Energy Fuels 2010, 24, 3831–3839 Published on Web 06/17/2010
: DOI:10.1021/ef100397v
Chemistry of Organic Nitrates: Thermal Oxidative and Catalytic Chemistry of Organic Nitrates M. A. Francisco* and J. Krylowski ExxonMobil Research and Engineering Company, 1545 Route 22 East, Clinton Township, Annandale, New Jersey 08801 Received March 31, 2010. Revised Manuscript Received June 7, 2010
Our previous research showed that the rates for unimolecular thermal fragmentation of the O-N bond of organic nitrates are the same in air as they are under a nitrogen atmosphere. This is contrary to what literature studies have reported. When the rate of reaction was followed by infrared (IR) spectroscopy, as in previous literature studies, the IR absorption of the unsaturated carbonyls generated during the reaction complicated the IR analysis and the rates deceptively appeared to slow in air. The current study shows that unsaturated carbonyls can also be formed under a nitrogen atmosphere when hydroperoxide is added. Hydroperoxides created naturally most likely give rise to the unsaturated carbonyl compounds when the reaction is carried out in air. Evidence suggests that the products of organic nitrate thermal chemistry accelerate the decomposition of the model hydroperoxide. The details of this chemistry are critical for controlling the oxidation of hydrocarbons. The reaction of organic nitrates with various catalytic compounds has also been studied. The catalysts fall into three categories: (1) no effect on the rate of loss of organic nitrate or the type of products generated over the thermal base case, (2) accelerate the loss of organic nitrate, but the products remain the same as the thermal base case, and (3) accelerate the loss of organic nitrate and generate different products than the thermal base case. Catalytic amounts (3 mol %) of copper(II) oleate and iron(III) acetyl acetonate each increased the rate of loss of organic nitrates at 170 °C under N2 by a factor of 1.5 and 2, respectively. The reactions remained first order (or pseudo-first order). Changes in product distribution, in some cases, indicate that the mechanism may be non-radical. Copper and iron compounds convert organic nitrates to stable products before they can cleave thermally and form radicals or react directly with other molecules. Hydroperoxides react with some of the same catalysts, but they also react with compounds that have no effect on organic nitrates. unsaturated carbonyl compounds, which can arise from olefins2,3 or carbonyls4,5 generated in the thermolysis of organic nitrates6,7 (Figure 2). The appearance and growth of the 1632 cm-1 absorption complicates the IR analysis, giving the appearance that the loss of organic nitrate slows. Deconvolution of the IR spectra showed that the rates of thermal decomposition are the same in nitrogen and air. GC/MS was used as a direct measure of the rates in air and for more complete characterization of the products in air and nitrogen. The details of the oxidation mechanism were shown to lead to the formation of the unsaturated carbonyl compounds. Hydroperoxides were identified as the reactive species involved in the formation of the R,β-unsaturated carbonyls. It was also determined that the loss of hydroperoxide was accelerated in the presence of the organic nitrate. The thermal decomposition products of the organic nitrate (NO) most likely are causing this acceleration through radical initiation reactions with the hydroperoxide.8 The objective of this study was to determine the details of the
Introduction The chemistry of model organic nitrates: ethylhexylnitrate (EHN) and octylnitrate (ON) were studied using infrared (IR) spectroscopy and gas chromatography with mass spectral detection (GC/MS). Our previous research using IR and GC/MS analysis confirmed literature studies showing that the thermal chemistry of organic nitrates involves homolytic cleavage of the weak N-O bond to form an alkoxy radical and NO2 (Figure 1).1 This homolytic cleavage is the rate-determining step (RDS) and is first-order. IR was used, in initial studies, to monitor the time-dependent loss of model organic nitrates (decrease in the -NO2 absorbance at 1638 cm-1 in the IR) at 170 °C and to characterize the products. The reaction becomes significant at 170 °C. The alkoxy radical and NO2, generated in the RDS, can engage in reactions with other molecules that may be present in the reaction mixture to generate carbonyl, nitro, and other compounds. The importance of organic nitrates and their chemical reactions was discussed in a previous publication.1 Organic nitrates generate a secondary product (IR absorbance at 1632 cm-1) in air that is not produced in a nitrogen atmosphere. The secondary products were identified as R,β-
(4) Landgrebe, J. A. Theory and Practice in the Organic Laboratory; Brooks Cole: Pacific Grove, CA, 1973; pp 257-259. (5) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 2ed ed.; Aldrich Chem. Co. Library: Milwaukee, WI, 1995 (6) Bohn, M. A. Proceedings of the 11th Symposium on Chemical Problems Connected with the Stability of Explosives, Sweden, May 24-28, 1998; pp 61-87. (7) Pivina, T. S.; Lushnikov, D. E.; Porollo, A. A.; Ivshin, V. P. Proceedings of the 23rd International Pyrotechnics Seminar, Tsukuba, Japan, 1997; pp 698-705. (8) Pryor, W. A.; Castle, L.; Church, D. F. J. Am. Chem. Soc. 1985, 107, 211–217.
