December, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Life-in fact, they are responsible for the most of it-but aside from that, friends are extremely useful. American chemists are fortunate in that they have been kept together in one allinclusive society, the meetings of which bring all kinds of chemists together. It is highly desirable that all of us should get out of our own little groups and get acquainted with chemists in other lines. One important function of the Xational Research Council is to be an intermediary between the industridists and the teachers and to serve as a clearing house for information. The future will doubtless show many excellent results from its efforts. I n addition to informal contacts, there are often advantages to be gained by teachers working in the plant laboratories during the summer. I n that way they get a real insight into the industrialist's problems and methods. The common objection is that one can accomplish little on a problem in three months. It is a short time, but three nionths is onethird of nine months and the teacher would hate to admit that
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he accomplishes nothing during the nine months session. One who is trained to think should be able to put his mind on a problem and accomplish something definite on it in three months. The amount undertaken should bear a proper relation to the time. il small problem, or one phase of a larger one, can be chosen. A teacher may serve as consultant even during the session, making occasional visits to the plant and keeping in touch .Ir-ith one or two problems. The industry gets the benefit of all the knowledge and experience that the teacher has and pays for only a part of his time. The teacher brings back to his classes much up-to-date information, and nearly every practical problem with which he comes in contact reveals some gap in fundamental knowledge or suggests some purely scientific investigation that is suitable for the university laboratory. To sum up, the university and the industry are mutually dependent and each can profit by better understanding and closer contact, yet each can serve the other best by doing its own work well.
The Development of Synthetic Anthraquinone By K e n n e t h H. Klipstein E. C. KLIPSTEIN&
SONS
Co.,N E W YORK, N. Y.
The A n t h r a c e n e Process
C h e m i s t r y of t h e S y n t h e t i c Process
FFORTS to develop the anthraquinone vat dye industry in the United States presented the vital problem of a source of supply of pure anthraquinone in large quantities and a t an economic cost. Up to the time when manufacturers in this country first attempted to produce the material, only one process had found general application on a n industrial scale. This was the chromic acid oxidation of anthracene. When coal tar is distilled, the fraction known as anthracene oil comes over above 270" C. It consists of anthracene, carbazole, phenanthrene, and other high-boiling constituents. The anthracene contained in the anthracene oil, because of the high percentage of impurities present, is unsatisfwtory as such for the manufacture of anthraquinone, and must be subjected to further purification. This consists of treatment with carbonate of potash, to remove the nonvolatile potassium derivative of carbazole, followed by one or two crystallizations from a solvent such as pyridine. The anthracene is then sublimed to render it in the proper physical form for oxidation. It is next treated with chromic acid until oxidation has been completed. This step is exceedingly simple chemically and can be accomplished with almost quantitative yields, based on the anthracene present. From an economic standpoint, however, a serious problem is encountered in connection with the oxidation in the recovery or disposal of the by-product, chromium sulfate. Although the liquors in some cases have been disposed of without further treatment, and in others have been recovered by regeneration of chromic acid through electrolytic oxid&ion, impurities in the liquors and maintenance of the cells in the electrolytic recovery are sources of unavoidable difficulties. The crude anthraquinone, after separation from the liquors, is dissolved in hot concentrated sulfuric acid to destroy certain of the impurities. The purified product is then precipitated by pouring the sulfuric acid solution into water. The crystals are filtered off and given a final purification by sublimation or crystallization from a suitable solvent.
Despite the apparently definite establishment of the anthracene process to the exclusion of all others, Liebermann, one of the early investigators of anthraquinone and its derivatives, pointed out over fifty years ago that commercialization of a synthetic process might be possible. He stated:'
E
Even though anthracene will surely be the raw material in the manufacture of alizarin colors for some time to come, nevertheless technical men should not even a t the present time lose sight of the possibility of the synthetic production of alizarin in other ways. Such a synthesis, for example, would be possible, and it is not a t all inconceivable, if satisfactory yields could be worked out for the methods involving benzyl chloride, benzyl toluene, and o-benzoylbenzoic acid.
It was a relatively simple matter to condense o-benzoylbenzoic acid to anthraquinone with a high yield by treatment with phosphorous pentoxide or, better, sulfuric acid. The key to a commercial synthesis of anthraquinone, therefore, was the preparation of the intermediate o-benzoylbenzoic acid. I n 1877 the use of anhydrous aluminum chloride as a catalyst in bringing about reaction between certain types of organic compounds with the elimination of hydrogen chloride was accidentally discovered by Friedel and Crafts as a result of observations made on the action of aluminum metal on amyl chloride. They synthesized first homologs of benzene from benzene and aliphatic chlorides in the presence of anhydrous aluminum chloride, and ketones from benzene and acid chlorides.2 Early the following year they found that acid anhydrides could be substituted for aliphatic and acid chloride^.^ Carbon dioxide, the anhydride of carbonic acid, reacted to give benzoic acid; sulfur dioxide, the anhydride of sulfurous acid, benzene sulfinic acid; acetic anhydride, benzoic acid, and acetic acid as a by-product; and phthalic anhydride, o-benzoylbenzoic acid : CsHiOa
AlC13 + CEHB--+C~~HIUOS
I n 1907 Heller showed that in order to obtain a nearly quantitative yield of o-benzoylbenzoic acid a slight excess
* Ber., 7, 805 (1874). 1
Comfit. rend., 84,1392, 1450 (1877); 86, 76 (1877).
a Zbid., 86, 1368 (1878).
