[CONTRIBUTION FROM
THE
CHEMICAL LABORATORY OE WASHINGTONUNIVERSITY ]
T H E ACTION OF GRIGNARD REAGENTS ON HEAVY METAL SALTS. IV. T H E MECHANISM OF T H E REACTION WITH SILVER BROMIDE E. ALLEN BICKLEY'
WITH
JOHN H. GARDNER
Received August 6, 1939
In the first paper of this series (l),it was shown that the reaction of silver bromide with certain Grignard reagents results in the coupling of the organic radicals of the Grignard reagents. In succeeding papers (2, 3), it has been shown that, with a mixture of two Grignard reagents, silver bromide reacts with the production of all possible coupling products. In this case, it was found that there is a systematic variation in the yields of the several products with the electronegativity of the radicals involved, the more nearly equal the relative electronegativities, the higher being the yield of the unsymmetrical coupling product. It was further noted that relatively stable organosilver compounds such as phenylsilver decompose more rapidly in the presence of a decomposing unstable organosilver compound than they do alone (3). This suggests that the reaction in its final stages is bimolecular, two molecules of organosilver compound reacting with the production of the coupling product and silver. The present investigation is a study of the mechanism of this reaction from a number of viewpoints. In order to determine whether the solvent plays an essential part, p-tolylsilver and p-anisylsilver were prepared free of unchanged Grignard reagent, magnesium salts, and solvent, and were then mixed and decomposed by heating. From the products of this reaction, after demethylation, bi-p-tolyl, 4,4'-dihydroxybiphenyl and p-tolylphenol were isolated, showing that all possible coupling products were formed in the absence of a solvent, and that the solvent, when present, probably plays no part in the reaction. In the earlier papers of this series, it was postulated that the coupling of organic radicals through organosilver compounds involved the initial formation of silver and free radicals followed by the union of the radicals to form the coupling products (1, 2). In the last paper (3), reasons for 1 Based upon a dissertation submitted by E. Allen Bickley in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Washington University, June, 1939. 126
ACTION OF GRXCNARD REAGENTS ON HEAVY METAL SALTS
127
discarding this view were presented. In the present investigation, an atternpt was made to obtain a more definite answer to this question. In a large number of reactions in which it seems certain that free phenyl radicals are produced in solution, it has been found that the radicals always react with the solvent and never with each other (4). Thus, with carbon tetrachloride, chlorobenzene is formed, with aliphatic hydrocarbons, benzene, and with aromatic liquids, derivatives of biphenyl. Biphenyl is formed only in the presence of benzene. In an effort to detect any sign of fme phenyl radicals in the decomposition of phenylsilver, samples of this compound were allowed to decompose under a number of solvents. In no case was there any evidence of reaction with the solvent, biphenyl a1wa:ys being obtained in good yield and high purity. While the criticism may be advanced that, since phenylsilver is extremely insoluble in all solvents used, the reaction may not be carried out under conditions such as to1 produce phenyl radicals in solution, it seems most probable that, since the phenylsilver was in intimate contact with the solvent in every case, some reaction with the solvent should have been detected had any free radicals been formed. This indicates that the reaction occurs through a binary reaction between two molecules of phenylsilver rather than through an initial dissociation of a single molecule into silver and a free radical. The entire reaction from Grignard reagent to coupling products can be divided into two distinct parts, the formation of the organosilver compounds, and their decomposition. Because of the instability of the silver compounds, the separate study of these two parts is virtually impossible in the majority of cases, but an observation by Joseph ( 5 ) that the results are sometimes affected by the nature of the halogen of the Grignard reagent suggests a means by which some information as to the first part of the reaction chain may be obtained. As an initial series of experiments, silver bromide was made to react in the usual way with n-butylmagnesium chloride, bromide and iodide separately. It was found that the yield of n-octane obtained from the bromide was about 70% higher than that from either of the others. Similarly, phenylmagnesium bromide gave nearly twice as great a yield of biphenyl as did the iodide. Similarly, using mixtures of phenyl- and n-butyl- magnesium halides or mixtures of benzylmagnesium chloride and sec.