Base-Catalyzed Autoxidation of 9, 10-Dihydroanthracene and Related

15 Base-Catalyzed Autoxidation of 9, 10Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0075.ch015

Dihydroanthracene and Related Compounds J. O. H A W T H O R N E , K. A . S C H O W A L T E R , A. W . SIMON, and M . H. W I L T Applied Research Laboratory, United States Steel Corp., Monroeville, Pa. M . S. M O R G A N Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pa.

The autoxidation mechanism by which 9,10-dihydroanthracene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyltrimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initiated free-radical chain mechanism that involves a single-electron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported.

A number of studies have been reported in which organic compounds with acidic hydrogens attached to carbon have been oxidized in a basic medium (5). It was not generally recognized until recently (6, 7) that a methylene hydrogen of 9,10-dihydroanthracene could dissociate i n certain solvents in the presence of a base to form a carbanion, which, in contact with molecular oxygen, would yield oxidation products of dihydroanthracene. The investigation presented i n this paper was conducted to establish the mechanism by which the autoxidation of 9,10-dihydroanthracene takes place under homogeneous basic conditions. Under the conditions used here, the two major oxidation products were anthracene and anthraquinone. 203 In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

204

OXIDATION OF ORGANIC COMPOUNDS

1

Ο

o

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0075.ch015

H ^ H

2

Base Solvent Ο

Either compound could be the major oxidation product by selecting suitable solvent and base. Since anthraquinone is a more valuable product than anthracene, the major portion of this investigation was directed toward identifying the factors that are important in producing anthra quinone and minimizing the formation of anthracene. A homogeneous reaction system was used in which benzyltrimethylammonium hydroxide was the base and pyridine the solvent. A n oxida tion mechanism is proposed that is consistent with observations on the reaction variables and possible oxidation intermediates of dihydroanthracene. Experimental Apparatus. The reaction system was a M i n i - L a b assembly (Ace Glass, Inc., 50-ml. capacity) with a hollow-bore stirring rod through which oxygen or air could be introduced. The gases were metered b y passing them through a flowmeter. Temperature was manually controlled b y supplying heat with an electric heating mantle, or, i n the event of an exothermic reaction, by directing a cold air blast against the outside of the reactor. Procedure. The general procedure was to add a 4 0 % pyridine solu tion (1.7 ml.) of the catalyst (benzyltrimethylammonium hydroxide, 3 mmoles) to a solution of 9,10-dihydroanthracene (9.0 grams, 50 mmoles) in anhydrous pyridine (50 m l . ) . The solution was heated to 50° to 60°C. and excess oxygen (0.29 to 1.05 liters per minute) was passed into the stirred solution. The heat of the reaction increased the temperature to 7 0 ° C , and this temperature was maintained b y external cooling (air) for the first 30 minutes. (Anthraquinone began to crystallize after 10 minutes.) Heat was applied thereafter to maintain the 70°C. tempera ture for a total reaction time of 2 hours. The mixture was then cooled to 2 5 ° C , and the catalyst was neutralized with acetic acid (marked b y a color change from red to light yellow). The solvent was evaporated under reduced pressure (bath temperature, 3 0 ° C ) , and the residue was washed with water, collected on a tared filter, dried at room temperature under reduced pressure, and weighed. Tests were also conducted i n which the anthraquinone that crystallized from the reaction solution was collected b y filtration, washed with pyridine (15 m l . ) , and dried. The mother liquor was evaporated to dryness to recover material remaining in solution. The products were analyzed for anthraquinone, anthracene, and unreacted dihydroanthracene.

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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HAWTHORNE ET AL.

205

9,10-Dihydroanthracene

W i t h the above procedure, variables such as reaction temperature, mole ratio of catalyst to dihydroanthracene, reaction solvent, and oxygen concentration were examined.


Results Temperature Effects. The oxidation of 9,10-dihydroanthraôene to anthraquinone in anhydrous pyridine solvent with benzyltrimethylammonium hydroxide as the base occurs over a wide temperature range ( Table I ) . Some oxidation takes place at a temperature as low as — 2 0 ° C , but maximum anthraquinone conversions (about 7 0 % ) occur between 50° and 70°C. Above 7 0 ° C , the conversion decreases, probably as a result of thermal decomposition of the benzyltrimethylammonium hydroxide. Table I.

