Nitration of Diphenylmethane and the Isomeric Nitrodiphenylmethanes

Ind. Eng. Chem. Res. 2002, 41, 1929-1934

1929

Nitration of Diphenylmethane and the Isomeric Nitrodiphenylmethanes in Dichloromethane Angelo G. Giumanini,* Paola Geatti, and Giancarlo Verardo Department of Chemical Sciences and Technologies, University of Udine, Via del Cotonificio 108, I-33100 Udine, Italy

Diphenylmethane could be mononitrated to 2- and 4-nitrodiphenylmethane by nitric acid in dichloromethane. Adjustment of experimental conditions gave a mixture of 2,2′-, 2,4′-, and 4,4′dinitrodiphenylmethane. The occurrence of side reactions was negligible, and the conversions were quantitative in the latter reaction. In both processes conventional separation of the pure isomers appeared realistic from a practical point of view. The reactions ran smoothly and did not necessitate stringent controls against dangerous runaways. This nitration procedure was applied also to the three isomeric mononitrodiphenylmethanes which underwent quantitative mononitration in the unsubstituted ring. Introduction The perhemptorious sentence by which the nitration of diphenylmethane (1) is dismissed in the organic chemist’s sacred book1 (“Die Reaktionsfa¨higkeit ist, zumindest in symmetrischen Verbindungen, in den einzelnen Phenylresten gleich gross, so dass Monosubstitutionsprodukte nicht zu erhalten sind”) held its full validity to date. Perusal of the literature let one find only two works of academic nature,2,3 using nitric acid in acetic anhydride, in which polynitration seemingly did not occur. In view of the industrial interest of the nitration of aromatics and, in particular, of the synthesis of diamines of diphenylmethane, which may be pursued from the corresponding dinitro derivatives, we wished to take a close look at the process and see what could be its outcome by using the soft nitrating system nitric acid in dichloromethane (DCM; see Scheme 1).4,5 The interest in this alternative route to the diamines of diphenylmethane is documented in a suitable number of patents (see, e.g., refs 6-11), which were recently issued covering the topic of the synthesis of x,y′dinitrodiphenylmethanes, via either direct nitration or Friedel-Crafts reactions, which were eventually converted by reduction into diaminodiphenylmethanes, in turn used for the production of diisocyanates, which are the building blocks for the manufacture of polyurethanes. Experimental Section Materials. Diphenylmethane (Aldrich, Milwaukee, WI) was used as received. DCM of high technical grade was kindly supplied by Dow Europe (Horgen, Switzerland) and employed as such. Fuming nitric acid was gratiously supplied by Pravisani Esplosivi (Sequals, Italy). The mononitro isomers 2-4 were prepared by Friedel-Crafts reaction of the corresponding nitrobenzyl chlorides (Aldrich) according to the described method for the preparation of 1.12 3,3′-Dinitrodiphenylmethane (8) was prepared with a modification of the literature method,13,14 by heating nitrobenzene (Ald* Corresponding author. Tel: +39-0432-558839. Fax: +390432-558803. E-mail: [email protected].

