I
WILLIAM
H. CLINGMAN, Jr., and FRANCIS T. WADSWORTH
American Oil Co., Texas City, Tex.
Oxidation of tert=ButyIcyclohexane to Dibasic Acids with Nitrogen Dioxide Several techniques have been found which improve selectivity of oxidation. Best results are obtained at superatmospheric pressure of oxygen in presence of acetic-sulfuric acid mixtures
DIBASIC
ACIDS and, in particular, alkyladipic acids are intermediates for the production of synthetic diester lubricants and plasticizers. The purpose of this study was to improve the yield of such acids in the oxidation of naphthenes with nitrogen dioxide. Hoot (5) oxidized cyclohexane with nitrogen dioxide and obtained a high yield of adipic acid based on total product formed. When nearly all of the nitrogen dioxide had reacted, however, only 1 to 2.5% of the cyclohexane was converted to product. Doummani, Coe, and Attan6 ( 3 ) found that when methylcyclohexane is oxidized with nitrogen dioxide plus nitric acid, succinic rather than methyladipic acid is formed. The mechanism of the nitrogen dioxidecyclohexane reaction has been investigated by Titov (7, a), while Brand (7) has recently described the mechanism of the nitrogen pentoxide-cyclohexane reaction. In the oxidation to dibasic acids, nitrogen-containing compounds are formed as intermediates. Some nitro compounds are by-products. For example, Doummani (3) has stated that a classical procedure involves oxidation of cyclohexane with nitric acid to a mixture of nitrocyclohexane and adipic acid. After isolation, the nitrocyclohexane is converted to its sodium salt and hydrolyzed, and the resulting cyclohexanone oxidized to a mixture of dibasic acids. In the present study, the oxidation of tert-butylcyclohexane has been investigated, with emphasis on finding a reaction system that would give good selectivity-Le., a high ratio of dibasic acids to other oxidation products. The effects of variables such as pressure, temperature, and nature of the reaction medium have been investigated, and several techniques have been found which greatly improve the selectivity of the oxidation. The best results are obtained by carrying out the reaction
under superatmospheric pressures of oxygen in the presence of acetic acidsulfuric acid mixtures.
Experimental The acetic and sulfuric acids were Mallinckrodt analytical reagent grade. The nitrogen dioxide was supplied by the Matheson Co.; the Linde Air Products Co. supplied the argon and U ,9.P. oxygen. tert-Butylcy clohexane was prepared by hydrogenation of Phillips pure grade tert-butylbenzene over a nickel-on-kieselguhr catalyst. The infrared spectrum of the tert-butylcyclohexane indicated that it was spectroscopically pure. Method. I n a typical run, No. 71, 15.4 grams of 95% sulfuric acid, 31.0 grams of nitrogen dioxide, 67.3 grams of glacial acetic acid, and 69.3 grams of tert-butylcyclohexane were charged directly to a 1410-ml. stainless steel (No. 316) high pressure bomb. Argon (to facilitate analysis) and oxygen were added to the reactor. The charging pressure of argon at room temperature was 30 p.s.i.; that of oxygen was 300 p.s.i. The reactants were then heated with agitation to 79' G. for 1 hour, during which time the total pressure increased to 380 p.s.i. As the reaction proceeded the pressure decreased, but was kept between 325 and 380 p.s.i. by addition of oxygen once during the run. At the end of 3.75 hours at 79' C. the reaction was quenched by pressuring into the reactor approximately 100 ml. of water. In the presence of oxygen, the water converted unreacted nitrogen dioxide to dilute nitric acid. A gas sample was taken from the bomb and cooled in a dry ice-kerosine slush (approximately -50' C.). Any organic material or water present in the original sample was frozen out, and the remainder of the gas was collected for
mass spectrographic analysis. From the mole ratio of argon to carbon monoxide, carbon dioxide, nitrogen, and nitrogen dioxide in this sample, the partial pressure of the latter four components in the bomb could be calculated, as the argon partial pressure was known. The liquid portion of the product was cooled; it consisted of an aqueous phase and an organic phase. Dibasic acid, sulfuric acid, and acetic acid were washed out of the organic phase with water, and washings were combined with the aqueous phase from the bomb. Aliquant portions of this solution were taken for determination of nitric, sulfuric, and dibasic acids. Unreacted nitrogen dioxide was determined by titrating the total acid present in this aqueous solution and correcting for the dibasic, acetic, and sulfuric acids. Sulfuric acid was determined by titrating with barium ion. The analysis for run 71 showed 147.9 mmoles of sulfuric acid compared with 149.0 mmoles added to the charge. This agreement is well within the experimental error and indicates that no sulfonation occurs during the reaction. The number of moles of dibasic acid was determined by first evaporating off the acetic acid and water at room temperature and then redissolving the residue of sulfuric acid and dibasic acid in water. Sulfate ion was then removed as barium sulfate from a neutral solution. The silver salt of the acid was precipitated from a neutral solution, washed with isopropyl alcohol, and dissolved in I N nitric acid. Silver was determined in the resulting silver nitrate solution by precipitating and weighing silver chloride or by volumetrically precipitating silver chloride. This procedure is similar to that used by Hoot
(5).