Effect of Initial Reaction Temperature on Oxidation of Dihydroanthracene to Anthraquinone" Temp., °C.

Conversion to Anthraquinone, Wt. %

-20 0 30 50 70 90

8 10 13 71 70 40

Reaction mixture and conditions: anhydrous pyridine, 50 ml., benzyltrimethylammonium hydroxide, dihydroanthracene, 9.0 grams, 50 mmoles; reaction time, 2 hrs.

w

Table II. Effect of Base Concentration on Oxidation of Dihydroanthracene to Anthraquinone α

Mole Ratio of Base to Dihydroanthracene

Conversion to Anthraquinone, Wt. %

0.015 0.03 0.06 0.09

19 29 70 67

Reaction mixture and conditions: anhydrous pyridine, 50 ml.; benzyltrimethylammo nium hydroxide; dihydroanthracene, 9.0 grams; temperature, 70°C; reaction time, 2 hrs.

a

Effect of Base Concentration. The effect of base concentration was studied by varying the mole ratio of base to dihydroanthracene (Table II). The maximum anthraquinone conversion is obtained with a mole ratio of catalyst to dihydroanthracene of 0.06.



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1

Solvent Effects. The conversion of dihydroanthracene could be increased by adding water to the pyridine solvent (Table III). A n 86% conversion to anthraquinone was obtained when 95% aqueous pyridine was used as the solvent. Furthermore, methanol could be substituted for the water with equivalent results. Other solvents were tried in place of pyridine (Table IV). The data indicate that 95% aqueous pyridine gave the best yields, although aniline gave nearly similar results. W h e n acetonitrile and dimethylformamide were used, the large amounts of unreacted starting material indicate that these solvents may have de activated the base by undergoing a hydrolysis reaction. Table III.

Effect of Water Content in Pyridine on Oxidation of Dihydroanthracene to Anthraquinone" Conversion to Anthraquinone, Wt. %

Water in Pyridine, Wt. % 0 2 5 10

71 67 87 75

Reaction mixture and conditions: solvent, 50 ml.; benzyltrimethylammonium hydrox ide, 3 mmoles; dihydroanthracene, 9.0 grams; temperature, 70°C; reaction time, 2 hrs. a

Table IV. Effect of Various Solvents on Oxidation of Dihydroanthracene to Anthraquinone

Solvent

To Anthraquinone

To Anthracene

Unreacted DihydroAnthracene, Wt. %

95% Aqueous pyridine 2V,N-Dimethylaniline Aniline Morpholine N-Methylmorpholine Quinoline Ν,Ν-Dimethylaminopropylamine Diethylenetriamine Cyclohexylamine n-Hexylamine Acetonitrile 2- Picoline 3- Picoline 4-Picoline IV,N-Dimethylform amide

85 31 81 54 53 15

15 13 20 37 33 13

0 60 0 8 14 71

74 43 59 65 15 52 59 60 14

26 55 40 35 3 46 39 36 3

0 1 0 0 81 0 0 0 87

Conversion," Wt. %

a

"Totals deviate from 100% in some instances because of differences in methods of analysis.


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207



Effect of Oxygen Concentration. The effect of oxygen concentra tion on the conversion to anthraquinone and anthracene was also deter mined. As the oxygen partial pressure was increased, the ratio of anthraquinone to anthracene formed increased significantly (Table V ) . Thus, the data indicate that higher oxygen concentrations favor anthra quinone formation.

Table V.

Effect of Oxygen Concentration on Oxidation of Dihydroanthracene 0

Conversion, Wt. %

Λ

Oxygen Pressure, atm. 0.2 1.0 4.0

To Anthraquinone 60 72 90

6

To Anthracene 40 28 10

. AnthraqutnoneAnthracene Ratio 1.5 2.6 9.0

"Reaction mixture and conditions: anhydrous pyridine, 50 ml.; benzyltrimethylammo nium hydroxide, 3 mmoles; dihydroanthracene, 9.0 grams; temperature, 70°C.; reac tion time, 2 hrs. Carbon dioxide-free air at 1 atm. 6

Table VI. Oxidation of Various Compounds in Pyridine with Benzyltrimethylammonium Hydroxide Catalyst a

