Article pubs.acs.org/IECR
Preparation of Nitroform from Isopropanol and Study of the Reaction Mechanism Peng Ding, Liang Wen, Huiying Wang, Guangbin Cheng, Chunxu Lu, and Hongwei Yang* School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ABSTRACT: The mechanism of the oxidation, nitration, and hydrolysis process of isopropanol with fuming nitric acid (98%, white) was studied by 13C NMR spectroscopy. The effect of the nitronium (NO2+) on the preparation of nitroform from isopropanol was investigated by using fuming nitric acid and H2SO4−HNO3 as nitrating agents. The results indicated that fuming nitric acid showed a better efficiency for the studied reaction. Furthermore, the effect of specific factors, such as reaction temperature, reaction time, and molar ratio of nitrating agent to isopropanol, on the yield of nitroform was investigated. Pure nitroform can be obtained under optimized condition with excellent yield of 51.3%, about 26% higher than that reported in literature. Orthogonal experiments were also carried out to further support the effectiveness.
1. INTRODUCTION Hydrazine nitroformate (HNF) is of interest as a potential oxidizer for energetic solid rocket propellants with environmentally combustion products.1,2 As the crucial raw material of hydrazinium nitroformate (HNF), nitroform (NF) has attracted considerable attention in the search for new energetic materials3 since it was first synthesized by Shishkov.4 Synthetic methods for NF have been explored by nitration of various substrates including acetylene, acetic anhydride, pyrimidine-4,6diol, and isopropanol,5−9 as shown in Scheme 1. However,
not accepted due to the difficulty of obtaining pure nitroform. In addition, although mechanism has always been the crucial part in the promotion of yield of target compounds, the mechanism for the preparation of nitroform from isopropanol has never been referred to in related literatures. Herein, we reported syntheses of pure nitroform from isopropanol using different nitrating agents and optimized conditions in fuming nitric acid. The reaction mechanism was studied using 13C NMR spectroscopy.
2. EXPERIMENTAL SECTION 2.1. General Information. All reagents were commercially available. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 300 spectrometer operating at 300.13 and 75.48 MHz, respectively, using DMSO-d6 or CDCl3 as solvent. Although no explosion or detonation happened in the course of the research, the intermediate KC(NO2)3 should be handled with care since it may be subject to detonation by heat or friction. KC(NO2)3 and NF should be stored at low temperature (0 °C) to preclude the decomposition. 2.2. General Procedure for the Synthesis of Nitroform from Isopropanol. To a vigorously stirred nitrating system was added isopropanol (1.56 g, 0.026 mol) at room temperature, and temperature was strictly controlled below 35 °C during the feeding course. Then the system was heated up to setted temperature and reacted for specific time (Tables 1−6). The reaction was quenched by lowering the temperature of the reaction system with ice−water bath and was extracted with dichloromethane (30 mL × 4). The organic phase was isolated, and the organic solutions were combined, washed with saturated sodium carbonate aqueous solution, and dried over MgSO4. To the purified organic solution was added a solution of potassium hydroxide (KOH) in methanol until no yellow precipitate was formed, and then crude KC(NO2)3 was
Scheme 1. Synthetic Routes for the Synthesis of Nitroform Using Different Substrates
industrial production of nitroform lags far behind laboratory research because of the inevitable problems exist in these synthetic procedures, such as poor safety for synthetic route using acetylene, high costs for synthetic route using pyrimidine4,6-diol, and environmentally unfriendly synthesis for synthetic route using acetic anhydride as substrate. Isopropanol shows certain advantages over the substrates used in synthesis of nitroform, for example, safety, low costs, and environmentally friendly synthesis. Accordingly, the synthetic route from isopropanol is potentially interesting for industrial scale production. Synthesis of nitroform from isopropanol was reported by Frankel et al.10 first in 1978 by using concentrated nitric acid as the nitrating agent, but further study on optimal reaction condition to improve the yield of NF from isopropanol seemed to be balky.11 And the quantification for nitroform was © 2014 American Chemical Society
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April 21, 2014 June 19, 2014 June 19, 2014 June 19, 2014 dx.doi.org/10.1021/ie5011993 | Ind. Eng. Chem. Res. 2014, 53, 10886−10891
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Isopropanol (1.56 g, 26.0 mmol); reaction temperature was 60 °C; molar ratio of anhydrous nitric acid to isopropanol was 12:1. b KC(NO2)3 was obtained after recrystallization. cThe yield was calculated by the equation: yield% = n(NF)/n(isopropanol).
