Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978 247
Regioselective Substitution in Hindered Side Chain Aromatics. p-Nitrobenzoic Acid from Nitration and Oxidation of Styrene Residues Anatoll Onopchenko," Johann G. D. Schulz," and John D. Bacha Gulf Research & Development Company, Catalyst & Chemicals Research, Pittsburgh, Pennsylvania 75230
Nitration of trans-stilbene, bibenzyl, styrene, polystyrene, or residues from styrene manufacture, followed by nitric acid oxidation of the resulting nitrated intermediates, affords good yields of nitrobenzoic acids with high selectivity to the para isomer. This selectivity is in contrast to predominant ortho isomer formation with simple alkylbenzenes. Bulkiness of side chains attached to the aromatic entity as present in styrene oligomers and similar structures leads to substitution at the less hindered para position. While far more general in scope, one application of this technique is the production of p-aminobenzoic acid, easily obtained by hydrogenation of p-nitrobenzoic acid. p-Aminobenzoic acid is widely used in the preparation of pharmaceuticals and polymers.
Introduction This paper relates to the selective preparation of pnitrobenzoic acid from styrene oligomers with nitric acid, taking advantage of steric hindrance in polymers to direct the entering NOz+ electrophile to the para position. Several model substrates including styrene itself, bibenzyl, and trans-stilbene were also converted to p-nitrobenzoic acid in high yield. Hydrogenation of p-nitrobenzoic acid produced the highly useful p-aminobenzoic acid. Experimental Section Nitration and Oxidation Apparatus. Nitration reactions were carried out in standard laboratory glassware, while oxidations and hydrogenation were done in a 1-L 316 stainless steel magnetically stirred autoclave (Autoclave Engineers, Inc., Erie, Pa.). The autoclave was equipped with a cooling coil and heaters and was connected to a gas supply (nitrogen or hydrogen), temperature and pressure controllers, and recording instruments. NMR spectra were obtained on a Varian T-60 spectrometer (Me2SO-d6, Me,Si). Chemical shifts are in 6 units, in parts per million. IR spectra were recorded on a Perkin-Elmer Model 237B spectrometer. Nitrobenzoic acids were analyzed by gas liquid chromatography (GLC) as n-propyl esters on a 25-ft X 0.25 in. 25% SE-31 on Porapak Q column at 155 "C. The order of nitrobenzoic acid emergence followed the sequence: ortho, para, and meta. Procedure. In a typical experiment, 30 g of styrene was incrementally added, while stirring, to 180 g of 90% nitric acid, maintaining a temperature around 10 f 2 "C. After 1 h, the homogeneous solution was poured over 500 g of cracked ice-water. The white precipitate that formed was filtered, air-dried, transferred to the autoclave, and oxidized with 300 g of 30% nitric acid (180 "C, 1 h). After cooling and depressuring, the reaction mixture was filtered to recover 29.8 g of product (first crop). The filtrate was evaporated to dryness to give 11.6 g of a second crop. Analysis of the combined product by GLC showed the isomer distribution of nitrobenzoic acids to be 90% para, 7.5% meta, and 2.5% ortho (86% yield). Recrystallization from water afforded p-nitrobenzoic acid, mp 239-241 "C (Ullmanns, 1953; mp 242.2 "C for para, 141.1 "C for meta, and 147-148 "C for ortho). The above procedure was used with all substrates with good reproducibility, except that 0019-7890/78/1217-0247$01,00/0
in some experiments nitric acid was added with a pump. A total of 25 g of p-nitrobenzoic acid prepared above was hydrogenated in 500 mL of tetrahydrofuran over Raney Ni (10 g) at 50 "C, 1000 psig H2 for 1h. Filtration, followed by evaporation of the filtrate to dryness, and recrystallization from water gave 14.4 g (70%) of p-aminobenzoic acid, mp 185-188 "C, neut equiv 136, whose IR and NMR spectra were identical with those of an authentic sample (Ullmanns, 1953; mp 187-188 "C). Results and Discussion p-Nitrobenzoic acid is commercially prepared through oxidation of p-nitrotoluene. As nitration of toluene produces less than 40% of the desired para isomer, its availability consequently depends on the utilization of the predominant ortho product. While alkylbenzenes with longer or bulkier side chains will yield greater proportions of the p-nitro isomer, difficulties in the subsequent oxidation as well as economics will offset any advantage. Availability of low cost aromatic residues from commercial styrene manufacture suggested their utilization in various areas, of which synthesis of p-aminobenzoic acid had particular interest. Detailed composition of styrene residues is not known, although they contain varying amounts of styrene oligomers, alkylated styrenes, stilbene, bibenzyl, and styrene monomer, in addition to other oneand two-ring compounds. Nitration of these species, followed by oxidation, could be expected to produce nitrobenzoic acids. As isolation of individual constituents appeared to be impractical, initial experiments were carried out with composite residues. The transformation of basic single ring units present in all constituents of the mixture is shown in eq 1.
