[CONTRIBUTION
FROM THE
CHEMICAL LABORATORY OF WASHINGTON
UNIVERSITY]
THE REACTIVITY OF PARA-FLUORINE I N TRIARYLMETHYL CHLORIDES WITH JOHN H. GARDNER Received June IS, 1938
FRANK BACON
Soon after the discovery of triphenylmethyl it was found that para halogens in the triphenylmethyl group may be removed or replaced under conditions which indicate a quinoidation in the benzene nucleus containing the reacting halogen. The behavior of halogenated triarylmethyl chlorides in liquid sulfur dioxide alone and in the presence of silver chloride, and also in benzene solution in the presence of finely divided silver will be discussed in this paper. p-Bromotriarylmethyl chlorides rearrange in liquid sulfur dioxide solution to yield p-chlorotriarylmethyl bromides. Gombergl investigated numerous examples of this rearrangement, obtaining for example, an 85 per cent. yield of p-chloro-di-p-bromotriphenylmethyl bromide from tri-p-bromotriphenylmethyl chloride. Long agitation of p-bromotriarylmethyl chlorides or bromides in this solvent with silver chloride resu1t)s in their conversion into p-chlorotriarylmethyl chlorides2. The mechanism of these reactions must involve the quinoidation of the triarylmethyl halide (or the corresponding positive ion), placing the halogen atom from the methyl carbon and the nuclear halogen in the same position in the para-quinoidal structure. In the case of direct rearrangement, the halogen which was on the aromatic nucleus goes to the methyl carbon when the compound reverts to the benzenoidal form. Replacement is explained by ionization of the quinoidal form which permits either or both of the halogen atoms on the para quinoidal carbon to be replaced by chlorine atoms from the silver chloride. Gomberg and Cone3found that when a triarylmethyl chloride containing chlorine, bromine, or iodine on an aromatic nucleus is shaken with silver, the chlorine atom is quantitatively removed from the methane carbon atom in about half an hour. After several weeks, more or less of the halogen on the aromatic nucleus is removed if it was present in the para position. Halogen atoms in the ortho or meta position do not react. The 1 2
3
GOMBERG, Ber., 43, 406 (1909). GOMBERQ, Ber., 40, 1847 (1907); GOMBERT AND CONE,Ann., 370,142 (1909). GOMBERG AND CONE,Ber., S9,3274 (1906). 281
282
FRANK BACON WITH JOHN H. GARDNER
experiments of Gomberg and Blicke4 showed rather conclusively that in the case of several para-brominated triarylmethyl chlorides, exactly onehalf atom of bromine per molecule is removed by prolonged shaking and free radicals of the “second order” result. These reactions may be formulated as follows for the case of p-bromotriphenylmethyl chloride: 2(BrC6H4)(CeH&CCl
+ 2Ag
-+
’
2(BrC6H4)(CeHs)&
+ 2AgC1
\ 2
The present investigation was undertaken to study the reactivity of fluorine as compared to the other halogens in these reactions. For this purpose, the mono-, di-, and tri-p-fluorotriphenylmethyl chlorides were synthesized, the last two for the first time. In the first series of experiments, solutions of these compounds in liquid sulfur dioxide were agitated for a considerable number of days. Analysis of the products after evaporation of the sulfur dioxide for ionizable fluorine indicated that rearrangement had taken place to the extent of about one to eight per cent. A similar series of experiments carried out in the presence of silver chloride gave results of the same order, indicating that the silver chloride was without any appreciable effect. Agitation of the fluorinated triphenylmethyl chlorides in benzene solution with finely divided silver yielded from 0.13 to 0.43 atoms of ionizable fluorine per molecule in the ten experiments conducted. The results and conditions were rather irregular and the differences between the average results for the three compounds were small as compared to the deviation between individual experiments. The fact that ionizable fluorine was found in appreciable amounts indicates the formation of radicals of the second order such as Gomberg and Blicke4 showed to result from similar treatment of p-bromotriphenylmethyl chloride. The results obtained in the three groups of experiments vary widely and have little quantitative significance, but they show positively that fluorine in the para position in triarylmethyl chlorides has reactions similar to the other halogens, although it does not react as readily. EXPERIMENTAL
Methyl p-jZuorobenzoate.-p-Fluorobenaoic acid was prepared by the method of Schiemann and Winkelmuller6 and recrystallized from 25% alcohol. The product GOMBERQ AND BLICKE, J. Am. Chem. Soc., 46, 1765 (1923). AND WINKELM~~LLER, Organic Syntheses, 13, 52 (1933).