Compound 9,10-Dihydroanthracene Xanthene 5,10-Dihydrotetracene

Product (Conversion, % ) Anthraquinone (70), anthracene (30) Xanthone (79) 5,10-Tetracenequinone ( 39 ), tetracene (61) Acridine (47), acridone (0) 1- Methylanthraquinone (30) 2- Methylanthraquinone (35) Anthraquinone (40)

Acridan 1- Methyl-9,10-dihydroanthracene 2- Methyl-9,10-dihydroanthracene Anthrone Reaction mixture and conditions: anhydrous pyridine, 50 ml.; benzyltrimethylammo nium hydroxide, 3 mmoles; substrate, 50 mmoles; temperature, 70°C.; reaction time, 2 hrs. a

Oxidation of Related Compounds. Several other compounds related to dihydroanthracene i n structure were oxidized i n pyridine solvent (Table V I ) . No attempt was made to optimize the yields in any instance except with dihydroanthracene. It was surprising that anthrone reacted much more slowly than dihydroanthracene and that only a 40% yield of anthraquinone was obtained.


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1


Discussion Mechanism for Base-Catalyzed Autoxidation of 9,10-Dihydroanthracene. The autoxidation of 9,10-dihydroanthracene i n pyridine as the solvent and in the presence of benzyltrimethylammonium hydroxide, a strong base, is believed to involve the reaction of a carbanion and molecular oxygen. Indirect evidence of the existence of the carbanion of d i hydroanthracene in pyridine solution comes from the color that forms in the presence of the base. W h e n dihydroanthracene is added to a pyridine solution of the base, a deep blood-red color develops immediately. This color is not completely attributable to carbanions since a trace of anthraquinone alone w i l l produce it. However, under an inert atmosphere (nitrogen) in which no anthraquinone can be formed, a deep red color is also formed. Background and Possible Intermediates. Accepting the premise of carbanion formation in the basic media, the mode of reaction with molecular oxygen can now be considered. Sprinzak ( 8 ) reported that the autoxidation of fluorene in basic media proceeds by direct reaction of the fluorenyl carbanion with oxygen to form initially the hydroperoxide, which decomposes to yield 9-fluorenone, as depicted below.

H 0 2

OH©

If the 9,10-dihydro-9-anthranyl carbanion were to react directly with oxygen, 9,10-dihydro-9-anthranylhydroperoxide would be formed. This could decompose to give anthrone and/or anthracene. Anthrone, which would exist mainly as anthranol in a basic medium, generally is oxidized easily to anthraquinone. The following equations illustrate this reaction.



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O0D-

209

+ H 0 2

However, when anthrone was oxidized under the same conditions as the dihydroanthracene, the conversion to anthraquinone was estimated to be only 40%, and that value was probably high because of interference by unreacted anthrone during analysis. Anthrone, then, was not readily oxidized, contrary to expectation if it were the intermediate to the quinone. A sample of the monohydroperoxide, previously reported by Bickel and Kooyman (2), was obtained by autoxidation of 9,10-dihydroanthracene in benzene under ultraviolet irradiation. When this compound was treated under nitrogen with benzyltrimethylammonium hydroxide, it decomposed to give a mixture of anthracene and anthrone. (Under acidic conditions, it decomposed entirely to anthracene. ) A fresh sample of the hydroperoxide was then oxidized. The physical appearance of the reaction mixture was similar to that in the oxidation of anthrone. The product was analyzed, and the conversion to anthraquinone was only 59%. Again, other oxidation products or anthrone may have contributed to the anthraquinone estimate. Both Russell (5) and Barton ( I ) have examined the oxidation of dihydroanthracene in a solvent system consisting of 80% dimethyl sulfoxide and 20% tert-butyl alcohol and with potassium ferf-butoxide as the base. In both studies, a large excess of base was used, so that there is a possibility of dicarbanion formation. In the present investigation, only catalytic amounts of base were used, which makes it unlikely that a