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
over two molecules of aluminum chloride to one molecule of phthalic anhydride was r e q ~ i r e d . ~The aluminum chloride, in this particular reaction, was found not to function catalytically in the sense that small quantities of the chloride could activate large quantities of the anhydride and hydrocarbon. Up to this time the proportion of chloride and anhydride used had been that originally recommended by Friedel and Craftsnamely, approximately one and a half molecules of chloride to one molecule of anhydride. I n the following year, 1908, Heller and Schiilke concluded that? The aluminum chloride and phthalic anhydride slowly go into solution in the benzene, hydrogen chloride is evolved, and presently there separates a thick mass which does not crystallize and which on treatment with water yields an aluminum salt of o-benzoylbenzoic acid. ***The first step is the formation of a n addition compound from one molecule of anhydride, two molecules of aluminum chloride, and one molecule of benzene. The intermediate addition product splits out hydrogen chloride and is converted into a complex compound containing o-benzoylbenzoic acid.
I n other words, an addition took place between all the reactants, and hydrogen chloride was split out from this addition compound to form a new compound containing 0benzoylbenzoic acid, some aluminum chloride, and possibly some hydrocarbon :
I n recent years the work started by Heller and Schiilke has been carried to completion by the able researches of Prof. F. B. Allen, of the University of Toronto, and his students. Friedel and Crafts, in the preparation of p-toluylbenzoic acid, the methyl homolog of o-benzoylbenzoic acid, from phthalic anhydride and toluene, using only one and a half molecules of aluminum chloride to one molecule of anhydride, had noticed the formation of small quantities of a resinous by-product. Von Pechmann, on repeating the work of Friedel and Crafts, had found that the by-product was ditolylphthalide.s Rubidge and Qua, under the direction of Professor Allen, found that a phthalide was formed, not only in the reaction between phthalic anhydride and toluene, but also in the reaction between phthalic anhydride and benzene, if an insufficient quantity of aluminum chloride was employed. They found further that the formation of phthalides in either case was due to the action of an excess of anhydride on the intermediate addition product containing 0benzoylbenzoic acid or its homolog, some aluminum chloride and possibly some hydrocarbon.’ The various researches described above and others, also excellent, contributed much to our knowledge of the steps of the reaction. Attempts have been made to explain the exact catalytic effect of the anhydrous aluminum chloride, but it has been only recently, with the advance in the knowledge of the structure of, and forces within, the atom that a somewhat deeper penetration into the actual working of the catalyst has been made possible. As a result of an exhaustive study of the problem, Dr. Gregg Dougherty, of Princeton University, concludes: B y utilizing the Lewis-Langmuir theories of valence, together with T. M. Lowry’s8 ideas concerning intramolecular ionization, we can perhaps get a start toward a working hypothesis of the Friedel and Crafts reaction mechanism. G. N. Lewis accounts for the additive properties of aluminum chloride on the basis of 2. angew. Chem., 19, 669 (1906). 6 Ber., 41, 3627 (1908). 8 I b i d . , 14, 1865 (1881). 7 J . Am: Chcm. SOC., S6,736 (1914). 8 J . Chem. SOC.(London),123, 822 (1923); PhiZ. Mag., 46, 964, 1013 (1923). 4
Vol. 18, No. 12
the electronic structure of the molecule.# An atom has a tendency t o surround itself with a n outer shell of eight electrons, a condition of considerable stability. In aluminum chloride the aluminum atom has only six electrons: C!
+:c1 CI
In order t o obtain the other two, it will share a pair previously belonging totally to some other atom or molecule. We thus get
a bond between the aluminum chloride and the other atom or molecule, since unit valence, according to Lewis, consists of a shared pair of electrons:
-c I
‘
c1 :Ai: CI
-c
or
Cl
1
‘
.. C1.. c1
::?:A!:
c1
Lewis does not explain the reason for the activity or nonactivity of these addition complexes. We might expect t h a t the addition of the aluminum chloride would at first upset the electronic equilibrium of the other compound. We might also expect t h a t in some cases a rearrangement would take place within the new molecule t o restore this equilibrium. This would result in a stable, inactive addition compound. In other cases, for some reason, the equilibrium would not be restored, and we would have a n unstable and active complex. This latter would have polar character or, in other words, i t would have a positive part and a negative part. Stability and electrical neutrality of the system could then be effected by interaction of the polar molecule with a nonpolar molecule or a different b u t also polar molecule. Lowry has not dealt with polarization b y means of catalysts, b u t he attributes chemical activity in general among organic compounds to the existence of molecules which are polarized or internally ionized. Let us now apply these theoretical considerations to, for example, the interaction of benzene and a n alkyl chloride in the presence of aluminum chloride. Without going into detail as to the place of attachment or t o the mechanism of the formation of the polar bond, we may say that the aluminum chloride adds t o the benzene and t o the alkyl chloride, causing polarization in these two molecules :
p]+..
AlClr]’+
[C.Hs.
pa]+..p,
AlCb]’
Neutralization then occurs, yielding addition compounds of methyl benzene and aluminum chloride, and of hydrogen chloride and aluminum chloride, which split up into methyl benzene, hydrogen chloride and free aluminum chloride:
1+ 1
[CHGHr.AICb
1
f HCI
HC1, NClr +CHaOHs
+ 2AlCL
Although in some cases more complicated, this explanation may be used t o give a logical interpretation of any of the Friedel and Crafts syntheses. The formation of o-benzoylbenzoic acid can be outlined along similar lines. The addition complex of Heller and Schiilke, consisting of one molecule of phthalic anhydride, two molecules of aluminum chloride, and one molecule of benzene, gives, probably, first a compound of the acid chloride type:10
[ 1 [GH,