rich), paraformaldehyde, and 96% sulfuric acid for 2 days at 120 °C; we obtained low conversions in a 13%, 74%, and 13% solid mixture of respectively 2,3′-dinitrodiphenylmethane (7), 8, and 3,4′-dinitrodiphenylmethane (9) from which the wanted isomer could be easily purified by crystallization from EtOAc [mp 174.8 °C (lit.13 174 °C)]. Qualitative and Quantitative Analyses. All of the quenched reaction mixtures were weighed after complete solvent evaporation in order to evaluate the efficacy of the workup and the material balance. The intact mixtures were carefully analyzed by the hyphenated procedures of Fourier transform infrared (FTIR)gas chromatograph (GC)-mass spectrometer (MS) described in detail somewhere else;15 accurate quantitative data were obtained by 1H NMR, using CDCl3 as the solvent. Table 1 collects essential 1H and 13C NMR data. GC analyses on a short fused silica capillary column (a laboratory-prepared fused silica capillary column, 3 m × 250 µm i.d., coated with OV-1 stationary phase, 0.25 film thickness) of high separative power, taking less than 10 min (programmed between 150 and 310 °C with a temperature gradient of 10 °C min-1), were perfectly adequate to obtain excellent and complete separations of all isomers and satisfactory quantitative analyses without precalibration or the use of external standards. Caution in GC analyses must be taken because the 4,4′dinitrodiphenylmethane may be somewhat retained on some otherwise well-separating columns. GC-MS analyses were performed with a Fisons TRIO 2000 GC-MS, operating in the electron impact mode at 70 eV. A 30 m silica gel capillary column (Supelco MDN-5S, i.d. 0.25 mm, 0.50 µm film thickness) was operating between 100 and 310 °C with a temperature program of 10 °C min-1 (injector at 250 °C). Preliminary identifications were provided using the spectral library of the instrument. The GC quantitative data were utilized as a double check for the more reliable 1H NMR data, based on the integral values for the individual peaks, which showed more or less occasionally some fluctuation of location, causing overlapping, owing to concentration effects.16 When this problem showed up for a particular mixture, we resumed to the anyhow very reliable GC quantitative data. The 1H and 13C NMR patterns of all of the

10.1021/ie0109298 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/19/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002

Scheme 1. Expected Pattern of Mono- and Dinitrationa

a

DPM (1): six reactive positions (*). Mononitro-DPM (2-4): three reactive positions.

separated nitro derivatives 2-11 confirmed their attributions. Selected 13C NMR peaks occasionally showed a 1:1 intensity ratio for identical concentrations of some nitro derivative pairs without the need of time-consuming operations at the spectrometer. NMR spectra were recorded at room temperature on a Bruker AC-F 200 spectrometer at 50 MHz. Melting points were determined with an automatic Mettler (model FP61) apparatus and are not corrected. All of the products mentioned in this paper were known and described with sufficient information. Experiments. Essential details about the experiments of nitration of 1-4 in H2SO4, acetic anhydride,

and acetic acid are to be found in the footnotes of Tables 2 and 3. The reaction mixtures were quenched by pouring them onto ice, and the organic material was extracted with CH2Cl2. Solutions of HNO3 in CH2Cl2 were obtained by adding the acid to the solvent: the process is endothermal. We used fuming nitric acid (δ ) 1.51) either directly as such or after degassing from nitrogen oxides at the water pump at room temperature until a colorless solution was obtained or by distilling it before use: we did not find any essential difference in behavior. In any case the acid was titrated before use; concentrations down to 92% were found to be perfectly adequate.

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1931 Table 1. NMR Data of 2- (2), 3- (3), and 4-Nitrodiphenylmethane (4) and 2,2′- (6), 2,4′- (5), and 4,4′-Dinitrodiphenylmethane (10) 1H

compd 2 3 4 6 5 10

13C

NMR (CDCl3/TMS): δ (ppm), J (Hz)

δ 4.25 (s, 2 H), 7.05-7.50 (m, 8 H), 7.85 (dd, 1 H, J ) 8.0, 1.4) δ 4.06, (s, 2 H), 7.12-7.55 (m, 7 H), 7.99-8.10 (m, 2 H)

NMR (CDCl3/TMS): δ (ppm)

δ 38.1, 124.5, 126.4, 127.2, 128.4, 128.8, 132.2, 132.8, 135.5, 138.5, 149.1 δ 41.4, 121.2, 123.6, 126.6, 128.7, 128.8, 129.3, 135.0, 139.3, 143.1, 148.3 δ 41.6, 123.6, 126.6, 128.7, 128.9, 129.5, 139.1, 146.3, 148.8 δ 36.0, 125.1, 127.9, 131.9, 133.3, 134.2, 149.1