(300 MHz, CDCl3), 7.49 (s, 1H); 13C NMR (75 MHz, CDCl3), 113.3. 2.3. General Procedure for the Mechanistic Studies Using 13C NMR. 2.3.1. Procedure for Taking Samples at the End of the Feeding Course and the End of the Reaction of Isopropanol. Isopropanol (2 mL) was added dropwise into fuming nitric acid (14 mL). During the feeding course, temperature was controlled below 35 °C. And then the reaction system was heated to 60 °C and refluxed for 3 h. The mixed liquid (0.5 mL) was sampled at the end of the feeding course and at the end of the reaction, respectively, following by mixing with 1 mL of DMSO-d6. The 13C NMR spectra were recorded to afford Figure 1a and 1c. 13C NMR (75 MHz, DMSO-d6): 22.9, 27.3, 32.6, 153.1, 166.0, 175.9, 189.5, 211.2 ppm (Figure 1a, for NMR sample at the end of the feeding course). 13C NMR (75 MHz, DMSO-d6): 181.7, 119.0, 23.8 ppm (Figure 1c, for NMR sample at end of the reaction). 2.3.2. Procedure for Taking Samples at the End of the Reaction of Acetone. The sample was taking at the end of the feeding course of acetone following same procedure with isopropanol. Two milliliters of acetone was used and the 13C NMR spectra was recorded to afford Figure 1b. 13C NMR (75 MHz, DMSO-d6): 23.0, 27.3, 32.6, 91.4, 153.2, 175.4, 189.5, 210.2 ppm.
obtained by filtration and purified by recrystallization from methanol. And pure nitroform as white crystals prepared by the acidification of KC(NO2)3 with concentrated sulfuric acid in bromoethane and then extracted with bromoethane. 1H NMR
3. RESULTS AND DISCUSSION 3.1. Mechanism Study of Synthesis of Nitroform from Isopropanol in Fuming Nitric Acid. The reaction mechanism involved three elementary steps (Scheme 2). At the first step, hydroxy (OH) in isopropanol could be oxidized
Table 1. Yield of Nitroform Produced from Isopropanol in H2SO4−HNO3a
entry
time (h)
n(H2SO4)/n(HNO3)
KC(NO2)3 (g)b
yield (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13
2 4 6 2 4 6 2 4 6 2 6 2 6
0:1 0:1 0:1 1:10 1:10 1:10 1:5 1:5 1:5 1:1 1:1 3.5:1 3.5:1
2.23 2.40 1.08 1.12 1.21 1.09 0.92 0.73 0.67 − − − −
41.8 44.1 19.1 23.0 20.7 18.6 15.3 12.2 10.2 − − − −
a
Figure 1. (a) 13C NMR spectrum recorded at the end of the feeding course of isopropanol into fuming nitric acid. (b) 13C NMR spectrum recorded at the end of the feeding course of acetone into fuming nitric acid. (c) 13C NMR spectrum recorded at the end of the reaction of isopropanol in fuming nitric acid. 10887
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Scheme 2. Mechanism of the Synthesis of Nitroform from Isopropanol in Fuming Nitric Acid
Scheme 3. Formation of Nitronium (NO2+) in HNO3−H2SO4 and HNO3
13
to generate carbonyl (CO), thus acetone would be formed by the oxidation of isopropanol in fuming nitric acid. The second step would involve the formation of series of polynitro compounds. The generation of acetone made the nitration of C−H single-bond possible because of the strong electron withdrawing ability of the carbonyl (CO). 1,1,1-trinitroacetone (intermediate A) was formed by the substitution of three hydrogen atoms in acetone with NO2+ step by step. At the last step, nitroform would be generated by hydrolysis of intermediate A. The strong electron withdrawing ability of trinitromethyl (-C(NO2)3) in A contribute to be hydrolyzed by H2O to generate nitroform and acetic acid. Considering the great differences in the “carbon chemical environment” of varied intermediates, 13C NMR spectral tests were conducted to support the proposed mechanism above. Because of the formation of acetone in the possible oxidation step of isopropanol, we took the sample to record 13C NMR spectral during the feeding course and end of reaction with isopropanol and acetone as substrate, respectively. As shown in Figure 1a,1b, obviously, carbon atoms observed at 189.5 and 211.2 ppm was assigned to the carbonyl (CO), while the carbon atoms observed at 153.1, 166.0, 175.9 ppm could be assigned to the derivatives derived from the nitration process of acetone, which presented a direct testimony to the oxidation of isopropanol into acetone (step 1). 13C NMR spectral during the feeding course in acetone as substrate shows great similarity with that of isopropanol. The only difference in the two 13C NMR spectra is located at the carbon atoms observed at 91.4 ppm for acetone system and 166.0 ppm for isopropanol system, which could be attributed to nitration process accelerated by heat released from the oxidation step involved in isopropanol system. The carbon atom observed at 91.4 ppm might be assigned to mononitroacetone and corresponding carbon atom at 166.0 ppm would be assigned to trinitroacetone. This great similarity proved further the formation of acetone (step 1) in the proposed mechanism.