The nitration step can be carried out with nitric acid alone or in combination with sulfuric acid. As nitric acid is also used for the ensuing oxidation, it was convenient to combine both steps. Following nitration with fuming nitric acid, water was added to dilute the acid down to the level needed for oxidation. Experiments with styrene residues are summarized in Tables 1-111. If the residue basically is composed of styrene units (91.69% C and 0 1978 American Chemical Society
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Ind. Eng. Chern. Prod. Res. Dev., Vol. 17, No. 3, 1978
8.31% H), a total of 4.3 mol of nitric acid is needed to produce nitrobenzoic acids (eq 2). In practice, however,
do2 +
3 ~ c & t 132 H U C j~ -3
3 CO2
+ 10 NO +
11 r l 2 0
(2)
coot-
6.8 mol was consumed, of which 4.5 mol was used during the oxidation, a larger amount than expected. The product consisted of two fractions. The first crop, recovered by filtration, contained predominantly p-nitrobenzoic acid and little meta isomer. The second crop, obtained by evaporation of the filtrate to dryness, contained mostly the more soluble ortho and meta isomers. The combined product, produced in amount of 1.2 g/g of residue, is a bright yellow to tan,free-flowing solid, with a melting point ranging from 208 to 230 "C. The NMR spectrum showed a broad absorption in the range of 7.2 to 9.0 ppm for aromatic protons, while GLC indicated the presence of nitrobenzoic acids only. Use of an internal standard, however, showed that only 70% of the product was accounted for. The remainder which did not elute is still unidentified. Recovery of nitrobenzoic acid from the crude product by crystallization from acetic acid or water gave 0.7 to 0.8 g of p-nitrobenzoic acid/g of residue. Only small amounts of the corresponding ortho and meta isomers were found in the filtrate. Analysis of the acids was carried out by GLC, after converting to their n-propyl esters with 1-propanol/BF, (Appleby and Mayne, 19671, on a silicone oil column (Smith and Vernon, 1969). The isomer distribution of total product was 88% p-, 890 m-, and less than 4 % o-nitrobenzoic acids. To eliminate underoxidation as the cause of possible incomplete conversion to nitrobenzoic acids, a batch of nitrated styrene residue was oxidized under more severe conditions (200 "C, 6.5 h, large excess of nitric acid). No significant increase in product was observed, which indicates that unidentified structures, not convertible to benzoic acids, make up part of these residues, a fact also suggested by GLC analysis (biphenyl, naphthalene, diphenylmethanes, etc.). To optimize parameters for the nitration and oxidation of styrene residues, conditions for styrene itself had to be developed. Nitration of styrene with concentrated nitric acid was reported to give a tarry substance from which on steam distillation crystalline w-nitrostyrene was obtained in small amount (Simon, 1839). Improved yields of this product were apparently obtained with fuming nitric acid (Blyth and Hofman, 1845). Product isolated by us from styrene nitration with fuming nitric acid did not resemble w-nitrostyrene, but rather the structure of a nitrated oligomer, Elemental analysis gave the empirical formula C8H8NO2.1.5H2Ofor the nitration intermediate, corresponding to one nitro group per unit structure. The NMR spectrum indicated the presence of water, corresponding to about 15% of the sample (exchangeable 5.1 ppm peak). It showed only a small peak at 5.9 ppm for the ArCH(ONOz) structure (Tadashi et al., 1971), indicating that most groups introduced were NO2 rather than ON02. Gel permeation chromatography gave a molecular weight distribution from about 300 (dimer) to 400 000 (polymer, -2700 units). Oligomer distribution was as follows: 3.4% of dimer (n = 2), 38.3% of oligomers with n = 3-5,46.8% of oligomer with n = 6, and 11.5% of polymers with n >> 6. The bulk of the product (85%) consisted therefore largely of oligomers with n = 3-6. The reaction of styrene with fuming nitric acid at temperatures above 20 "C is extremely violent. No dif-
r l r l r l
i 0
rl
/I
3
-O.)