210


1

dicarbanion could be formed. Barton and Russell both proposed reaction schemes in which a hydroperoxide is formed. The hydroperoxide is converted to anthrone, which is then oxidized to anthraquinone. However, Russell's rate data (5), which agree with our observations, indicate that anthrone oxidizes at a significantly slower rate than dihydroanthracene itself. Since both the hydroperoxide and anthrone oxidize under our reaction conditions at a slower rate than dihydroanthracene, it does not seem likely that they are intermediates in the oxidation. Attempts were made to identify oxidation intermediates by quenching the oxidation products of dihydroanthracene before the reaction had gone to completion, followed by an examination of the reaction products by gas chromatography. There was no evidence of anthrone or any other oxidation intermediates. Only unreacted dihydroanthracene, anthraquinone, and anthracene were found. The direct reaction of oxygen with the carbanion from dihydroanthracene does not seem likely. Russell (5) has indicated a preference for a one-electron transfer process to convert the carbanion to a free radical, which then reacts with oxygen to form an oxygenated species. Therefore, we considered a mechanism involving one-electron transfer to form a free radical from the carbanion, which would lead to the formation of anthraquinone and anthracene without having either the hydroperoxide or anthrone as an intermediate. Postulated Mechanism. The first phase of the oxidation of 9,10dihydroanthracene involving a free-radical process would be the following chain initiation.

Reaction 1 is again the abstraction of a proton by a base in a reversible reaction to give the monocarbanion. The carbanion reacts with oxygen (Reaction 2) by a one-electron transfer to give the free radical and the charged oxygen molecule, which can react again to become a peroxide


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211


ion. [ A similar step has been presented for the autoxidation of p-nitrotoluene under basic conditions (4).] The chain propagation is expressed as follows.

W i t h the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains i n forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical ( Reaction 4 ). [The possibility of such an intramolecular attack has been demonstrated i n an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions ( 9 ) ] .

The hydroperoxide radical reacts with another molecule of oxygen (Reaction 5) to give the hydroperoxide-peroxy radical. This radical i n turn reacts with a molecule of dihydroanthracene (Reaction 6 ) , to give the dihydroperoxide and generate a radical to propagate the chain. H o w ever, the hydroperoxide radical formed in Reaction 4 may be decomposed by a carbanion to the anthracene diradical (Reaction 7 ) . [ A n example of the decomposition of an unstable hydroperoxide by reaction with an anion is found in the basic autoxidation of 2-nitropropane (3).]


212


1


The diradical can decay to form anthracene with termination of a chain.

(8)

In Reaction 6, one of the products was the dihydroperoxide, which is decomposed by the base to give anthraquinone ( Reaction 9 ).

+ 2H 0 2

The reactions are summarized as follows, where R equals dihydro-9,10-anthrylene :

(9)

9,10-

1. H — R — H + O H - ^ H — R : " + H 0 2

2. H — R : " + 0 - » H—R- + 2

0 " 2

3. H — R - + 0 -> H — R O O 2

4. H — R O O - ->

ROOH

5.

R O O H + 0 ->

6.

O O R O O H + H — R — H -> H O O R O O H + H — R -

7.

ROOH + H—R:"

8.

-R- —> Anthracene

2

OOROOH

R- + H O O : " + H — R -

9. H O O R O O H -> Anthraquinone + 2 H 0 2

Supporting Evidence. Reactions 5 and 7 indicate that the hydroperoxide radical can react with either oxygen or a carbanion to give anthraquinone or anthracene, respectively. Thus, high oxygen and low carbanion concentrations would favor the formation of anthraquinone;



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213

the reverse order would favor a greater proportion of anthracene. Experimentally, the ratio of anthraquinone to anthracene in the product was directly related to the oxygen concentration (Table V ) . As Equation 1 indicates, adding water would decrease the concentration of carbanions by shifting the equilibrium to the left. Experimentally, the ratio of anthraquinone to anthracene was 2.6 to 1 i n anhydrous pyridine with oxygen at 1 atm., but was increased to 7.2 to 1 i n 95 volume % aqueous pyridine. Further addition of water to 10% decreased the over-all reaction rate. Water had no effect at higher oxygen concentrations ( oxygen pressure of 4 atm. ). The stoichiometry of the reaction was examined by measuring the amount of oxygen consumed. If one disregards the small amount of oxygen which reacted i n the initiation step (Reaction 2 ) , the reactions involved are 10 and 11.

