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THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY‘
sis,” and others, as a simple apparatus for measuring hydro; gen electrode potentials. With a clear understanding of these matters he has a “stock in trade” which will save him
Vol. 14, No. 5
much perplexity and which will aid him in clear insight into many of the problems which he must tackle when he begins the practice of his profession.
T h e Reactions of Phosgene with Benzene and m-Xylene in the Presence of Aluminium Chloride”’ By Robert E. Wilson3 and Everett W. Fuller RESEARCHLABORATORY OF APPLIEDCHEMISTRY, ~ ~ A S S A C H U S E T T SINSTITUTE OF TECHNOLOGY CAMBRIDGE,MASSACHUSETTS
’ On account of the great reduction in the cost of producing phos-
gene, which was brought about by war-time deuelopments, this Laboratory has undertaken a study of possible new methods of utilizing this reactive compound i n certain syntheses which now appear tp be within the range of commercial possibility. The initial aim of the work described in this article was to produce anthraquinone f r o m phosgene and benzene by the Friedel and Crafts reaction: 0
/I
/‘\p,
2HC1
2HC1 0 Although anthraquinone was not secured, the results obtained are of interest, us they represent a much more thorough study of the reactions between phosgene and benzene (and its homologs) than has been published preoiously. The reaction between phosgene and benzene i n the presence anhydrous AICl, gioes benzoyl chloride and benzophenone but no trace of anthraquinone. The reaction fakes place in two stages, with the intermediate formation of the compound C6H6COCl:AIClawhich can be hydrolyzed to gicc benzoic acid. This reacts aery rapidly with more benzene, howeoer, and the final product is almost entirely benzophenone irrcspectioe of changes in temperature, method of mixing, ratio of reacting substances, etc. When CS2 is used as a diluent a large part of the product Can be obtained as benzoic acid. Eoidence is presented to show that this is due to the slight solubility of the intermediate benzoyl chlorideAICla compound in CS2 which is thus removed before it has opportunity to react with more benzene. m-xylene gioes with phosgene a di-xylyl ketone with at least two of the methyl groups ortho to the carbonyl group. Attempts to change this di-xylyl ketone f o a derioatioe of anthraquinone by oxidation were not successful.
I
N one of their early papers, Friedel and Crafts4mention
that by adding anhydrous aluminium chloride to a mixture of benzene and Phosgene, they mxud benzophenone and a small amount of benzoic acid. No trace of anthraquinone was found. Later experimenters5 used xylene in place of benzene and obtained the corresponding phenones and acids, but no evidence of a quinone linkage was reported. No further work on the reactions of phosgene with benzene, 1 Presented in preliminary form before the Division of Organic Chemist r y a t the 00th Meeting of the American Chemical Society, Chicago, Ill., September 0 t o 10, 1920. a Published as Contribution No. 42 from the Research Laboratory of Applied Chemistry, M. I. T. a Director, Research Laboratory of Applied Chemistry, M. I. T. 4 Friedel, Crafts and Ador, Ber., 10 (1S77),1854. 8 Ador and Pelliet. I b i d . , 11 (1878),399.