" E
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
249
Table 11. Effect of Time and Temperature on Nitration of Styrene Residue conditions charge expt no.
g of styrene residue (mol)"
8
30 (0.288)
5 9 10 11
20 30 30 30
time after temp, residue addtn "C to acid, h
g o f 90% HNO, (mol)
HNO, consumed during nitration g (mol)
202 (3.21) 22 1.3 react for 2 h a t 30 " C react for 56 h at 22 C 135 (2.14) 10 1.0 202 (3.21) 26 1.0 202 (3.21) 40 1.o 202 (3.21) 30 2.0
(0.192) (0.288) (0.288) (0.288)
42.9 43.4 42.9 29.6 36.5 40.9 40.3
g/g of residue
(0.68) (0.69) (0.68) (0.47) (0.58) (0.65) (0.64)
1.43 1.45 1.43 1.48 1.22 1.37 1.34
(Average) 1.4 (2.3 mol/mol)
" Assume MW = 104. Table 111. Nitration and Oxidation of Styrene Residue, "0, charge expt no.
styrene residue, g (mol)
1 8 5 9 10
30 (0.288) 30 (0.288) 30 (0.288) 30 (0.288) 30 (0.288)
nitration "03,
g (mol) 148 202 202 202 265
(2.35) (3.21) (3.21) (3.21) (4.21)
average 30 (0.288)
Material Balance oxidation
3'"
total HNO, consumed/g of residue
HNO consumed, g (mol)
concn used in oxidn, wt %
HNO, consumed, g (mol)
product, g
g/g
mol/mol
1st crop
2nd crop
34.4 (0.55) 42.9 (0.68) 44.2 (0.70) 40.3(0.64) 41.6 (0.66)
31.9 40.0 39.0 40.0 47.7
79.4 (1.26) 90.1 (1.43) 78.1 (1.24) 81.3(1.29) 81.9 (1.30)
3.8 4.4 4.1 4.1 4.1
6.3 7.3 6.7 6.7 6.8
27.2 26.9 25.8 29.6 25.9
12.9 12.4 10.5 12.3 16.7
40.7 (0.65)
_-
82.1 (1.30)
4.1
6.8
27.1
12.9
Comoosition of Products (Comuosite) IGLC. Area 5%) product
wt, g
p-NBA (%)
m -NBA
0-NBA
others
first crop second crop
27.1 12.9
97.0 10.1
3.0 33.5
5.5
50.9
Table IV. Nitrobenzoic Acids from Model Substrates
(styrene )n n n n n
= l = l = l =6
n = 1058 bibenz yl trans-stilbene residue residue " Number average.