6 SCHIEMANN
PARA-FLUORINE I N TRIARYLMETHYL CHLORIDES
283
was dissolved in a large excess of methyl alcohol, saturated with dry hydrogen chloride, and allowed to stand for one day. The resulting solution was distilled under 8mm. pressure, and the fraction boiling at 73-79" was collected. This material gave a capillary boiling point of 197". Meyer and Hub* give 197". p-Fluorotriphenylmethyl chloride.-A Grignard reaction was carried out with 37 g. of bromobenzene and 5.9 g. of magnesium turnings in 100 cc. of ether and 44 g. of p-fluorobenzophenone (prepared by the method of Dunlop and Gardner?) in 100 cc. of benzene. The product was decomposed with ice and hydrochloric acid, and extracted with ether. The extract was concentrated, and some impurities removed by steam distillation. The residue, which solidified in a brown sticky mass, was recrystallized twice from carbon tetrachloride with the use of calcium chloride and an activated charcoal, yielding 10.5 g. of yellowish material. This was dissolved in benzene, placed over calcium chloride, and treated with dry hydrogen chloride for several hours. The solution was filtered and concentrated until the boiling point reached 100". An equal volume of petroleum ether was added, and crystals were obtained by holding a t a temperature below 0' for two days. The product was recrystallized from petroleum ether in which i t dissolves very slowly. Yield, 6.5 g. (lo%), m.p. 90-91". Nixon and Branch* give 87". Anal. Calc'd for C19HI4C1F:C1, 11.95. Found: C1, 11.81, 12.26. Di-p-fluorotriphenylmethyl chloride.-Di-p-fluorotriphenylcarbinol was prepared similarly to p-fluorotriphenylcarbinol from 175 g. of p-fluorobromobenzene in 500 cc. of ether, 27 g. of magnesium, and 62 g. of methyl benzoate in 150 cc. of ether. Two recrystallizations from carbon tetrachloride yielded 37.7 g. of yellowish crystals of impure carbinol. A solution of 10 g. of this material in petroleum ether was placed over calcium chloride and treated for four hours with dry hydrogen chloride. The resulting solution was decolorized by boiling with fresh calcium chloride and an activated charcoal, was filtered, concentrated on the steam bath, then held below 0" over night after seeding with a minute crystal of p-fluorotriphenylmethyl chloride. White crystals of di-p-fluorotriphenylmethyl chloride separated. Yield, 5.5 g. (14%), m.p. 56-57'. This compound is much more soluble in benzene and petroleum ether than p-fluorotriphenylmethyl chloride. Anal. Calc'd for ClsHlsClF2: C1, 11.27. Found: C1, 11.11, 11.31. Tri-p-fluorotriphenylmethyl chloride.-Tri-p-fluorotriphenylcarbinol was prepared similarly to p-fluorotriphenylcarbinol from 36.5 g. of p-fluorobromobenzene in 120 cc. of ether, 5. g. of magnesium, and 15 g. of methyl p-fluorobenzoate in 60 cc. of ether. The product was recrystallized three times from petroleum ether with the aid of an activated charcoal and powdered calcium chloride. White crystals resulted. A solution of these crystals in petroleum ether was placed over calcium chloride and treated for six hours with dry hydrogen chloride. The resulting solution was boiled with an activated charcoal and powdered calcium chloride, then filtered and concentrated on a steam bath. A small portion of this liquid when exposed to the air in a thin film solidified, and when this solid was scraped into the main portion of liquid, i t solidified completely. The solid mass was recrystallized from petroleum ether with the use of an activated charcoal and calcium chloride. Crystals with a slight yellowish tinge separated after seeding with a reserved crystal. Yield, 1.1 g. (4%), m.p. 81-82'. The slight coloration and low analysis indicate t h a t some impurity was still present.