*To whom correspondence should be addressed. E-mail: manuel.a.
[email protected]. (1) Francisco, M. A.; Krylowski, J. Ind. Eng. Chem. Res. 2005, 44, 5439–5446. (2) MacLean, A. F.; Stautzenberger, A. L. Oxidation of ethylenically unsaturated compounds. BE 635407 19640127, 1964; p 9. (3) Theissen, R. J. J. Org. Chem. 1971, 36, 752–757. r 2010 American Chemical Society
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: DOI:10.1021/ef100397v
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Figure 1. Thermal fragmentation of the O-N bond of EHN.
Figure 3. IR spectra of EHN thermolysis with 5 equiv of CHP.
Figure 2. Generation of R,β-unsaturated carbonyl compounds.
The ionization was accomplished by electron impact at 70 eV. Products were identified and characterized on a time-of-flight GC/MS (Leco Pegasus II) equipped with a HP6890 GC. The GC was equipped with a RTX-5 column (10 m 0.18 mm inside diameter 20 μm film thickness). The injection port temperature started at 50 °C for 0.1 min and was increased at a rate of 700 °C/ min to 300 °C (held for 2 min). The initial GC oven temperature was 0 °C and increased at a rate of 50 °C/min to 300 °C (held for 3 min). The reactions with cumene hydroperoxide (CHP) and catalyst were conducted in the same way as the pure thermal experiments, except that 1-5 equiv of CHP and 1-5 mol % of catalyst were added to the reaction mixture before heating to the desired temperature. CHP, catalysts, EHN, and ON were purchased from Aldrich Chemical Co.
oxidation mechanism leading to the formation of the unsaturated carbonyl compound and to identify the reactive species involved. The details of this chemistry are critical for controlling the oxidation of hydrocarbons (i.e., in lubricants and other petroleum products). Another objective of this study was to determine the effects of different types of catalysts and additives typically found in petroleum products and lubricants on the thermal chemistry of organic nitrates. Experimental Section Thermolysis reactions were carried out with 1 wt % EHN and ON in hexadecane as the solvent. The entire reaction mixture is about 20 g and was heated at 170 °C in a three-neck glass flask. Limited experiments were also conducted at 160 and 180 °C. The flask was equipped with a magnetic stir bar, two thermometers (one to record the temperature of the reaction mixture and the other to record the temperature of the hot oil bath), and a reflux condenser to condense and return any volatiles back to the reaction mixture. The stirring rate was adjusted to 400 rpm for every experiment, and the heater was adjusted to heat the oil bath and maintain the reaction mixture at a constant temperature. The temperature control was quite good and varied less than a degree. The condenser was cooled with a flow of chilled water from the laboratory chilled water system. The atmosphere in the condenser and reaction flask was controlled by keeping the entire system closed, except for the outlet at the top of the condenser, which was allowed to exit through an oil trap. A slight positive pressure of air or nitrogen was introduced through one of the three necks of the flask. The flow of air or nitrogen blankets the reaction mixture and flows up the condenser and through the oil trap, maintaining a slight positive pressure of the atmosphere of choice throughout the system. The reaction mixture was always purged with nitrogen to remove any dissolved air in the case of the nitrogen atmosphere experiments. Samples (1.5 g) were removed periodically for IR analysis. The samples were loaded into KBr cells (0.1 mm path length), and full IR spectra (4000-500 cm-1) were recorded on a Mattson series FTIR model Rs2 infrared spectrometer (class IIa laser product). GC/MS analyses were conducted on the following instruments. Rates were analyzed on a HewlettPackard 5890 series II gas chromatograph/mass spectrometer with a Supelco SPB capillary column (30 m 0.25 mm inside diameter 0.25 μm film thickness; catalog number = 2-4034; column number = 12537-02B). The injection port temperature was set at 200 °C, and the initial oven temperature was set at 70 °C and increased to 290 °C at a rate of 15 °C/min. The column effluent to the detector was split 100:1. Helium was the carrier gas at a flow rate of 0.8 mL/min (6.3 psig). The total run time was 19.67 min. The detector temperature was set at 300 °C.