toluene, or xylene is reported in the literature to the knowledge of the authors. ATTEMPTS TO MAKEANTHRAQUINONE By varying the experimental conditions the attempt was made to carry the reaction beyond the stage of the formation of benzophenone and thus secure anthraquinone. However, by none of the methods tried was it found possible to cause more than one phosgene molecule to react with a single benzene molecule even when a large excess of phosgene was used. For additional proof, benzophenone was treated with phosgene and aluminium chloride at different temperatures. There was no evidence of any reaction taking place and the benzophenone was recovered unchanged. Also, benzoyl treated with an equivalent amount of aluminium chloride in the hope that two molecules might mutually react with each other and thus give anthraquinone. But no reaction was noted up to near the boiling point of the benzoyl chloride, when charring and decomposition of the latter took place. No definite new could be i d a t e d . These results are in accord with the customary observation that it is very difficultto make two negative groups enter an aromatic ring ortho t’o each other. They also agree with the results of v. Meyer6 who found it impossible to put two acetyl groups on benzene by the Friedel and Crafts reaction. By reducing the benzophenone to diPhenylmethane i t was, however, found possible to make it react with more phosgene, but the resulting product was tarry, and was not identified. This indicates again that the negativity of the keto group is what prevented further reaction with the phosgene. BETWEEN PHOSGENE AND BENZENE REACTIOKS Although there seemed to be little likelihood of being able to prepare anthraquinone by this method, a study was made of the principle factors influencing the reaction between phosgene and benzene to form benzoyl chloride and benzophenone. The mechanism of the action of aluminium chloride in the Friedel and Crafts reactions has been studied by a number of inb-estigators.7 For reactions of the above type, the chloride appears to form a compound with one or both of the reacting substances and thus to render them active, I n the case of phosgene, three fairly stable molecular compounds with aluminium chloride are claimed to have been isolated,8 while with benzoyl chloride and benzophenone molecular compounds, containing mole for mole, have been found. The aluminium chloride thus bound up renders its own molecule active but is not available for activating other molecules. Ber., 29 (1890),1413 Boeseken, Rec. tuav. chem., 19 (IQOO), 19, 20 (1901), 102; 24 (1905), 6; Perrier, Ber., 88 (IQOO),815; Krarberg, J. prakt. chew., 6 1 (lWO),494. 8 Band, Compl. rend., 140 (1905),1088. 8
THE JOURNAL OF INDUSTRIAL A N D ENGIXEERIIYG CHEMISTRY
May, 1922
TABW I-RISACTIONS KO.
1
2 3
4
5 6 7 8 9
10
11 12 13 14 15 16 17 18 19 20 21 22 23 24
Benzene 4 4 4 2 2.5 2 2.5 2 2 1 1
Phosgene Excess Excess 2 1 1 1
1 1 1
1 1
1
1 1
1 1 1 1 2
.. .. *. ..
..
....
. e
..
.. .. .. .. .. ..
.. ,. ....
.. ...... .. ..
..
....
.. ..
PHOSGENE
..
..
1 1.5
Excess 1
BENZENE AND
(All quantities are given in relative moles) Benzoic Acid CSP Benzoic Acid Bemophenone Benzophenone METHOD 0.01 0.89 Phosgene into benzene and AlCla a t 4 ' C. 0.01 0.87 .. Phosgene into benzene and AlCla a t 80° C. 0.01 0.55 Benzene to mixture of phosgene a n d AlCls 0.01 0.55 AlCla to mixture of benzene and phosgene 0.01 0.63 AlCla t o mixture of benzene and phosgene 0.01 0.70 Same a s 4 and kept under 1 atmos. gage pressure .. 0.01 0.75 Same as 4 and kept under 1 atmos. gage pressure 0.01 0.18 A1Ch t o mixture of benzene and phosgene 0.01 0.27 A1Ch t o mixture of benzene and phosgene 0.01 0.48 O:b21 AlCh t o mixture of benzene and phosgene 6: 10 0.05 0.43 0.116 AlCls to mixture of benzene and phosgene 0.50 0.10 0.41 0,244 AICh t o mixture of benzene and phosgene 1 0.26 0.33 0.760 AlC1a t o mixture of benzene and phosgene 2 0.36 0.29 1,24 AlCh t o mixture of benzene and phosgene 2.5 0.33 0.23 1.43 A1c1a to mixture of benzene and phosgene 4 0.38 0.25 1.52 AlCla to mixture of benzene and phosgene 4 0.59 0.17 3.47 Benzene added drop by drop t o others-kept cold 4 0.01 0.39 AICh added very slowly t o others 4 0.01 0.25 .. Phosgene added very slowly t o others 4 0.03 0.52 AlCla t o mixture of benzene and phosgene 0.01 0.24 A1 amalgam in benzene+RCl and then phosgene 0.01 0.16 AltHgC12 in benzene t phosgene 0.01 0.08 AI in benzene HC1 phosgene No action Anhydrous FeC13(1 mole)
.. .. .. .. ..