C NMR spectrum recorded at the end of the reaction of isopropanol in fuming nitric acid is shown in Figure 1c. The carbon signal of nitroform was found at 119.1 ppm, while other signals observed at 23.9 and 181.9 ppm were in good agreement with that of acetic acid. The generation of nitroform and acetic acid presented a direct testimony to the formation (step 2) and hydrolysis (step 3) of intermediate A. 3.2. Preparation of Nitroform from Isopropanol in Fuming Nitric Acid. 3.2.1. Preparation of Nitroform from Isopropanol in H2SO4−HNO3 Nitrating System: Selection of the Nitrating Agents. The oxidation capacity and the nitration capacity of nitrating agents would effect on the yield of nitroform from isopropanol according to the reaction mechanism. Therefore, the effect of the nitronium (NO2+) on the preparation of nitroform from isopropanol was investigated by using fuming nitric acid and H2SO4−HNO3 as nitrating agents. The formation of NO2+ in fuming nitric acid and H2SO4−HNO3 is shown in Scheme 3. Compared to the fuming nitric acid without addition of concentrated sulfuric acid, the ionization process was greatly promoted in nitrating system because of the strong dehydration of concentrated sulfuric acid.12,13 Thus, the nitration capacity for H2SO4−HNO3 was strengthened. However, the oxidation capacity for HNO3 was weakened when concentrated sulfuric acid was added to fuming nitric acid.14−16 The results are shown in Table 1. In our preliminary investigation, we carried out the nitration of isopropanol with fuming nitric acid only with reaction temperature of 60 °C when the molar ratio of fuming nitric acid to isopropanol was 12:1. The hydrazine nitroformate (KC(NO2)3) stood as an important intermediate in preparation process of NF. The output of KC(NO2)3 in the synthetic strategy directly symbolized extremum of yield of NF. Yield of nitroform experienced an increase and a decrease when reaction time changed from 2 to 4 h (entries 1−3). Lengthen of reaction time to 6 h will cut down yield of nitroform, which was probably because of a side reaction accompanying the nitration reaction. In comparison, H2SO4−HNO3 nitrating system with 10888
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the settled molar ratio was adopted as the nitrating system, and the results are shown in Table 1. Yields of nitroform went down when concentrated sulfuric acid was added to the fuming nitric acid nitrating agent. The yields of nitroform decreased from 23.0% to 10.2% as molar ratio of concentrated sulfuric acid to fuming nitric acid increased from 1:10 to 1:5 (entries 4−9). Furthermore, nitroform can scarcely be obtained when the molar ratio increased to 1:1 or 3.5:1 (entries 10−13). The decrease of the yield could be attributed to the characteristic of sulfuric acid in the nitrating system. Although the nitration capacity was strengthened, the capacity for oxidation of the nitrating system was weakened when concentrated sulfuric acid was added to the fuming nitric acid. Moreover, the hydrolysis of intermediate A would be greatly inhibited because of dehydration of concentrated sulfuric acid. Thus, we conclude that fuming nitric acid has higher efficiency than H2SO4− HNO3 nitrating system for preparation of nitroform from isopropanol. 3.2.2. Preparation of Nitroform from Isopropanol in Fuming HNO3: Conditions Screening. On the basis of nitrating agents we investigated, we studied the effect of reaction temperature in the range of 40−90 °C on the yield of nitroform produced by isopropanol in fuming nitric acid. As shown in Table 2, nitration of isopropanol with fuming nitric acid
Table 3. Effect of Reaction Time on the Yield of Nitroform in Fuming HNO3a
temperature (°C)
KC(NO2)3 (g)b
yield (%)c
1 2 3 4 5 6
40 50 60 70 80 90
1.08 2.47 2.65 2.26 1.09 0.50
19.1 45.1 49.5 41.8 18.6 7.4
reaction time (h)
KC(NO2)3 (g)b
yield (%)c
1 2 3 4 5
2 3 4 5 6
2.23 2.65 2.40 1.73 1.08
41.8 49.5 44.1 32.1 19.1
Isopropanol (1.56 g, 26.0 mmol); reaction temperature was 60 °C; molar ratio of anhydrous nitric acid to isopropanol was 12:1. b KC(NO2)3 was obtained after recrystallization. cThe yield was calculated by the equation: yield% = n(NF)/n(isopropanol). a
The effect of molar ratio of fuming nitric acid to isopropanol on the yield of nitroform in fuming nitric acid was investigated with different molar ratio values, and the results are shown in Table 4. Yield of nitroform was improved continuously with the Table 4. Effect of Molar Ratio of Fuming Nitric Acid to Isopropanol on the Yield of Nitroform in Fuming HNO3a
Table 2. Effect of Temperature on the Yield of Nitroform in Fuming HNO3a entry
entry
entry
molar ratio
KC(NO2)3 (g)b
yield (%)c
1 2 3 4 5 6 7 8
3:1 6:1 9:1 12:1 15:1 18:1 21:1 24:1
0.29 0.59 1.76 2.66 2.70 2.77 2.87 2.92
2.8 8.7 32.4 50.5 51.3 53.0 54.1 54.8
Isopropanol (1.56 g, 26.0 mmol); reaction temperature was 60 °C; and reaction time was 3 h. bKC(NO2)3 was obtained after recrystallization. cThe yield was calculated by the equation: yield% = n(NF)/n(isopropanol). a
a
Isopropanol (1.56 g, 26.0 mmol); reaction time was 3 h; molar ratio of anhydrous nitric acid to isopropanol was 12:1. bKC(NO2)3 was obtained after recrystallization. cThe yield was calculated by the equation: yield% = n(NF)/n(isopropanol).