Results and Discussion Impact of Air and Model Hydroperoxide. The same 1632 cm-1 secondary product that is generated in air can also be generated in a nitrogen atmosphere when a model hydroperoxide, such as CHP, is added. When 5 equiv of CHP (relative to organic nitrate) are added to a 1% solution of EHN in hexadecane and heated at 170 °C under N2, the 1632 cm-1 product reappears, as shown by the IR spectra in Figure 3 (duplicate runs). This suggests that, when the reaction is run in air, the 1632 cm-1 product is probably produced by the reaction of natural hydroperoxides that are generated as a result of air oxidation reactions. The GC/MS analysis provides1 evidence for hydroperoxide when the reaction is conducted in air. The 1632 cm-1 product appears early on in the reaction (about 45 min). The absorptions between 1665 and 1725 cm-1 are due to carbonyl compounds formed from oxidation reactions (Figure 3). Zero time is the upper most IR line in Figure 3, and 120 min is the lower most IR line in Figure 3. The rate of the reaction as analyzed by IR also slows, as in the case of air, but not to the same extent (Figure 4). The rate is slower in air, probably because there is a constant formation of hydroperoxide2 and a greater production of the 1632 cm-1 product, as opposed to a fixed concentration of hydroperoxide, which is eventually depleted in the CHP experiment. Figure 4 shows the rate (IR) of the EHN reaction at 170 °C in N2, air, and N2 with 5 equiv of CHP present. The value of k in Figure 4 is the average of two experiments with CHP. The error at the 95% confidence level is 1.4% or 0.009 h-1. The left bar in Figure 4 refers to thermolysis performed in nitrogen; the middle bar in Figure 4 refers to 3832
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CHP and close to the rate in air without CHP (Figure 7). The GC/MS analysis of the ON reaction with 5 equiv of CHP shows that the reaction is indeed first-order and the rate is not slowed as the IR measurement indicates. The GC/MS analysis of ON with 5 equiv of CHP gave a rate of 0.582 h-1. This is on the high side for ON rate measurements made by GC/MS (air, N2, or N2 with CHP) or IR (N2 only), but the result does demonstrate that the rate is not slowed as the IR measurement would indicate. The left bar in Figure 7 refers to thermolysis performed in nitrogen; the middle bar in Figure 7 refers to thermolysis performed in air; and the right bar refers in Figure 7 refers to thermolysis performed in nitrogen with 5 equiv of CHP. Products of the Thermal Chemistry of Organic Nitrates. IR analysis of the reaction of 1% EHN with 5 equiv of CHP shows that carbonyl and nitro compound formation is greater with CHP under nitrogen than it is in air without CHP (Figure 8). Unlike in the air case, the carbonyl and nitro compound concentration increases with time up to about 2 and 1 h, respectively, and then levels out. This is most likely due to the fact that there is a fixed initial concentration of CHP, which is decreasing as the reaction progresses and is eventually depleted. The upper most IR lines in Figure 8 show the growth products of organic nitrate thermolysis in nitrogen with 5 equiv of CHP. The lower most IR lines in Figure 8 show the growth products of organic nitrate thermolysis in air. The middle IR lines in Figure 8 show the growth products of organic nitrate thermolysis in nitrogen. Figure 9 shows that the GC/MS rate for the thermal chemistry of 1% CHP in hexadecane at 170 °C under N2 is apparently a first-order reaction, as expected (homolytic cleavage of the O-O bond to form alkoxy and hydroxyl radicals; Figure 9). The rate of the reaction is 1.98 h-1, more than twice the rate of the thermal reaction of EHN or ON. The high thermal activity of CHP9 under these conditions means that 96.7% of the initial CHP charged has