7
MEYERAND HUB,Monatsh., 31, 935 (1910). DUNLOPAND GARDNER, J . Am. Chem. Soc., 66, 1665 (1933). NIXONAND BRANCH,Ibid., 68, 492 (1936).
284
FRANK BACON WITH JOHN H . GARDNER
Anal. Calc'd for ClsH1d21Fa; C1, 10.67. Found: C1, 9.96, 9.90, 10.02. Rearrangement of JEuorinated triphenylmethyl chlorides i n liquid sulfur dioxide.This was studied by dissolving approximately 20-mg. portions of the compounds in about 0.5 cc. of liquid sulfur dioxide in sealed tubes, agitating the tubes for a number of days, and then analyzing the contents for ionizable fluorine. The sulfur dioxide was permitted to vaporize slowly from each tube at the end of its period of agitation, and the residue was dissolved in 50 cc. of 50% alcohol. Fluorine was determined in this solution by the Willard and Winter titration* using sodium alizarin sulfonate alone as indicator. A sample of triphenylmethyl chloride which had been dissolved in and recovered from liquid sulfur dioxide gave an appreciable titer, indicating that a little sulfur dioxide remained, and precipitated thorium. Experiments with known solutions showed that in the presence of triphenylmethyl chlorides the error of the method is never more than 10% of the correct value. The data and results for these experiments are shown in Table I. TABLE I BEHAVIOROF TRIPHENYLMETHYL CHLORIDES IN LIQUIDSULFURDIOXIDE TRIPHINYLMETHYL CHLOBIDE
Unsubstituted. . . . . . . . . . . . . . . . . . . p-Fluoro- ....................... Di-p-fluoro ...................... Di-p-fluoro- ..................... Di-p-fluoro-. .................... Di-p-fluoro- ..................... Tri-p-fluoro- .................... Tri-p-fluoro- .................... Tri-p-fluoro- .................... Tri-p-fluoro- .................... 0
WEIQHT,
Th(NO& ca.
ATOMS ONIZABLI F PER MOL.
0.04 0.10 0.16 0.26 0.20 0.40 0.08 0.09 0.13 0.19
0.007 0.019" 0.030 0.049 0.037 0 .>OS0 0.016 0.019 0.026 0.037
0.0128 N
ma
TIME, DAYS
FINAL COLOR
20.2 20.5 21.6 21.3 21.8 19.5 20.9 20.3 19.9 20.2
0 56 46 59 59 63 19 19 23 23
Yellow Orange Orange Orange Orange Dp. Or. Orange Orange Orange Orange
This value was low because a portion of the sample was lost.
Behavior of the jluorinated triphenylmethyl chlorides in liquid sulfur dioxide in the presence of silver chloride.-The procedure here was the same as in the study of rearrangement in sulfur dioxide except that 99-107 mg. of silver chloride was added to each tube on filling, and this silver chloride had to be filtered out of the product before titrating the fluorine. The data and results for these experiments are given in Table 11. Behavior of the Jluorinated triphenylmethyl chlorides in benzene i n the presence of silver.-The procedure here was similar to that used in the study of rearrangement in liquid sulfur dioxide. The compounds were dissolved in about 0.5 cc. of benzene in the presence of 99-103 mg. of finely divided silver, and this mixture was sealed in the tube under an atmosphere of carbon dioxide. At the conclusion of its period of agitation, the contents of each tube were mixed with 35 cc. of alcohol, then filtered t o remove the excess silver and the silver chloride formed. The filtrate was diluted 9 WILLARDAND WINTER,Ind. Eng. Chem., Anal. Ed., 6, 7 (1933); REYNOLDS, KPRSHAW,AND JACOB,J . Assoc. Oficial Agr. Chem., 19, 159 (1936).
285
PARA-FLUORINE I N TRIARYLMETHYL CHLORIDES
with 15 cc. of water and then titrated. Titration of known sodium fluoride samples in the presence of benzene showed that the error does not exceed 10% of the correct value under these conditions. In the presence of benzene and triphenylmethyl chloride, but no fluoride, the first drop of titrating solution gave the end-point. The data and results for these experiments are given in Table 111. TABLE I1 TREATMENT OF TRIPHENYLMETHYL CHLORIDESIN LIQUID SULFURDIOXIDEWITH SILVERCHLORIDE TRIPHENYLMETHYL CHLORIDE
WEIQHT, MQ.