-butylmagnesium halides, very considerable variations in the yields of the various coupling products were noted. Since all of these results involve the halogen atom, they must be due to occurrences during the first part of the reaction chain, and may be connected with differences in reaction velocity. A number of attempts were made to estimate roughly the relative velocities of the reactions between various Grignard reagents and silver
138
E. ALLEN BICKLEY WITH JOHN H . GARDNER
bromide by the disappearance of the Michler’s ketone test (6). Nearly always, the reaction proceeded too rapidly to be measured in this way, so indirect methods had to be found. Such a method was provided by the reaction between pairs of Grignard reagents and one-half of the amount of silver bromide theoretically required t o react with them. After completion of the reaction, the unchanged Grignard reagents were decomposed with dilute acid and the amount of each radical coupled was determined by isolating the reaction products. It is obvious that in this procedure, when two Grignard reagents react with silver bromide a t equal rates, the amounts of the radicals coupled will be equal. Further, when the reaction rates are unequal, the radical of the reagent reacting most rapidly will be coupled to the greatest extent. By carrying out this experiment with suitably selected pairs of Grignard reagents, it is possible to set up a series in terms of reaction velocity. This was done for phenylmagnesium bromide with n-butylmagnesium chloride, bromide and iodide and for phenylmagnesium iodide with n-butylmagnesium bromide. In this way, it was found that these reagents react with silver bromide in the order of increasing velocity: C6H6MgI
< C6&MgBr < C4HgMgBr< C4H9MgC1< C4HgMgI
The detailed results upon which this series is based are given in the Experimental Part. With this series in mind, the results obtained in coupling experiments in which silver bromide was present in excess become understandable. Using phenyl- and n-butyl-magnesium halides, the largest yield of n-butylbenzene was obtained when both were bromides, that is, in the case in which the relative reaction velocities with silver bromide were most nearly equal. The yields with n-butylmagnesium chloride and iodide decreased in that order. When phenylmagnesium iodide was used, the yield of n-butylbenzene was always less than with the bromide, and again the yields decreased in the order n-butylmagnesium bromide > chloride > iodide. It is probable that a similar correlation between reaction velocity and yield of the unsymmetrical coupling product could be shown in the series of reactions between benzylmagnesium chloride and the three n-butylmagnesium halides, but time did not permit determination of the relative rates in this case. An interesting and perhaps important side-reaction was noted in the course of this work. It was found that, whenever iodides were used, the octane fraction always contained considerable quantities of the corresponding butyl iodide. In order to determine the source of this material, n-butylmagnesium bromide was mixed with an ethereal solution of magnesium iodide, with and without the addition of silver bromide. It
ACTION OF GRIGNARD REAGENTS ON HEAVY METAL SALTS
129
was found that when silver bromide was present, n-butyl iodide was formed in large yield, but none was detected in the absence of silver broimide. This may be accounted for on the basis of the equation: n-C4HsAg
+ MgIz = n-C4HgI + Ag + MgI
Similarly, phenylsilver reacted with magnesium iodide to give iodobenzene and a phenylmagnesium halide. No evidence of similar reactions was noted with the other halides. These reactions are to be further investigated. TABLE I n-BUTYLMAGNESIUM HALIDESAND SILVER BROMIDE YIELD OF 11-OCTANI QRIQNARDREAQENT
__
n-C4HpMgC1.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-C4HoMgBr.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-C4H9MgI..................................
-
~
CeHsMgBr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CsHsMgI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
g.