MW 104 104 104 600
110000"
--_ ___-_
source Gulf Gulf Gulf Pressure Chemical Co. Arc0 Polymers
nitration oxidn temp, temp, "C (av) "C (av)
nitrobenzoic acid distribution (GLC) p-
m-
0-
% yield
-5 - 10 20 20
180 170 170 180
90.0 90.0 85.8 93.8
7.5 9.5 8.4 6.2
2.5 0.5 5.8 tr
86 83 80 71
25 10 10 10 35
180 175 180 180 180
94.0 86.0 87.0 88.0 87.6
6.0 13.0 10.7 8.5 8.2
tr 1.0 2.3 3.5 4.2
62 77 80 0.77 g/g of residue 0.83 g/g of residue
ficulties, however, were encountered when styrene was incrementally added to nitric acid in the temperature range of -10 to 10 "C. The reaction proceeds in homogeneous phase to afford, on dilution with ice-water, a white solid which was oxidized to nitrobenzoic acids at 180 "C. The yield of nitrobenzoic acids varies from about 60 to 86%, depending on conditions. For maximum efficiency, it is important to nitrate styrene as completely as possible and to allow sufficient time in the oxidation step. Incomplete nitration tends to produce a higher amount of the meta isomer as well as benzoic acid. Incomplete oxidation gave a lower yield and a tacky product, although its isomer distribution did not change. Isomer distribution is obviously determined in the nitration step, while overall acid yields are dependent on oxidation conditions. It is reasonable to assume that the initial reaction of styrene with fuming nitric acid is oligomerization, followed by nitration, as similar selectivities to nitrobenzoic acids are obtained from styrene, oligomeric styrene, and polystyrene (Table IV). The more recent literature indicates that styrene
reacts with 7 0 4 0 % nitric acid to give partially nitrated oligostyrenes of t h e type 0 2 N P h C H ( O N 0 2 ) (CH2CHPh),CH3, where n = 1 or 2 (Tadashi et al., 1971). With a mixture of fuming nitric acid and sulfuric acid, nitrated oligostyrenes are produced, but no mention is made of nitration with fuming nitric acid alone. It is also reported that both nitration (Bachman et al., 1947) and oxidation (Fortina and Passerini, 1959) of polystyrene separately with nitric acid have been carried out, but to our knowledge oxidation of nitrated polystyrene has not been studied. After this paper was submitted for publication, a report on the preparation of p-nitrobenzoic acid from polystyrene appeared in this journal (Rondestvedt et al., 1977). Reactions of model compounds are summarized in Table IV. Each experiment produced a high ratio of the desired p-nitrobenzoic acid. This high ratio of para to ortho isomer is rationalized on the basis of steric factors. To be certain, however, that some o-nitrobenzoic acid formed did not degrade under oxidation conditions (Gould, 1959),a sample
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
of pure ortho isomer was treated with nitric acid at 180 "C, without appreciable loss of product, evidencing stability of this isomer under the reaction conditions. In another experiment, a sample containing 20% of 0- and 80% of p-nitrobenzoic acids was reacted for 1 h first, and then for an additional hour, without any loss in selectivity a t 175 "C. Reduction of p-nitrobenzoic acid to the more useful p-aminobenzoic acid is a well known, high-yield reaction (Mallonee, 1967; Woodbury, 1960), and has therefore not been studied by us. Literature Cited
Blyth, J., Hofman, A., Justus Liebigs Ann. Chem., 53, 289 (1845). Fortina, L., Passerini, R., Boll. Sci. Fac. Chim. Ind. Bologna, 17, 1 (1959); Chem. Abstr., 53, 17928a (1959). Gould, E. S.,"Mechanism and Structure in Organic Chemistry", p 346, Henry Holt and Co., New York, N.Y., 1959. Mallonee, J. E., U. S. Patent 3324 175 (1967). Marechal, E., Ruault, J. P., C . R . Acad. Sci. Paris, Ser. C , 267, 1115 (1968); Chem. Abstr., 70, 2 0 5 8 6 ~(1969). Rondestvedt, C . S., Jeffrey, J. R., Miller, J. E., Ind. Eng. Chem. Prod. Res. Dev., 16, 309 (1977). Simon, E., Justus Liebigs Ann. Chem., 31, 269 (1839). Smith, A., Vernon, F., J . Chromatogr., 43, 503 (1969). Tadashi, Y., Katsutoshi, N., Tsuyoshi, M., Takenori, A,, Kogyo Kagaku Zasshi, 74 (3), 528 (1971); Chem. Abstr., 75, 21144p(1971). Woodbury, L. S., U. S. Patent 2947781 (1960). "Ullmanns Encyklopadie der technischen Chemie", 3rd ed, Vol. IV, p 267; Vol. 111, p 467, 1953.