Separate experiments with pure dibasic acids showed that tert-butyladipic and adipic acids are quantitatively determined by this procedure. ComVOL. 50, NO. 5
0
M A Y 1958
777
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parison of analyses by the silver ion method and by chromatography indicated that the carboxylic acid determined by the silver ion method was approximately equal to the total teributyladipic, adipic, glutaric, and succinic acids present in the product. This fact is supported by the data in Table IV, where a complete carboxylic
acid analysis by chromatography is reported for run 71. The carboxylic acids in the aqueous product from run 71 were chromatographed on a silica gel column, using nbutyl alcohol-chloroform solutions as eluent. The procedure was similar to that reported by Bulen, Varner, and Burrell ( 2 ) . T o determine both the
Table I.
Effect of Nitrogen Dioxide Concentration and Oxygen Pressure Illustrates Competition for Initially Formed Intermediates
Run
NOz/Mole
Moles No.
TBC5
28 11 26 14 35 38
0.13 0.12 0.11
14
23 41 42
45
Time, Hr.
0.12
17.5 17 17.5 22.25
0.11
17.75
0.12 0.13 1.38 0.48 2.02 3.94
22.25 3.5 13 3.5 4.25
19
Initial OZ Press., P.S.I. at 25' C. 30 60 90 120 200 500 120 120 470 470 470
tert-Butylcyclohexane.
778
monobasic and dibasic acids, however, a modification was necessary. First, the chromatogram for all the carboxylic acids in the product was obtained. Then the entire effluent, which had been neutralized with sodium hydroxide during titration, was evaporated to dryness. T o remove monobasic acids, the residue of sodium salts was acidified with con-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Temp., OC.
78 78 79 79
76 75 79 77 77-94 77-81 76-82
Mole Dibasic
Mole %
BcidlMole NO2 Reacted
Reacted
0.10 0.12 0.13 0.19 0.26 0.22 0.19 0.11 0.26 0.15 0.07
TBC
...
... . . a
... 5.1 60 13 47 79
Additives (Moles Additive/Mole TBC in Parentheses)
... ... ... ... ... ... . I .
HOAC (2.35); HzO (0.170) cc14 (6.91) cc14 (15.5) H O A c (8.77); CC14 (31.2)
HOAc (2.31); HOAC (4.50);
BUTYLCYCLOHEXANE OXIDATION centrated hydrochloric acid and evaporated to dryness at room temperature in vacuo. The remaining dibasic acids were then rechromatographed by the same procedure as for the total acids. By comparing the two chromatograms, both the monobasic and dibasic acids could be characterized and determined. In experiments with pure dicarboxylic acids, it was shown that recovery of tertbutyladipic, adipic, glutaric, and succinic acids is quantitative after such hydrochloric acid treatment. The individual acids separated by the above procedure were identified by their characteristic peak effluent volumes. This chromatographic procedure will be described in more detail later. For the runs given in Table 111, the unreacted tert-butylcyclohexane in the organic phase of the product was determined by extracting the nitrated compounds into concentrated sulfuric acid and weighing the residue of pure naphthene. For the other runs, the number of moles of reacted naphthene was calculated from the moles of dibasic acid and nitro compounds formed. The latter were estimated by the method of Hoot ( 5 ) , the naphthene being evaporated at room temperature and the “nitro” bottoms weighed. These bottoms were assumed to be nitro-tertbu tylcyclohexane.