Per Cent
10.8 18.4 10.4
37.8 64.4 36.4
I 25'0 10.5
1
64.8 27.2
EXPERIMENTAL
Thermal decomposition of a mixture of p-tolylsilver and p-anisy1silver.-The silver compounds were prepared separately by the action of the Grignard reagents on silver bromide, washed with dry ether until Michler's ketone test was no longer given, centrifuged from the ether and mixed. The mixture was heated for one-half hour a t 100". The products were extracted from the silver residue and demethylated with constant boiling hydriodic acid in an equal volume of glacial acetic acid. The precipitate formed on dilution with water was extracted with 2% sodium hydroxide. The residue was di-p-tolyl, m.p. 119-120", from alcohol. Ullmann and Meyer give the m.p. 121-122" (7). Acidification of the extract gave a precipitate which was crystallized from benzene. The fraction more soluble in benzene was 4-methyl-4'-hydroxybiphenyl, m.p. 152-153", benzoate, m.p. 185-186". Kliegel and Huber give the respective melting points 154-155" and 188" (8). The fraction less soluble in benzene waa 4,4'-dihydroxybiphenyl, m.p. 266-268" from acetic acid, diacetate, m.p. 157159". Schmidt and Schultz give the respective melting points 272" and 159-160" (9). Attempts to capture free radicals i n the decomposition of phenylsi1ver.-Phenylsilver was prepared by the reaction of phenylmagnesium bromide and silver bromide and washed with dry ether until no test waa given for a Grignard reagent. Portions were
130
E. ALLEN BICKLGY W I W JOHN H . GARDNER
then decomposed successively under carbon tetrachloride, chlorobenzene and nitrobenzene. I n all experiments the only product isolated was biphenyl; no evidence of reaction with the solvent was found. Reaction between n-but ylmagnesium halides and silver bromide.-These experiments were carried out according to the procedure of Gardner and Borgstrom (1) ,using onehalf mole of alkyl halide in each. The n-octane from n-butylmagnesium iodide was contaminated with n-butyl iodide, and the actual yield was estimated from a composition-refractive index curve for n-octane and n-butyl iodide. The results are given in Table I. Reaction between phenylmagnesium halides and silver bromide.-These experiments were conducted according to the procedure of Gardner and Borgstrom (11, using onehalf mole of the halide in each case. The results are given in Table 11. Reaction of mixtures of phenyl- and n-butyl- magnesium halides with silver bromide.Grignard reagents were prepared separately from one-half mole of each halide, TABLE I11 COUPLING PRODUCTS
FROM PHENYL- AND
n-BUTYL-MAGNESIUM HALIDES
I REAQENTB
YIELDS, 0 ,
n-Octane, B.P. 125-8"
n-Butylbenzene, B.P. 177-8"
Biphenyl, M.P. 65-7'
CBHsMgBrwith n-CdHsMgBr. ............................. n-C4HgMgCl.............................. n-C4HgMgI...............................
6.0 4.8 10.7
25.0 18.0 1.5
12.0 13.3 27.0
CeHsMgI with n-C4HoMgBr... . . . . . . . . . . . . . . . . . . . . . . . . . . . n-C4HgMgCl.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-C4HsMgI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2 8.5 8.8
4.0 2.0 1.0
23.0 23.5 23.5
mixed, and caused to react with one mole of silver bromide according to the procedure of Joseph and Gardner (3). Whenever an iodide was used, the n-octane was contaminated with n-butyl iodide and the yield was estimated from a compositionrefractive index curve. The results are given in Table 111. Reaction of mixtures of benzylmagnesiurn chloride and sec.-butylmagnesium halides with silver bromide.-These experiments were carried out according t o the procedure of Joseph and Gardner (3), using one-half mole of each halide and one mole of silver bromide in each. The 3,4-dimethylhexane from the runs in which see.-butyl iodide was used was contaminated with sec.-butyl iodide. The yield in this case was estimated from a refractive index-composition curve. The results are given in Table IV. Relative rates of reaction of phenyl- and n-butyl- magnesium halides with silver bromide.-These experiments were carried out as before except t h a t only one-half mole of silver bromide was used for a mixture of Grignard reagents prepared from one-half mole of each of two halides. The results are expressed in terms of moles of phenyl hydrolysed, detected as benzene in the final product, moles of phenyl coupled (biphenyl n-butylbenzene), and moles of n-butyl coupled (n-butyl-