δ 4.06 (s, 2 H), 7.11-7.39 (m, 7 H), 8.05-8.16 (m, 2 H) δ 4.63 (s, 2 H), 7.12 (d, 2 H, J ) 7.5), 7.32-7.45 (m, 2 H), 7.45-7.58 (m, 2 H), 8.01 (d, 2 H, J ) 8.2) δ 4.42 (s, 2 H), 7.25-7.41 (m, 3 H), 7.43-7.53 (m, 1 H), 7.57-7.68 (m, 1 H), 8.01 (dd, 1 H, J ) 8.1, 1.5), 8.07-8.20 (m, 2 H) δ 4.20 (s, 2 H), 7.30-7.40 (m, 4 H), 8.13-8.22 (m, 4H)

δ 38.5, 123.7, 125.2, 128.2, 129.4, 132.7, 133.5, 133.7, 146.5, 146.6, 148.9 δ 41.3, 124.0, 129.7, 146.7, 147.0

Table 2. Action of HNO3 on 1 in Different Environments exp

amount of 1 used (mmol)

solvent (mL)

HNO3 (mmol)

mode of addition

convn (%)

products (rel %)

1 2 3 4

61a 100d 3 18

80% H2SO4b (22) 96% H2SO4b (75) Ac2O (10) AcOH (3)

57 50 3 18

c e f g

55 very low 59 50

2 (12), 3 (2), 4 (18), 6 (8), 7 (3), 5 (25), 9 (3), 10 (25), 12 (4) complex mixture 2 (34), 3 (2), 4 (64) no nitration occurredh

a Nitrobenzene (1 mL) was added to keep 1 a liquid during the experiment. b The acid solvent does not appreciably dissolve 1 at room temperature; their mixture stirred for 1 day has caused the complete disappearance of 1 from the final homogeneous solution because of the sulfonation of the substrate.11,18 c The mixture of the acid was added to 1 under vigorous stirring at a rate compatible with temperature (ca. 30 °C) self-control and then stirred for 48 h at ambient temperature. d Nitrobenzene added: 2 mL (12.5 mmol). e Dropwise addition for 1 h at ambient temperature and then stirring for an additional 2 h. f A solution of HNO3 in Ac2O was rapidly added at 5 °C to a solution of 1 in Ac2O. After the solution was stirred for 1 h, the solution was poured into cold water (25 mL) and then refluxed for 30 min; then the volatile materials were distilled away under reduced pressure, and the mixture was extracted as usual. g A solution of HNO3 in AcOH was rapidly added at room temperature to a solution of 1 in AcOH; then the mixture was stirred for 7 days. h The reaction mixture contained unreacted 1 (49%), benzophenone (12; 46%), benzhydryl acetate (13; 2%), tetraphenylethene (14; 2%), and 1,1,2,2tetraphenylethane (15; 1%).

Table 3. Nitration of the Isomeric Mononitrodiphenylmethanes 2-4 in H2SO4 and DCM expa

substrate

S/HNO3 (mol/mol)

time (h)

convn (%)

1c 2 3c 4 5c 6 6′ 6′′ 7 8 8′ 9 10

2 2 3 3 4 4 4 4 2 + 4e 2 + 4e 2 + 4e 1 + 2e 1 + 3e

1.20 0.19 1.20 0.19 1.20 0.19 0.19 0.19 1.80 0.40 0.40 0.40 0.26

1 3 1 16 1 3 5 23 1 0.3 20 4 16

100 100 94.2 100 100 67 77 100

solventb

products (rel %)

H2SO4 (96%)d CH2Cl2 H2SO4 (96%)d CH2Cl2 H2SO4 (96%)d CH2Cl2 CH2Cl2 CH2Cl2 H2SO4 (96%) CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

