4- -4-fluorobenzene

Fluorobenzene [5] A solution of phenylmagnesium bromide (from bromobenzene (1.57 g, 10 mmol) in ether (50 ml)) is added dropwise during 30 min to a stirred slurry of /V-fluoroquinuclidinium fluoride (1.49 g, 10 mmol) in ether (15 ml), under an inert atmosphere. An immediate reaction gives a fine white precipitate. The mixture is stirred for 2 h, filtered, and the filtrate added to chloroform, resulting in further precipitation. The mixture is filtered, and the filtrate is concentrated under reduced pressure, and filtered to remove solid quinuclidine. The filtrate is carefully concentrated and the residual liquid is carefully fractionally distilled to give fluorobenzene (0.249 g, 23%), b.p. 84°. 

Although the isolated yield from the above preparation was modest, the true yield was higher, and the work-up procedure should be capable of improvement, and the method should be applicable to other organomagnesium compounds [6]. 

Banks and E. Tsiliopoulos, unpublished work R. E. Banks, personal communication. 

A mixture of 1350 cc. of water and 1650 cc. (20 moles) of concentrated hydrochloric acid (sp. gr. 1.19) is placed in a large glass jar (30 by 30 cm.) or a 40-I. crock and stirred mechanically (Note 1) while strongly cooled by an ice-salt mixture. Two thousand and seventy-five grams (16 moles) of aniline hydrochloride (Note 2) and a solution of 1200 g. (17 moles) of sodium nitrite in 1500 cc. of water are made ready. When the temperature of the acid has reached 50 or below, about one-third of the aniline hydrochloride is added to it, and diazotization is begun by the slow addition of the nitrite solution, the temperature being held below 7°. Additional aniline hydrochloride is added from time to time in such amounts that an excess of crystals is always present. The entire amount may be added by the time that half of the nitrite has been added. The diazotization is stopped when a positive test for free nitrous acid is obtained with potassium iodide starch paper. This should require nearly all of the nitrite solution. 

Fluoboric acid is made concurrently with the diazotization by the addition, in small amounts, of 1000 g. (16.2 moles) of boric acid (u. s. p. crystals) to 2150 g. (65 moles) of 60 per cent hydrofluoric acid. (Handle with care Note 3) (Note 4). The addi- 

The combined distillate is separated from any phenol which may have settled out. It is washed four or five times with 10 

A slow-speed, paddle stirrer with several blades is preferable because of the large amount of suspended matter present at several times in the process. A stirrer of wood or metal protected with acid-proof paint is satisfactory, although the paint does not last well. Rubber-covered stirrers may also be used. 

Aniline (1485 g.) and hydrochloric acid (3000 cc.) without any added water can be used in place of the aniline hydrochloride,. water, and acid. The separation of aniline hydrochloride in a hard cake on the side of the jar, however, leads to difficulty in cooling and stirring. 

Dissolve 46-5 g. (45-5 ml.) of aniUne in a mixture of 126 ml. of concentrated hydrochloric acid and 126 ml. of water contained in a 1-htre beaker. Cool to 0-5° in a bath of ice and salt, and add a solution of 36-5 g. of sodium nitrite in 75 ml. of water in small portions stir vigorously with a thermometer (1) and maintain the temperature below 10°, but preferably at about 5° by the addition of a httle crushed ice if necessary. The diazotisation is complete when a drop of the solution diluted with 3-4 drops of water gives an immediate blue colouration with potassium iodide - starch paper the test should be performed 3-4 minutes after the last addition of the nitrite solution. Prepare a solution of 76 g. of sodium fluoborate (2) in 150 ml. of water, cool, and add the chilled solution slowly to the diazonium salt solution the latter must be kept well stirred (1) and the temperature controlled so that it is below 10°. Allow to stand for 10 minutes with frequent stirring. Filter 

Assemble the apparatus shown in Fig. 1 V, 67, 1 this is self-explanatory. The distilling flask has a capacity of 250 ml. and the beaker contains 150 ml. of 10 per cent, sodium hydroxide solution. All corks must fit well and should be coated with paraflSn wax (by dipping into molten wax, and allowing to drain). Place half of the yield of the dry phenyldiazonium fluoborate in the distilling flask. Heat the solid gently with a small luminous flame at one point near its surface until decomposition begins withdraw the flame and allow the reaction to continue 

If it is desired to employ fluoboric acid HBF4, it can be prepared by adding 100 g. of A.R. boric acid in small proportions to 325 g. of A.R. hydrofluoric acid (40 p er cent. HF) cooled in ice the hydrofluoric acid is contained in a Bakelite beaker, a beaker coated with wax or in a lead vessel. A simple container may also be prepared by cutting of the neck of the wax bottle (in which the hydrofluoric acid is supplied) with a large e.g., a butcher s ) knife which has been slightly warmed. One-third of the above solution should be employed in the preparation. Handle unth great care. 