,
Results Nitrogen dioxide can undergo three general types of reaction in the oxidation of naphthenes. First, it can be reduced to nitric oxide. The nitric oxide, in turn, is oxidized to nitrogen dioxide by the oxygen in the system; thus, this first type of reaction results in no net loss of nitrogen dioxide. Second, the nitrogen dioxide can react with the naphthene and its oxidation products to form nitro compounds, resulting in a loss of nitrogen dioxide. Third, it can be reduced to nitrous oxide or nitrogen, which are not oxidized by the oxygen present. Either of the last two reactions is undesirable
Run No. 48 61 66 65 69 74
71 73 72 a
*
Moles NOz/Mole TBC” 0.38 0.38 1.4 1.4 1.4 1.6 1.4 1.5 1.5
Time, Hr. 3.5 3.5 3.5 3.5 3.5 3.5 3.75 3.5 3.5
Table
Run No. 11 13 26 27 19 20
II.
Acetic Acid Improves the Efficiency of Nitrogen Dioxide for Oxidizing tert-Butylcyclohexane Mole yo Initial Mole Mole Dibasic Moles OS Added Dibasic Acid CO f Mole Press., Acetic Acid/Mole Based on COa/Mole NOz/Mole Time, Temp., P.S.I. Acid/Mole NOZ TBC in Dibasio TBC Hr. O C. at 2b0 C. TBC Reacted Charge Acid 0.12 17 78 60 0.00 1.3 1.8 0.12 0.12 16.25 78 60 0.20 0.24 2.5 1.0 0.11 0.13 0.12 0.13
17.5 17.5 17.5 17.5
79 79 87 87
90 90 60 60
and will decrease the oxidation efficiency of the nitrogen dioxide-Le., more nitrogen dioxide must react to obtain the same amount of dibasic acid. The effect of nitrogen dioxide concentration and oxygen pressure on the reaction was first investigated. The results have been expressed in terms of the number of moles of dibasic acid formed per mole of reacted nitrogen dioxide. A comparison of the first six runs in Table I shows that as the oxygen pressure increases to 200 p d . , the oxidation efficiency of the nitrogen dioxide also increases. Above 200 p.s.i. no further increase in dibasic acid yield is obtained. I t is believed that the oxygen and nitrogen dioxide compete for the initially formed free alkyl radicals, thus explaining the increase in yield with increasing oxygen pressure. Further evidence for this competition is provided by the remainder of the runs in Table I, in which, the nitrogen dioxide concentratiop was varied under different conditions. As Hoot (5) reported an explosion with a nitrogen dioxide-cyclohexane mixture containing 70 mole % nitrogen dioxide, a solvent such as acetic acid or carbon tetrachloride was used to decrease the explosion hazard when the nitrogen dioxidenaphthene ‘ratio was greater than 0.13. I n general, at constant oxygen pressure, increasing the nitrogen dioxide concentration decreases the dibasic acid yield.