Under basic conditions, the hydrogen peroxide would be decomposed as follows: H 0 2

2

- » 1/2 0 + H 0 2

2

Oxidation of the dihydroanthracene (50 mmoles) by oxygen at 4 atm. consumed 1.80 molecular equivalents (90 mmoles) of oxygen. This amount of oxygen corresponds to an 8 7 % conversion to anthraquinone and a 13% conversion to anthracene. Analysis of the product gave corresponding values of 90 and 10%. The difference between calculated and experimental conversions may well be within experimental error. A n attempt was made to determine the amount of hydrogen peroxide formed and correlate it with the amount of anthracene. A n experiment was made with oxygen at atmospheric pressure and a reaction temperature of — 20 ° C , so that any hydrogen peroxide formed would be less likely to decompose. The solid product (88% recovery of dihydroanthracene) was isolated and found to contain 1 mmole of anthracene. The


214


1

aqueous filtrate from the isolation of the product contained 1.2 mmoles of hydrogen peroxide as determined by active oxygen content. W i t h i n probable experimental error, the stoichiometry of the oxidation to anthra cene follows Reaction 11.


Conclusions This study indicates that the oxidation of dihydroanthracene in a basic medium involves the formation of a monocarbanion, which is then converted to a free radical by a one-electron transfer step. It is postulated that the free radical reacts with oxygen to form a peroxy free radical, which then attacks a hydrogen atom at the 10-position by an intramolecu lar reaction. The reaction then proceeds by a free-radical chain mecha nism. This mechanism has been used as a basis for optimizing the yield of anthraquinone and minimizing the formation of anthracene. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9)

Barton, D. H. R., Jones, D . W., J. Chem. Soc. 1965, 3563. Bickel, Α., Kooyman, E., Ibid., 1956, 2215. Russell, G . Α., J. Am. Chem. Soc. 76,1595 (1954). Russell, G. Α., "Oxidation to Produce Petrochemicals," Symposium Pre prints, Division of Petroleum Chemistry, 137th Meeting, American Chemical Society, Cleveland, Ohio, April 1960, Vol. 5, No. 2-C, p. C-25. Russell, G. Α., Janzen, E . G., Bemis, A. G., Geels, E. J., Moye, A. J., Mak, S., Strom, E. T., A D V A N . C H E M . SER., No. 51, 112-171 (1965). Russell, G. Α., Janzen, E. G., Becker, H - D . , Smentowski, F. J., J. Am. Chem. Soc. 84, 2652 (1962). Simon, A. W., Morgan, M . S., Wilt, M . H., U. S. Patent 3,163,657 (Dec. 29, 1964). Sprinzak, Y., J. Am. Chem. Soc. 80, 5449 (1958). Wibaut, J., Strang, Α., Koninkl. Ned. Akad. Wetenschap. Proc. B54, 102 (1951).

RECEIVED October 9,

1967.

Discussion G . A . Russell (Iowa State University, Ames, Iowa): I find it difficult to accept the intramolecular hydrogen transfer. W h y do you think this occurs? K . A . Schowalter: Our data indicate that both the hydroperoxide and anthrone oxidize at a slower rate than dihydroanthracene in our



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215

system. In the experiment where we quenched the reaction prematurely, neither the hydroperoxide nor anthrone was found among the reaction products. Based on this, we concluded that anthrone was not a reaction intermediate and the intramolecular hydrogen transfer appeared to provide best for a mechanism that would not proceed through the hydroperoxide or anthrone. Dr. Russell: Is it possible that the oxidation of 9,10-dihydroanthracene is more exothermic than that of anthrone and that the heat generated could have accounted for the high yield of anthraquinone i n the 2-hour reaction? Dr. Schowaiter: The reaction was carefully conducted at 70°C. i n both cases; therefore, the difference i n rate could not have been the result of temperature difference. A. J. Moye ( California State College, Los Angeles, Calif. ) : A n alternative mechanism to the intramolecular hydrogen transfer could be the transfer of a single electron to give a radical-anion intermediate i n place of the radical-peroxide of Reaction 4. Have you considered this possibility? Dr. Scho waiter: It appears that this is another mechanism that could account for the oxidation of dihydroanthracene without intermediate anthrone formation.


Base-Catalyzed Autoxidation of 9, 10-Dihydroanthracene and Related

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