4 4 -Difluorodiphenyl. Bis-diazotise a solution of 46 g. of benzidine (Section IV,88) in 150 ml. of concentrated hydrochloric acid and 150 ml. of water by means of a solution of 35 g. of sodium nitrite in 60 ml. of water add about 200 g. of crushed ice during the process (compare p-Fbtorotoluene above). Filter the solution and add it to a filtered solution of 85 g. of sodium borofluoride in 150 ml. of water. Stir for several minutes, collect the precipitated bis-diazonium borofluoride by suction filtration, wash with 5 ml. of ice-cold water, and dry at 90-100°. Place the dry salt in a flask fitted with an air condenser, immerse the flask in an oil bath, and slowly raise the temperature to 150° (Fume Cupboard ). When decomposition of the salt is complete, steam distil the mixture collect the 4 4 difluoro-diphenyl which passes over and recrystallise it from ethanol. The yield is 21 g., m.p. 92-93°. 

Dissolve 34 g. of o-nitroaniline in a warm mixture of 63 ml. of concentrated hydrochloric acid and 63 ml. of water contained in a 600 ml. beaker. Place the beaker in an ice - salt bath, and cool to 0-6° whilst stirring mechanically the o-nitroaniline hydrochloride will separate in a finely-divided crystalline form. Add a cold solution of 18 g. of sodium nitrite in 40 ml. of water slowly and with stirring to an end point with potassium iodide starch paper do not allow the temperature to rise above 5-7. Introduce, whilst stirring vigorously, a solution of 40 g. of sodium borofluoride in 80 ml. of water. Stir for a further 10 minutes, and filter the solid diazonium fiuoborate with suction on a sintered glass funnel. Wash it immediately once with 25 ml. of cold 5 per cent, sodium borofluoride solution, then twice with 15 ml. portions of rectified (or methylated) spirit and several times with ether in each washing stir 

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The density of fluorobenzene is about 1 -025 at room temperature it is important to use the correct strength of sodium hydroxide solution in order to obtain a clear separation of the two layers. 

Fig. 2.4. Graph of [7 Flogjo (obs)/l mol" s ] against — (i/AH-loganjo). A, Benzene B, fluorobenzene C, bromobenzene D, ehlorobenzene (the ordinate in this ease is...

Under conditions in which benzene and its homologues were nitrated at the zeroth-order rate, the reactions of the halogenobenzenes ([aromatic] = c. o-1 mol 1 ) obeyed no simple kinetic law. The reactions of fluorobenzene and iodobenzene initially followed the same rates as that of benzene but, as the concentration of the aromatic was depleted by the progress of the reaction, the rate deviated to a dependence on the first power of the concentration of aromatic. The same situation was observed with chloro- andjbromo-benzene, but these compounds could not maintain a zeroth-order dependence as easily as the other halogenobenzenes, and the first-order character of the reaction was more marked. 

More typically its reactions showed an intermediacy of kinetic order like that observed with fluorobenzene or iodobenzene in nitromethane. 

This general behaviour is characteristic of type A, B and C bands and is further illustrated in Figure 6.34. This shows part of the infrared spectrum of fluorobenzene, a prolate asymmetric rotor. The bands at about 1156 cm, 1067 cm and 893 cm are type A, B and C bands, respectively. They show less resolved rotational stmcture than those of ethylene. The reason for this is that the molecule is much larger, resulting in far greater congestion of rotational transitions. Nevertheless, it is clear that observation of such rotational contours, and the consequent identification of the direction of the vibrational transition moment, is very useful in fhe assignmenf of vibrational modes. 

Figure 6.34 Part of the infrared spectmm of fluorobenzene showing typical type A, B and C bands...

The case of vibrational numbering in, say, fluorobenzene illustrates the point that we must be flexible when it may be helpful. Many of fhe vibrations of fluorobenzene sfrongly resemble fhose of benzene. In 1934, before fhe Mulliken recommendations of 1955, E. B. Wilson had devised a numbering scheme for fhe 30 vibrations of benzene. This was so well esfablished by 1955 fhaf ifs use has fended to continue ever since. In fluorobenzene fhere is fhe further complication fhaf, alfhough Mulliken s system provides if wifh ifs own numbering scheme, if is useful very often to use fhe same number for a benzene-like vibration as used for benzene ifself - for which fhere is a choice of Mulliken s or Wilson s numbering Clearly, nof all problems of conventions have been solved, and some are nof really soluble, buf we should all fry to make if clear to any reader jusf whaf choice we have made. 

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