Appleby, A. J., Mayne, J. E. O., J . Gas Chromatogr., 6 , 266 (1967). Bachman, G. B., Hellman, H., Robinson, K. R., Lewis, R. W., Micucci, D. D., J . Org. Chem.. 12, 108 (1947).
Received for review October 28, 1977 Accepted April 17, 1978
Determination of the Antioxidant Capacity of New and Used Lubricants; Method and Applications Lee R. Mahoney,' Stefan Korcek, Sylvia Hoffman, and Pierre A. Willermet Ford Motor Company, Engineering and Research Staff, Fuels and Lubricants Department, Dearborn, Michigan 48 12 1
A simple laboratory method has been developed for the determination of the total effective concentration of chain stopping antioxidant species present in new and used lubricants. The principle of the method is based upon the titration of antioxidant species by peroxy radicals formed at a constant rate from the decomposition of a free-radical initiator. Examples of the application of the method for the analyses of a variety of pure and commercial antioxidants and samples of new and aged lubricants derived from laboratory and service tests are presented. In agreement with theoretical predictions, it is observed that the decay of antioxidant species takes place in used samples before significant changes in the other properties of the lubricant occur. Rapid degradation of the lubricants takes place only when the antioxidant species decay to a low level. Accordingly, it is suggested that the method may be useful in establishing correlations between laboratory tests and service use and in the development of predictive chemical models for the useful lifetimes of lubricants.
Introduction Chain stopping antioxidants comprise an important class of additives utilized in the formulation of lubricants. Their presence at low concentration levels in the new oil results in the efficient suppression of oxidative deterioration processes and leads to an extension of the useful service life of the lubricant. Methods which yield information on the concentration of antioxidant species in both new and used lubricants would be of considerable value in the development of new antioxidant additives, in the assessment of the performance of lubricants in service, and in the correlation of laboratory and field testing. However, conventional methods of analyses of antioxidants are very time consuming (Kagler, 1973) and yield only values for specific antioxidant compounds not including natural inhibitors and antioxidants formed by secondary reactions of additives and base stocks in service. Thus the results derived from these methods are of questionable technological significance when applied to systems containing complex mixtures of aged commercial materials. The present work describes a rapid and specific method, requiring only very small samples, for the determination of total antioxidant species present in new and used lubricants. The principle of this method is derived from the results of fundamental studies of the low-temperature free-radical initiated reaction of oxygen with liquid hy0019-7890/78/1217-0250$01.00/0
drocarbons. Over 20 years ago, Hammond and co-workers reported the occurrence of a direct proportionality between the concentration of added antioxidants and the length of time before a rapid reaction of oxygen and a liquid hydrocarbon occurred in a laboratory system (Boozer et al., 1955). That observation along with the results of the many investigations carried out in the intervening period provides a firm theoretical basis for the analytical procedure described here (Mahoney, 1969; Mahoney and DaRooge, 1975). Experimental Section Materials. Cyclohexene (Eastman Organic Chemicals) was distilled from calcium hydride and passed through activated alumina immediately before use (bp 83 "C). Azobisisobutyronitrile (AIBN) (Eastman Organic Chemicals) was recrystallized twice from methanol and dried: mp 106-107 "C. The antioxidants 2,6-di-tertbutyl-4-methylphenol and 4,4'-methylenebis(2,6-di-tertbutylphenol) (Aldrich Chemical Co.) were recrystallized twice from methanol and ethanol, respectively: mp 70 and 155 "C, respectively. p,p'-Dioctyldiphenylamine (Vanlube 81) was obtained from R. T. Vanderbilt Co. and recrystallized three times from methanol: mp 103 "C. The samples of di-1-pentyl and di-1-octyl zinc dithiophosphates were prepared by reported methods (Willermet et al., 1978). The diisopropyl and di-n-butyl derivatives of zinc
0 1978 American
Chemical Society