0.00 0.24 0.00 0.23
0.13 0.25 0.11 0.15
1.3 2.3 1.2 1.8
5.6 1.2 2.6 1.4
The implications of these results concerning the reaction mechanism are discussed below. The effect of water on the reaction was also studied. When the water and nitrogen dioxide concentrations became of the same order, the reaction essentially ceased. T o verify the hypothesis that cessation of the oxidation reaction was due to dilution of the nitrogen dioxide with water, a run was made in which the initial charge contained the same concentration of nitrogen dioxide and water as in the product of the previous run, in which the reaction had stopped. No dibasic acid was produced and only a small amount of nitration was observed. The nitrogen dioxide will react with product water to form nitric acid, which probably is not as strong an oxidizing agent for the naphthene as nitrogen dioxide itself (6). In addition to carrying out the reaction under superatmospheric pressures of oxygen, it was found desirable to include acetic acid in the charge. For temperatures greater than 70’ C. and oxygen pressures greater than 30 p.s.i., the presence of acetic acid in the reaction mixtures increases the amount of dibasic acid formed per mole of nitrogen dioxide reacted and decreases the amount of carbon monoxide and carbon dioxide produced. This is illustrated by the data in Table 11, where runs with and without acetic acid in the charge are
Table 111. Effect of Sulfuric Acid on Oxidation Reaction (Mixture of sulfuric acid and acetic acid is optimum) Mole % Mole Initial OZ Yield DBAb Mole % DBA/Mole Press., P.S.I. Temp., Based oh TBC NO2 Additives (Moles Additive/Mole at 2b0 C. C. TBC Reacted Reacted Reacted TBC in Parentheses) 120 120 120 120 120 300 300 120 120
75-80 77-81 79 77-83 75-81 74-79 79 71-80 78-80
28 28 15 48 49 43 67 42 52
22 12 37 31 31 34 29 31 30
0.24 0.11 0.11 0.24 0.20 0.20 0.36 0.17 0.21
... &SO4 (o.ossj . HOAc (2.33) HOAC (2.31); &So4 HOAC (2.31); HzSO4 HOAc (2.35) HOAC (2.27); HzSOa HOAc (4.60) HOAC (4.55); %SO4
(0.298) (0.676) (0.300) (0.618)
tert-Butylcyclohexane. Dibasic acid. .
VOL. 50, NO. 5
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compared. The data in Table I1 at 60 p.s.i. oxygen pressure also show that as the temperature increases the selectivity of the oxidation decreases. Hoot (5) reported that lower temperatures favored higher yields of dibasic acids. Although increasing the oxygen pressure and introducing acetic acid into the charge increased the oxidizing efficiency of the nitrogen dioxide, nitro compounds were still major by-products of the reaction. That these nitro compounds were not intermediates in the production of dibasic acids was shown by carrying out two runs in which nitrated by-products from previous reactions were added to the charge. KO significant yield increase of dibasic acids was observed. To decompose these unreactive nitro compounds during the reaction, experiments were made with sulfuric acid added to the charge. Mixtures of sulfuric and acetic acids were very effective in increasing the nitrogen dioxide oxidation efficiency and the yield of dibasic acid based on naphthene reacted (Table 111). From runs 48 and 61 it can be seen that sulfuric acid alone is not beneficial to the reaction and in fact decreases the dibasic acid yield based on nitrogen dioxide reacted. I n other words, both sulfuric and acetic acids must be added to the charge to obtain the best results. The yield of dibasic acids was highest in run 71 a t a n oxygen pressure of 255 to 300 p.s.i. and in the presence of a sulfuric-acetic acid mixture. A complete analysis of the carboxylic acids produced in this run is given in Table IV. The total mole per cent yield of carboxylic acid based on naphthene reacted was 87y0; the yield of dibasic acids was 7870. Although several different acids were produced, the major products were tert-butyladipic, adipic, glutaric, and succinic acids; more than twice as much tert-butyladipic acid was produced than any other single acid.
Table IV. Analysis of Oxidation Products Shows tert-Butyladipic Acid Is Predominant Component Mole % Yield"
Based on Naphthene Acid tert-Butyladipic Adipic Glutaric (plus unknown) Succinic Total Dibasic acid by silver ion method Trimethylace tic Butyric Acetic Formic Oxalic (plus unknown) Unknown dibasic acid Run 71.
780
Reacted 30.7 14.0 13.9 10.0 69
67 2.4 3.1 2.0 1.8
6.7 2.7
The distribution of nitrogen compounds in the product of run 71 was also measured. Of the nitrogen dioxide charged, 59.8% was recovered as such, 9.2% as nitrogen, and 20.0% appeared as nitrous oxide. The remaining 11.O7, formed nitrated by-products.