increase of the molar ratio of fuming nitric acid to isopropanol in the range of 3:1 to 24:1. The reaction rarely proceeded with yield of nitroform 2.81% when molar ratio of fuming nitric acid to isopropanol was 3:1 (entry 1). When the value of molar ratio increased to 6:1, which was the theoretical amount of fuming nitric acid, the yield of nitroform was only 8.7% (entry 2). This was probably due to the dilution of the fuming nitric acid in the nitration procedure. By further increasing the ratio to 9:1 or 12:1, the yield of nitroform was improved dramatically and reached up 50.5%(entries 1−4). Only 4% of yield improvement was achieved when the molar ratio of fuming nitric acid to isopropanol increased from 12:1 to 24:1 (entries 4−8). The continuous enhancement of yield is attributed to improvement of the nitration capacity and the oxidative capacity with an increasing concentration of fuming nitric acid in isopropanol. It also could be seen that 12:1 was optimized ratio in consideration of economical efficiency and effectiveness. 3.2.3. Orthogonal Experiments. To further confirm the high efficiency under optimized condition, orthogonal experiments L9(34) were designed to define the factors such as reaction temperature, reaction time and molar ratio of fuming HNO3 to isopropanol. For “L9(34)”, “9” represents the number of experiments, “3” is the number of factor levels, and “4” is the number of factors. Value of the selected factors in L9(34) orthogonal table are shown in Table 5, and results are shown in Table 6. Kij, kij, and Rj (i = 1, 2, 3; j = A, B, C) in the table were obtained from the experimental data with respect to the yield (%) of NF (kij = Kij/3). The value for Rj displayed the effect of
nitrating system afford nitroform in good yields of 49.5% (entries 2 and 3) with an excess amount of fuming nitric acid (12 equiv), 60 °C of reaction temperature and 3 h of reaction time. Yield of nitroform goes up when reaction temperature changed from 40 to 60 °C (entries 1−3), which could be attributed to the low reactivity of fuming nitric acid when the reaction system was under relative low temperatures. And further increase of the reaction temperature witnessed a sharp decrease for the yield of nitroform (entries 3−6). When reaction temperature was 90 °C, nitroform was obtained with only 7.4% of yield. This was probably because of losses of target compound derived from its high volatility and instability. The effect of reaction time on the yield of nitroform in fuming nitric acid was investigated with time in the range of 2− 6 h, and the results are shown in Table 3. It could be seen from the table that the yield of nitroform was as high as 49.5% when reaction time was 3 h (entry 2). If the reaction time was shorter or longer, yields of nitroform decreased. The conclusion is drawn that enough time is needed for the completion of nitration procedure and that increase of the reaction time could bring about the losses of nitroform synthesized in the reaction system because of side reaction. 10889
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nitric acid and H2SO4−HNO3 as nitrating agents. The fuming nitric acid showed good efficiency in the synthesis of nitroform with mild, efficient and time saving nitration procedure. Furthermore, the effect of specific factors such as reaction temperature, reaction time, and molar ratio of nitrating agent to isopropanol, on the yield of nitroform was investigated. Pure nitroform can be obtained under optimized condition with excellent yield of 51.3%, 26% higher than that reported in literature.10 Also, the orthogonal experiments were carried out to confirm the efficiency and practicality of the synthetic route for nitroform. In addition, a relatively convenient aftertreatment method to afford pure nitroform which involved the intermediate potassium nitroformate (KC(NO2)3) was utilized. In a word, the synthetic methodology and mechanistic studies will provide the basis for further studies on the industrial production of nitroform.