!:T
FINAL COLOR
?&g-)j, 1OE::LLE
00.
%E;*=
-___
Unsubstituted, . , . , , , . . , , . . . . . . . . p-Fluoro- . . . . . . . . . . . . . . . . . . . . . . . p-Fluoro- . . , , , , , , . , , , , , , . . , . . . . , p-Fluoro- . . . , , . . , , , , , . , , . , , . , . . . p-Fluoro- , . . , , , . , . , . , . . , . , . , . , , . Di-p-fluoro- . . . . . . . . . . . . . . , . . . . . . Di-p-fluoro- . . . , . . . , , . , , . . . . . . . . . Tri-p-fluoro- . . . . . . , . . . , . . . . . . . . . Tri-p-fluoro- . . . . . . . . . . . . . . . . . . . .
20.7 20.9 21.4 19.0 21 .o 19.1 20.9 20.3 20.7
0 56 56 73 77 59 59 19 23
Yellow Orange Orange Lt. Or. Orange Orange Orange Orange Orange
0.04 0.09 0.26 0.10 0.16 0.18 0.20 0.06 0.14
0.007 0.016 0.046 0.020 0.027 0.038 0.039 0.012 0.027
TABLE I11 TREATMENT OF TRIPHENYLMETHYL CHLORIDES IN BENZENE SOLUTION WITH SILVER TRIPHENYLMETHYL CHLOBIDE
WEIQHT,
TIME, DAY8
FINAL COLOR
21.5 20.6 23.1 20.8 21.6 20.3 20.5 20.5 20.6 20.0 20.6
56 56 77 42 42 57 57 23 23 23 29
Lt. Yel. Lt. Or. Orange Lt. Yel. Dp. Or. Dp. Or. Purple Co1orl.b Orange Lt. Or. Orange
-p-Fluoro- , . . . . . . , . . . . . . . . . . . . . . . p-Fluoro- ....................... p-Fluoro- . . . . . . . . . . . . . . . . . . . , . . . Di-p-fluoro- . . . . . . . . . . . . , . . . . . . , . Di-p-fluoro- , . . . . . . . . , . . . . . . . . . . . Di-p-fluoro- . . , , . . . . , . . , . . . . . , . . . Di-p-fluoro- . . . . . . . . . . . . . . . . . . . . . Tri-p-fluoro- . . . . . . . . . . . . . . . . . . . . Tri-p-fluoro- , . . . . . . . . . . . . . . . . . . . Tri-p-fluoro- . . . . . . . . . . . . . . . . . . . . Tri-p-fluoro-. . . . . . . . . . . . . . . . . . . .
0.92 1.6 1.26 1.00 1.46 1.1 0.86 1.18 1.0 2.2 0.66
0.16 0.30 0.21 0.19 0.28 0.22 0.17a 0.25 0.21 0.43 0.13
In this experiment the solvent was nitrobenzene instead of benzene. An orange color first developed in this experiment, then disappeared after several days. b
SUMMARY
1. Di- and tri-p-fluorotriphenylmethyl chlorides have been prepared and described, and p-fluorotriphenylmethyl chloride has been prepared by a new method which leaves no doubt as to its structure.
286
FRANK BACON WITH JOHN H. GARDNER
2. It has been shown that these three compounds rearrange in liquid sulfur dioxide to the extent of 1 to 8 per cent., depending on specific conditions, to yield chloro- and fluorochlorotriphenylmethyl fluorides, and that the presence of silver chloride has little or no effect on this reaction. 3. It has been shown that silver removes, on the average, 0.24 atoms of fluorine per molecule from the fluorinated triphenylmethyl chlorides in benzene solution and that a reaction resulting in the formation of “radicals of the second order” which goes halfway to completion is thus indicated. 4. These findings show that fluorine in a para position of the triphenylmethyl group has a mobility like that of the other halogens but in a lesser degree.