6 (24), 7 (6), 5 (70) 6 (48), 7 (6), 5 (52) 7 (24), 8 (5), 9 (71) 7 (25), 8 (5), 9 (70) 5 (24), 9 (5), 10 (71) 5 (24), 9 (nd), 10 (76) 5 (23), 9 (5), 10 (72) 5 (25), 9 (5), 10 (70) 6 (12), 7 (3), 5 (47), 9 (3), 10 (35) only traces of dinitro derivatives 6 (30), 7 (5), 5 (45), 9 (2), 10 (18) 3 (3), 4 (61), 6 (17), 5 (19) 2 (35), 4 (47), 12 (14), traces of dinitro derivatives (5-7, 9, and 10)

para/ortho ratio 2.9 1.1 3.0 2.8 3.0 3.2 3.2 2.8 nd 1.3f 2.6f 1.3g 2.4h

a All experiments were carried out at room temperature. b The solvent employed vs the amount of substrate was ca. 17 mL g-1 for the experiments in H2SO4 and 11 mL g-1 for those in CH2Cl2. c One lot of the organic substrate was added, and then the homogeneous mixture was stirred for 1 h. d The isomeric nitro derivatives 2-4 have been found to be pretty soluble in sulfuric acid; they undergo a rather rapid sulfonation by 96% sulfuric acid, so that it is essential to add these substrates to a stoichiometric amount of nitric acid in sulfuric acid in order to obtain essentially quantitative yield of dinitro derivatives. e Equimolar. f % unreacted 4 vs % unreacted 2. g % unreacted 2 vs % unreacted 1. h % unreacted 3 vs % unreacted 1.

Mononitration of 1 with HNO3 in DCM. Typical Experiment (Table 4, exp 1). A solution of HNO3 (670 mmol, 28.5 mL) in DCM (100 mL) was added dropwise to a well-stirred solution of 1 (167 mmol) in DCM (40 mL), keeping the temperature of the slightly exothermal process between 20 and 25 °C for 60 min. GC and 1H NMR analysis of a quenched aliquot of the reaction mixture showed the following composition: unreacted 1, 48%; mononitration vs dinitration products, ca. 8. When the full mixture was spent after 15 h, only 9% 1 had still to react and the above ratio dropped to ca. 2.3. The recovery was essentially quantitative, and no other products could be detected besides traces of benzophenone (12). Mononitration products could be neatly separated from anything else in the solution by vacuum distillation

in the range 120-135 °C at 0.1-0.01 Torr. A preliminary rough distillation test indicated the likely feasability of separating 2. The mixture did not show signs of decomposition during distillations. In larger preparations, the spent, slightly diluted, unreacted acid could be recovered by distillation under low pressure. Dinitration of 1 in DCM. Typical Experiment (Table 4, exp 6). Commercial fuming HNO3 (1 mol, 42.5 mL) was added for 30 min under vigorous stirring to a solution of 1 (149 mmol) in DCM (75 mL) initially chilled at -16 °C at a rate at which the exothermal reaction did not increase the temperature of the mixture above 0 °C. After some HNO3 was added, the solution became dark red-brown; after ca. 60 min of stirring at -10 °C after the addition, the color turned suddenly to light redorange, and the separation of a second liquid phase was

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Table 4. Nitrations of 1 with HNO3 in DCM [HNO3]/ [1]a exp 1 (mol/L)

T (°C)

time of react. addition timeb convn mononitro (min) (h) (%) (%)

mono (rel %)

products dinitro (%)

1 2

4.0 4.0

1.2 1.2

rt rt

60 60

0 15

52 91

89 70

2 (37), 3 (4), 4 (59) 2 (33), 3 (6), 4 (62)

11 30

3

2.2

1.7

-13

60

16

69

78

2 (34), 3 (3), 4 (63)

22

4

4.0

5.3

-13

60

16

100

35

2 (11), 3 (4), 4 (85)

65

5

4.3

1.1

-13e

40

48

100

21

2 (5), 3 (5), 4 (90)

79

6

7.0

0.2

30

3

100

0

0

100

dinitro (rel %) 6 (24), 5 (45), 10 (31) 6 (19), 7 (6), 5 (41), 9 (5), 10 (29) 6 (16), 7 (4), 5 (41), 9 (4), 10 (35) 6 (19), 7 (5), 5 (39), 9 (5), 10 (32) 6 (16), 7 (4), 5 (39), 9 (4), 10 (37) 6 (11), 7 (5), 5 (33), 9 (5), 10 (46)