Table 5. Factor-Level Table factor A
B
C
level
temperature (°C)
time (h)
molar ratio
1 2 3
50 60 70
2 3 4
9:1 12:1 15:1
Table 6. L9(34) Orthogonal Factor-Level Tablea
A entry 1 2 3 4 5 6 7 8 9 K1j K2j K3j K1j K2j K3j Rj optimum
temperature (°C) 50 50 50 60 60 60 70 70 70 121.9 129.8 115.7 40.63 43.27 38.57 4.70
B
C
time (h)
molar ratio
KC(NO2)3 (g)b
yield (%)c
9:1 12:1 15:1 12:1 15:1 9:1 15:1 9:1 12:1 100.2 126.9 140.3 33.40 42.30 46.77 13.37
1.59 2.47 2.56 2.23 2.70 2.03 2.28 1.90 2.23
29.1 45.1 47.7 41.8 51.3 36.7 41.3 34.4 40.0
2 3 4 2 3 4 2 3 4 112.2 130.8 124.4 37.40 43.60 41.47 6.20 A2B2C3
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +0086-25-84303286. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (BK2011696) and the National Natural Science Foundation of China (No. 21376121).
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REFERENCES
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Isopropanol (1.56 g, 26.0 mmol); reaction temperature was 60 °C; molar ratio of anhydrous nitric acid to isopropanol was 12:1. b KC(NO2)3 was obtained after recrystallization. cThe yield was calculated by the equation: yield% = n(NF)/n(isopropanol). a
different factors on the yield of NF. It can be seen from Table 6 that the extremums of level range value parameter: RC > RB > RA. Thus, molar ratio of fuming nitric acid to isopropanol affected the yield of nitroform mostly, followed by reaction temperature, and reaction time at the last. And each factor was studied independently to select the optimum condition for the synthesis of nitroform. A2 (60 °C), B2 (3 h), and C3 (15:1) was selected according to the sum of measuring values at same level for each factor of K1j, K2j, K3j (j = A, B, C), that K2A > K1A > K3A, so A2 was adopted, and B2 and C3 were selected for similar reasons. The optimum synthetic conditions could be selected directly by looking for the highest yield in entries 1−9 in Table 6. The optimum synthetic conditions for nitroform were A2B2C3, that is, 60 °C of reaction temperature, 3 h of reaction time, and 15:1 of molar ratio of fuming HNO3 to isopropanol. Under optimized condition, the yield of nitroform reached as high as 51.3% (entry 5).
4. CONCLUSIONS Synthetic route for nitroform from isopropanol was optimized. The proposed mechanism including the oxidation, nitration, and hydrolysis steps was rationalized by 13C NMR spectroscopy. The effect of the nitronium (NO2+) on the preparation of nitroform from isopropanol was investigated by using fuming 10890
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Acid, Sulfuric Acid, and Water. J. Chem. Soc. 1995, 91 (10), 1439− 1443. (13) Edwards, H. G.; Fawcett, V. Quantitative Raman Spectroscopic Studies of Nitronium Ion Concentration in Mixtures of Sulfuric Acid Nitric Acid. J. Mol. Struct. 1994, 326, 131−143. (14) Bernardi, F.; Cacace, F.; Pepi, F.; Rossi, I. Gaseous [N2O5] H+, [N2O4] H+, and Related Species from the Addition of NO2+ and NO+ Ions to Nitric Acid and Its Derivatives. J. Phys. Chem. A 1998, 102 (11), 1987−1994. (15) Olah, G. A.; Gupta, B.; Narang, S. C. Onium ions. 20. Ambient Reactivity of The Nitronium Ion. Nitration vs. Oxidation of Heterorganic (Sulfur, Selenium, Phosphorus, Arsenic, Antimony) Compounds. Preparation and NMR Spectroscopic (Carbon-13, Nitrogen-15, Phosphorus-31) Study of Nitro and Nitro Onium Ions. J. Am. Chem. Soc. 1979, 101 (18), 5317−5322. (16) Ross, D. S.; Kuhlmann, K. F.; Malhotra, R. Studies in Aromatic Nitration. 2. Nitrogen-14 NMR Study of The Nitric Acid/Nitronium Ion Equilibrium in Aqueous Sulfuric Acid. J. Am. Chem. Soc. 1983, 105 (13), 4299−4302.
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