1
1 1 1 1 1
BETWEEN
407
EXPERINENTAL STUDY O F BENZENE-PHOSGENE REACTIONS EXPERIMEKTAL METHOD-In studying the effect of changing various conditions, the general method of procedure was as follows, unless otherwise specified. A weighed amount of phosgene was absorbed in a known amount of benzene cooled with ice and salt. The aluminium chloride was then added slowly and the flask was allowed to stand in the ice bath for several hours until the reaction had proceeded to a considerable extent and then was left a t room temperature for 12 to 16 hrs. The mixture was then added to ice water, which decomposed the aluminium chloride compounds with the separation of an oil consisting of benzophenone, benzoyl chloride, and any excess benzene that might be present. The oil was treated with caustic soda solution to hydrolyze the benzoyl chloride and remove it as sodium benzoate. On acidification of the extract benzoic acid was precipitated and recovered. In the following work the benzoyl chloride is reported as benzoic acid. The remainder of the oil was dried and fractionally distilled to give benzene and benzophenone. EFFECT OF DIFFEREKT VARIhBLES-The effect Of Various conditions of the yields and on the course of the reactions may be determined from a study of the data given in Fig. 1 and in Table I. In considering the yields relative values should be given greater weight, as small differences in actual yields were unavoidable even under the same experimental conditions. All yields were based upon the purified product. Time-Fig. 1 indicates the results that were secured by removing 100-cc. samples a t various times from a mixture of relative moles of phosgene and aluminium chloride in an excess of benzene that was maintained a t 4" C., and analyzing for benzoyl chloride and benzophenone. The formation of excess benzoyl chloride, over that reacting with more benzene to give benzophenone, starts in slowly, gradually increases, and reaches a maximum in 5 to 6 hrs., after which it falls off again. Benzophenone also starts in very slowly but constantly increases to a maximum at somewhat less than 24 hrs. The reaction is evidently compIeted within 24 hrs., a t 4' C. A two-stage reaction with the intermediate formation of benzoyl chloride is clearly indicated. Thus it will be noted that at the very start of the reaction more benzoyl chloride than benzophenone is found. As time goes on the reaction as a whole accelerates, as measured by the total conversion of benzene to one of the two forms; but the quantity of intermediate product increases comparatively slowly while the end-product increases rapidly. The maximum rate of forming benzophenone coincides with the maximum momentary concentration of benzoyl chloride, as would be expected. Thereafter, as the rate of forming benzoyl chloride drops
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l
THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
408
Proportion of Constituents-The molecular proportions of constituents are given by the equations for the reaction and must be used to secure anything approaching 100 per cent yields. On account of the volatility of phosgene an excess of it or of benzene helps the yields per mole of the other constituent by maintaining sufficient gas throughout the run to give the theoretical yields. The effect of increasing amounts of aluminium chloride is shown in Expts. 8, 9, and 4. One mole of aluminium chloride is evidently united with and removed from its active state by each mole of product formed and so a theoretical amount of aluminium chloride must be used. As stated before, these molecular compounds of aluminium chloride have been isolated, but in further confirmation of their formation in these reactions a crystal obtained in one of the experiments was analyzed and found to correspond to the formula C6H6COC6Hs.AlC18. Substitutes for AZCZa-Several substitutes for aluminium chloride were tried, asisindicated in Expts. 21,22, 23, and 24, but none of these proved satisfactory. Anhydrous ferric chloride can be used in place of aluminium chloride in many of the Friedel and Crafts reactions, but with phosgene and benzene it appeared t o be inert. I
05
~
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l
'
l
1
I
I
I
1
04
03
z
O2 01
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2
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l 3
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,
, 4
Moles CS, FIG. 2
Use of CS2 as a Diluent-The result of using carbon disulfide as a diluent may be seen by comparing the experiments in which no disulfide was used with those in which varying amounts were used, Expts. 10 to 16. The comparison is brought out clearly in Fig. 2. A marked increase in the amount of benzene (proportion of benzene) remaining as benzoic acid (benzoyl chloride) is shown up to R carbon disulfide concentration of 2 moles to 1 of benzene, while beyond this point addition of carbon disulfide does not greatly affect the results. A corresponding decrease in benzophenone is found to the same concentration of carbon disulfide. This marked increase in the relative amount of benzoic acid to benzophenone with increasing carbon disulfide was further investigated, As stated before, the benzoyl chloride that is formed as an intermediate product in the preparation of benzophenone forms a definite molecular compound with aluminium chloride. It was suggested by Prof. J. F. Norris that this intermediate molecular compound might be rendered less soluble in the reacting medium by the presence of CSZ. It would thus be removed from the sphere of activity with a resulting increase in the relative amount of benzoic acid obtained in hydrolysis. Solubility data obtained in this Laboratoryg indicate this explanation to be correct. The benzoyl chloride-aluminium chloride compound was isolated as a yellow solid and its solubility determined for varying mixtures of carbon disulfide and benzene. The results follow: ' 9