Rc

recoveryd

8.1 excellent 2.3 excellent 3.5 excellent 0.5 good 0.3 good -

good

a Determined as 1 mol of 1 over the total volume of DCM used in the experiment. b After the end of addition. c Ratio of total mononitro to dinitro derivatives. d Poor: less than 70%. Good: 70-95%. Excellent: more than 95%. e -13 °C during addition and then room temperature.

observed. The mixture was then agitated for 3 h at room temperature before quenching it by addition of some water, which caused at first contact the instant precipitation of a bright yellow solid, whose dry weight was close to the calculated yield in dinitro derivatives of 1. After evaporation of the solvent from the dried (Na2SO4) solution, the residual solid added to the already separated material brought the full recovery to the theoretical weight for the production of dinitro derivatives. GC-MS-FTIR15 analysis of this solid allowed one to preidentify all of the components of the mixture: neither 1 nor any 2-4 nor 12 was present. Besides the six isomeric x,y′-dinitrodiphenylmethanes (5-10; see Table 4, exp 6, for quantitative composition), a tiny peak for 2,4-dinitrodiphenylmethane (11) could be determined by GC-FTIR-MS. Fractional crystallization from ethyl acetate and ethanol allowed one to promptly separate the 4,4′ derivative 10 and later the 2,4′ derivative 5. Attempts at further full separations and purifications were not made. Nitration of Mononitrodiphenylmethanes in DCM. 2-Nitrodiphenylmethane (2; Table 3, exp 2). A solution of HNO3 (47 mmol, 2 mL) in DCM (10 mL) was added in one lot to a well-stirred solution of 2 (9.1 mmol) in DCM (10 mL) at room temperature (22 °C): in a matter of ca. 10 min the temperature climbed to 29 °C with the homogeneous solution turning dark brown-red and then light orange while the temperature dropped to ca. 20 °C. After 3 h, the light yellow solution was quenched with water, and the organic light yellow solution was dried over Na2SO4 and analyzed by GC and GC-MS. Full evaporation of the solvent yielded a yellow solid (quantitative recovery as dinitro products), which was analyzed by 1H NMR. 13C NMR was informative as to the absence of sizable concentrations of starting material and products other than the expected dinitro derivatives. The deep yellow solid was recrystallized from propanol to yield 2,4′-dinitrodiphenylmethane (5) [mp 119.3 °C (lit.19,20 118)] as light yellow crystals. The 2,2′ derivative 6 could be obtained by careful fractional crystallization from methanol as yellow crystals [mp 84 °C (lit.21,22 83.5 °C, lit.23 85 °C)]. 3-Nitrodiphenylmethane (3; Table 3, exp 4). The reaction was carried out as described above for the 2 isomer. After 3 h of reaction, no original substrate was any more detectable. A quantitative transformation into dinitro derivatives was achieved. Crystallization from EtOAc-cyclohexane yielded a GC homogeneous white solid [mp 99.2 °C (lit.19 101.2)] identified by 1H and 13C