+
131
ACTION O F GRIGNARD REAGENTS ON HEAVY METAL SALTS
+
benzene n-octane). Since the unreacted n-butylmagnesium halide was finally converted into n-butane, the amount of unreacted n-butyl Grignard reagent was not determined. The results are given in Table V. The radical which is coupled t o the greater extent corresponds to the Grignard reagent in the pair which reacts the inore rapidly with silver bromide. Reaction of n-butylmagnesium bromide with magnesium iodide i n the presence of silver bromide.-Slightly more than the equivalent amount of silver bromide was added to an ethereal solution of 0.25 mole of n-butylmagnesium bromide and 0.25 mole of magnesium iodide. The mixture was stirred a t 0' for a half hour and then for 8.n hour a t the boiling point. The reaction mixture was treated with very dilute TABLE IV COUPLING
PRODUCTS FROM BENZYLMAQNESIUM CHLORIDE AND MAQNESIUM HALIDEB
sec.-BuTYL-
YIELDB, Q. REAQENTE
CaHbCHZMgCl with sec.-C4HoMgC1,........................... sec.-C4HgMgBr........................... sec.-C4H&fgI.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
3,4-Dimethylhexane, B.p. 1109"
2-Benzylbutane, B.p. 190-3'
Bibenzyl, M.P. 51-3'
34.0 32.2 13.0
11.0
1.5 13.7
11.0 21.0
PHENYL COUPLED YOLEB
It-BUTYL COUPLED YOLEB
TABLE V DATAFOR RELATIVERATES REAGENTS
PHENYL HYDROLYSBD MOLBB
~~
CsHsMgBr with n-CrH gMgBr............................. n-CdHpMgC1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-CdHpMgI.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.207 0.264 0.292
0.223 0.198 0.165
0.293 0.250 0.379
CaH&MgIwith n-C,H&fgBr. ............................
0.350
0.089
0.387
hydrochloric acid and the products were steam-distilled, dried over calcium chloride and distilled. There was obtained in this way 15 g. of a mixture of n-octane and n-butyl iodide, b.p. 122-129', d ; 1.200, n i 1.4480. The refractive index corresponds t o that of a mixture containing 32% n-octane and 68% n-butyl iodide. A blank experiment in which the silver bromide was omitted yielded neither n-octane nor n-butyl iodide. Reaction of phenylsilver and magnesium iodide.-Phenylsilver prepared from 0.20 mole of phenylmagnesium bromide was washed free of the Grignard reagent with dry ether and was then stirred with an excess of magnesium iodide in dry ether solution for two hours a t 0". The ether solution then gave a strong Michler's ketone test for Grignard reagent. After stirring an additional two hours a t the boiling
132
E. ALLEN BICKLEY WITH JOHN H. GARDNER
point, the suspended solids became brown. After hydrolysis with very dilute hydrochloric acid, there were isolated from the organic layer 3.0 g. of benzene, b.p. 78-80", n: 1.4980, 1.0 g. of iodobenzene, b.p. 180-190", n: 1.5962 and 2.0 g. of biphenyl m.p. 64-65". When phenylsilver was stirred in the cold for two hours with dry ether, the liquid gave a negative Michler's ketone test. SUMMARY
1. Organosilver compounds have been shown to decompose by a bimolecular reaction not involving free radicals. 2. It has been shown that the course of the reaction between Grignard reagents and silver bromide is determined by reaction velocities. 3. Various side-reactions have been studied. ST. LOUIS,Mo.
REFERENCE (1) GARDNER AND BORGSTROM, J . Am. Chem. SOC.,61, 3375 (1929). JOSEPH AND GOLLUB, J. Am. Chem. SOC.,69, 2583 (1937). (2) GARDNER, AND GARDNER, J. Org. Chem., 6, 61 (1940). (3) JOSEPH (4) HEY AND WATERS,Chem. Rev., 21, 193 (1937). (5) JOSEPH, Dissertation, Washington University (1937). J. Am. Chem. Soc., 47, 2002 (1925). (6) GILMANAND SCHULZE, AND MEYER,Ann., 333, 44 (1904). (7) ULLMANN (8) KLIEGELAND HUBER,Ber., 63, 1652 (1920). (9) SCHMIDT AND SCHULTZ, Ann., 207, 337 (1881).