D. I. Gross, undergraduate thesis, M . I. T.
Per cent CSz 100 75 50
Temperature 4' C. Per cent Benzene
0 25 50
Vol. 14, No. 5 Solubility
G.per 100 Cc. 1.4 2.6
7 7
Furthermore it was noticed that in runs where carbon disulfide was used and the yields of benzoic acid were high a heavy dark oil separated out in the bottom of the flask. Although this substance was a liquid where a solid would be expected, it undoubtedly contained the benzoyl chloridealuminium chloride compound which was thus separated t o some extent from the incoming benzene. The composition of the dark oil was not constant, but an analysis of that obtained at the end of one of the runs gave 9 per cent of benzoic acid, 53 per cent aluminium chloride, 17 per cent of benzophenone, and 21 per cent of phosgene, with possibly a little benzene. Further to prove this point, Expt. 17 was made, in which benzene was dropped very slowly into a cold mixture of carbon disulfide, phosgene, and aluminium chloride. This gave the best possible conditions for precipitating the intermediate molecular compound out of solution as it is foimed and before it could come in contact with more benzene. As was expected, the yield of benzoic acid was very large and that of ~ benzophenone was low, and some procedure similar to this is probably the best way to obtain a high yield of benzoic acid. On the other hand, when either aluminium chloride or phosgene was added very slowly to a mixture of the other three, it gave a chance for each new mole of benzoyl chloride to react with more benzene; thus the principal product was benzophenone, as shown in Expts. 22 and 23. The above experiments with carbon disulfide were all made with just one mole of benzene for each mole of aluminium chloride and phosgene and the discussion holds oiily for such a case. If, however, more than a, mole of benzene is used per mole of alumjnium chloride, the product will eventually be all benzophenone, since the intermediate compound will slowly redissolve and react with the excess benzene if sufficient time is allowed. Expt. 20 indicates this. Where no carbon disulfide is used, on the other hand, the much greater solubility of the benzoyl chloride-aluminium chloride compound in benzene brings about complete reaction to benzophenone, regardless of the ratio between benzene and aluminium chloride used (Expts. 1 to 10). COMMERCIAL PossIRILITIES1o-~hile the foregoing investigation has not been dikected primarily t o the commercial aspects of the process, it seems worth while to consider briefly this phase of the problem. Alt>houghthe listed price of benzophenone is quite high, it has little use except for scientific purposes and the demand is not large enough t o warrant a commercial installation. If its availability at much lower prices should ever develop a demand for this product, the above-described method is certainly admirably adapted for commercial product*ion,as t h e yields are high (85 to 90 per cent), the product clean, and t h e raw material cost quite low, a very small fraction of the present quotations for pure benzophenone. For benzoic acid, on the other hand, there is a very ready market, and material such as that prepared by the above reaction, which is entirely free from chlorine in the ring (as distinguished from the product made by chlorinating toluene, which gives trouble in this respect), has a large and active market. It is for this reason that the effort was made in the foregoing study to increase the yield of benzoic acid as compared with benzophenone, and this was found possible by slowly adding benzene to the other components dissolved The recommended process has been covered by U. S. Patent Application.