NMR and MS as ′9; the mother liquors after a few crystallizations yielded a GC homogeneous white solid (mp 54.2 °C) identified as the 2,3′ isomer 7 by 1H and 13C NMR. 4-Nitrodiphenylmethane (4; Table 3, exp 6, 6′, and 6′′). The reaction was carried out exactly as described above for the 2 isomer with a full recovery of a pale yellow solid consisting of the dinitro isomers. The 4,4′ isomer 10 was easily separated by crystallization from EtOAc [mp 188.1 °C (lit.19,20,24 183 °C)] and fully characterized by 1H and 13C NMR. The 2,4′ isomer 5 was purified by careful recrystallization from ethanol. Competitive Nitration of 2 and 4 with Nitric Acid in 96% Sulfuric Acid (Table 3, exp 7). A mixture of 2 (2.3 mmol) and 4 (2.3 mmol) was poured into a solution of HNO3 (2.6 mmol, 0.11 mL) in 96% H2SO4 (20 mL) maintained at room temperature, and the mixture was stirred for 1 h, quenched by pouring it onto ice, extracted with EtOAc, and washed with a 10% aqueous Na2SO4 solution. Competitive Nitrations with Nitric Acid in DCM (Table 3, exp 8, 8′, 9, and 10). In a typical experiment, a solution of HNO3 (12.2 mmol, 0.52 mL) in DCM (5 mL) was added in one lot to a well-stirred solution of equimolecular amounts (e.g., 2.4 mmol) of two substrates in DCM (e.g., 5 mL). The reaction was quenched by pouring it into cold water. Results and Discussion Our experiments in sulfuric acid (Table 2, exp 1 and 2) confirmed the impossibility of stopping the nitration of 1 at the stage of the introduction of a single nitro group. In protic media, namely, pure HNO3 and H2SO4, “irresistible” polynitrations1 were recorded, even when low concentrations of NO2+ were made available at any time during the nitrations of 1 in H2SO4 and the overall amount of HNO3 was equal or below stoichiometric for mononitration. A possible interpretation is that the fast nitration process shows an actual rate for the second and, in fact, third nitrations that is faster than the interphase transfer of fresh 1 into the nitrating mixture in replacement of the substrate which underwent the first nitration and did not leave yet the phase in which the initial reaction took place. The relative propensities of the ring positions of 1 to being nitrated were unveiled by performing the process in acetic anhydride (Table 2, exp 3): the isomeric ratio of 2 vs 3 vs 4 was found to be 34/2/64, in close approximation of previous findings by Dewar and Urch2

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1933

but in strong deviation from Sharnin and Falyakhov’s3 more recent data. In acetic acid, nitric acid acted only as an oxidant,25 yielding 12 (Table 2, exp 4). When the individual mononitro derivatives 2-4 were nitrated in H2SO4 (Table 3, exp 1, 3, and 5), the reactions proceeded to cause only nitration exclusively in the unsubstituted ring, essentially yielding the same substitution regiospecificity with a para/ortho ratio about 3.0. This observation leads to the conclusion that, as expected, there is no transmission of electronic effects from one ring to the other and that the steric requirements for substitution did not change with the position of the original nitro group. A higher proportion of para nitration in the nitration of 2-4 with respect to the analogous reactions in CH2Cl2 is perhaps an indication of some association of the nitrated ring with sulfuric acid, a fact increasing its bulkiness. A competitive experiment where 1 equiv of nitric acid competed for 1 equiv of 2 and 1 equiv of 4 (Table 3, exp 7) showed that the two isomers are nitrated at the same overall rate. Mononitration of 1 with a minor incursion of dinitration could be typically achieved by the use of nitric acid in a ca. 4-fold equivalent amount when DCM was used as a solvent for both nitric acid (to be more or less rapidly added) and 1, as would be seen from experiments 1 and 2 in Table 4. A large decrease of the temperature of the process did not show much influence in the composition of the final mixtures. The practically athermal, room temperature homogeneous reaction was rather slow. The para isomer (4) was the main product, being formed in almost twice the amount of the ortho isomer (2). As expected for the reaction, the meta isomer (3) was produced in a very tiny amount. The para/ortho ratio was also in this case the perfect reversal of the statistical factor or, if one prefers, of the orientation observed for the nitration of toluene.26 The common rationale for the fact is the operation of the steric hindrance by the PhCH2 group to the attack of the ortho positions. Nitric acid in DCM was found quite apt also at producing dinitration products from 1, essentially the 2,2′ (6), 2,4′ (5), and 4,4′ (10) isomers in a neat reaction (Table 4, exp 6). Only minor amounts of 12 were present in these systems. Isomer 10 can be easily separated by fractional crystallization in a high state of purity. From the mother liquors, isomer 5 could also be obtained. No attempts were made to separate 6, which we obtained from the reaction of 2 with nitric acid in DCM. The behavior of the three mononitro isomers 2-4 under the action of nitric acid in DCM was also examined (Table 3, exp 2, 4, 6, 6′, and 6′′). They underwent a neat mononitration in essentially quantitative yields. The nitro group was invariably introduced only in the second ring. A first surprising feature surfaced at once. Contrary to any expectation the ortho isomer 2 yielded a definitively lower para/ortho ratio (ca. 1) than the other two which showed an almost identical behavior (para/ortho ca. 3), a confirmation that the mesomeric effect of the p-nitro substituent is not felt in the other ring, with the latter figure being quite close to that found for the analogous experiment on 1 (Table 4). The competition experiment of nitration of equimolecular amounts of 2 and 4 (Table 3, exp 8 and 8′) using HNO3 as a limiting reagent resulted in the practically sole nitration of the former, even for long reaction times. The nitration of 2 was complete in 3 h,