Reaction Mechanism The mechanism of the reaction is undoubtedly complex, as a variety of compounds are formed. The effect of oxygen pressure and nitrogen dioxide concentration on the dibasic acid yield, however, can be explained by the following key reaction steps:
RH + NOz R. + HNOz + NOz intermediate
(1)
+
RH
-+
+
oxygen pressure and nitrogen dioxide concentration on the reaction. The exact mechanism by which acetic acid and sulfuric-acetic acid mixtures increase the yield in the nitrogen dioxide oxidation of tert-butylcyclohexane is not known. In studying the oxidation of cyclohexanol with nitric acid, however, Godtand Quinn (4)isolated a nitrolic acid intermediate, which can be readilyhydrolyzed by dilute hydrochloric acid to adipic acid. It is possible that in the nitrogen dioxide oxidation of tert-butylcyclohexane, the presence of acids in the naphthene phase catalyzes the decomposition or hydrolysis of nitrogencontaining intermediates. Sulfuric acid alone; which is not miscible with the naphthene phase, does not increase the dibasic acid yield.
nitro compounds (2) R.
+ NOz
-+
nitro compounds
(3j
R. f NO2 + intermediate + dibasic acid (4) R.
-+ 0
2
+
intermediate + dibasic acid ( 5 )
Reaction 2 is a direct reaction between the naphthene and nitrogen dioxide to form unreactive. nitrated compounds, Although this reaction may proceed in several stages, the slow step is assumed to be first order in both naphthene and nitrogen dioxide. Reaction 2 differs from Reaction 1 followed by 3 in that the intermediate in Reaction 2 cannot react with oxygen to forin dibasic acids. I t is necessary to postulate 2 to account for the limiting yield obtained with increasing oxygen pressure. I n Reactions 4 and 5 it is assumed that the intermediates formed are converted to dibasic acid very rapidly compared to their rate of formation-that at any time during the reaction no large concentration of these intermediates is present. The runs made with added nitro compounds provide evidence for this assumption. If the incremental yield of dibasic acid is derived from these key reactions alone, the following equation is obtained, assuming a steady-state concentration of
Conclusion The nitrogen dioxide oxidation of tert-butylcyclohexane produces a mixture of dibasic acids and nitro compounds. Water is also formed and converts unreacted nr'trogen dioxide to nitric acid; when sufficient water has formed, the reaction ceases. The effect of nitrogen dioxide concentration and oxygen pressure on the yield implies that nitrogen dioxide and oxygen compete for initially formed free alkyl radicals, thus making it desirable to conduct the reaction under superatmospheric pressures of oxygen. The yield of dibasic acid is substantially increased by adding acetic acid or a mixture of sulfuric and acetic acids to the charge. The yields were highest with the latter mixtures. A 78% yield of mixed dibasic acids has been obtained in this manner, the major products being tert-butyladipic, adipic, glutaric, and succinic acids.
Acknowledgment The authors wish to express their appreciation to H. H. Barber and N. H. Houghton for carrying out the chromatographic analyses.
R.:
literature Cited
d[dibasic acid]/d[nitro compounds] =
(1) Brand, J. C. D., J . A m . Chem. Soc. 77. 2703 11955). (2) Bule;, W. A.,Varner, J. E., Burrell, R. C., Anal. Chem. 24, 187 (1952). (3) Doummani, T. F., Coe, C. S., Attant, E. C., Jr. (to Union Oil Co. of California), U. S. Patent 2,459,690 (Jan. 18, 1949). (4) Godt, H. C., Jr., Quinn, J. F., J . A m . Chem. SOC.78. 1461 11956). Hoot, W. F., Kobe, K.A,', IND.ENG. CHEM.47, 782 (1955). Schmid, H., Maschka, A, Monatsh.
(A
+ B( [02l/[NOzl)i / { C + D( l
0 2 1 / ~ ~ 0 2 1 ) l
If k , is the rate constant of the ith reaction, constants A , B, C, and D may be expressed in terms of the rate constants as follows : A = ka; B = k s ; C = (kz/ki) (kg k ; ) 4-k3; D
+
=
kzkj/ki
As the ratio of oxygen to nitrogen dioxide is increased. a limiting ratio of dibasic acids to nitro compounds equal to B I D or k i / k z will be obtained. Thus, the above reaction scheme will account qualitatively for the observed effect of
INDUSTRIAL AND ENGINEERING CHEMISTRY
_
^
_
I
80, 253 (1949).
Titov, A. I., Matveeva, -M. K., Sbornzk Statei Obschei Khzm., Akad. L\-auk S.S.S.R. 1. 241 11953). ( 8 ) Zbid., p. 246 RECEIVED for review May 20, 1957 ACCEPTED September 27, 1957
