 D, 1.69 D and 1.60 D, respectively. These molecules are almost as polar as chloromethane (1.97 D) and bromomethane (1.81 D), where the polarity of the... 

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