whereas 4 was converted only to the extent of 64% after the same period under the same conditions. A competition experiment using 1 equiv of 1 and 2 competing for the amount normally used for the nitration of 1 equiv of substrate in DCM stopped at low conversions (22%; Table 3, exp 9) showed that the ratio of reacted 1 vs reacted 2 was 3. Because, in the absence of any effect from the nitrated ring in 2, the statistical factor would be 2 in favor of 1, we can conclude that perhaps there is some inductive effect. On the other hand, we observed that the ortho derivative 2 was nitrated much faster than the para isomer 4. Also, 3 appeared to be at least 7.5 times less reactive than 1 (Table 3, exp 10) considering the statistical factor. Conclusions The soft nitrating system nitric acid in DCM allowed one to control the stage of mononitration of diphenylmethane (1) operating in a homogeneous phase, avoiding the occurrence of other reactions. Preliminary tests of distillation indicated the possibility of obtaining pure the useful and costly o-nitrodiphenylmethane (2). Clean dinitration of 1 can also be performed under similar conditions. The nitration of the isomeric mononitrodiphenylmethanes 2-4 revealed unsuspected and surprising features, like the impressive difference in the rates of nitration between the 2 and 4 isomers and the lower para/ortho ratio observed in the products from the former. Nitration of simple mononitro derivatives 2-4, which did not exhibit the incursion of oxidation processes, yielded essentially mixtures of two isomers, which could be separated. The nitrating system nitric acid in DCM appears to be greatly superior on several obvious points to that using sulfuric acid as a catalyst solvent. More nitric acid is to be used but can be easily recovered and reused if needed. This procedure may be an industrially attractive alternative at least on two different points, when the aim is the final manufacture of either polyamides or polyurethanes. It makes available isomers obtainable in pure form, not easily prepared by other procedures, economically. Also, it avoids, in the case of the 4,4′ isomer 10, the now obligatory route (de facto 16 is the main component of a mixture of condensation products, which is used as such27 (eq 1)). HNO3

red.

CH2O

PhH 9 8 PhNO2 98 PhNH2 9 8 (p-NH2Ph)CH2 H2SO4 HCl 16 (1) Acknowledgment Financial support was obtained from Italian National Research Council and the Italian Ministry of University, Science and Technology (MURST). P.G. is the recipient of a 3-year postgraduation scholarship from the Department of Agriculture of the autonomous region Friuli Venezia Giulia. The authors are grateful to Dr. P. Martinuzzi for recording 1H and 13C NMR spectra and Mr. P. Padovani for expert instrumental maintenance. Literature Cited (1) Seidenfaden, W.; Pawellek, D. Aromatische Nitro-Verbindungen. In Houben Weyl, Methoden der Organischen Chemie; Mu¨ller, E., Ed.; Thieme: Stuttgart, Germany, 1971; Vol. X/1, p 539.

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Received for review November 20, 2001 Accepted November 21, 2001 IE0109298