reacted in 1.75 h. The GC/MS response of CHP relative to toluene as an internal standard is linear with the CHP concentration. The previous discussion pointed out that, at 5 equiv of CHP, the production of the 1632 cm1 absorption is enough to cause apparent slowing of the rate of thermal loss of EHN but the rate is not slowed as much as it is in air without the CHP. The explanation was that, in the air experiment, hydroperoxide is continuously generated and, therefore, the 1632 cm-1 product is formed. The CHP experiment has a finite amount of hydroperoxide, which is eventually used up during the course of the reaction. The rate data on CHP discussed above supports this argument. The higher concentration of carbonyl and nitro compounds in the case of the CHP experiment indicates that natural hydroperoxides generated in the air may be more effective at generating the 1632 cm-1 absorption than they are at generating carbonyl and nitro compounds. This may be due to differences in the structure and reactivity of natural hydroperoxide as compared to CHP.10 CHP is one of the most stable and, therefore, a less reactive hydroperoxide. As the concentration of CHP is lowered from 5 equiv, the production of carbonyl and nitro compounds with time
Figure 4. Rate constants for thermolysis of EHN.
thermolysis performed in air; and the right bar in Figure 4 refers to thermolysis performed in nitrogen with 5 equiv of CHP. This is significantly slower than the rate in N2 and closer to the rate in air. The plots of IR ln(a/a - x) as a function of time also look very similar for the reactions in air or N2 with 5 equiv of CHP, as shown in Figure 5 (a = initial IR absorbance at 1638 cm-1, and x = IR absorbance at 1638 cm-1 at various times during the thermal reaction). The same behavior was observed at 4 equiv of CHP. Only a trace of the 1632 cm-1 product appears at lower concentrations of CHP. There is not enough of the 1632 cm-1 product generated to interfere with the IR measurement of the rate. The reaction is still first-order, and the rate is the same as the reaction carried out in the absence of any CHP. A total of 2 equiv of CHP generate more of the 1632 cm-1 product but not enough to slow the rate of the reaction. It is not until the concentration of CHP is increased to 4 or 5 equiv that the generation of the 1632 cm-1 product is significant enough to create an apparent slowing of the rate of the reaction by IR analysis. A total of 3 equiv of CHP are similar to 1 and 2 equiv with more 1632 cm-1 product but not enough to create an apparent slowing of the rate as with 4 and 5 equiv. Analysis of the two reactions at 5 equiv of CHP by GC/MS shows that the rate of reaction at 5 equiv of CHP is 0.801 h-1, with a 0.0394 h-1 (4.9%) error at 99% completion (Figure 5 shows that the reaction is first-order). This is within experimental error of the IR measurement of the rate of the EHN reaction under N2 without CHP present (0.746 h-1 at 3.1% error). The same behavior was observed with 4 equiv of CHP when the reaction was analyzed by GC/MS. The GC/MS analysis is measuring the true overall rate of the thermal reaction of EHN regardless of the presence of air or hydroperoxide. The IR spectra of CHP in hexadecane at 170 °C under N2 without any organic nitrate present shows that there is no trace of a 1632 cm-1 product. This verifies that both organic nitrate and hydroperoxide are necessary for the production of the 1632 cm-1 product. The impact of CHP is also observed with other organic nitrates. Figure 6 shows the IR spectra of ON as a function of time at 170 °C in hexadecane under nitrogen with 5 equiv of CHP. Zero time is the upper most IR line in Figure 6, and 330 min is the lower most IR line in Figure 6. The growth of the 1632 cm-1 absorption is clear, particularly at 90 min. The 1632 cm-1 product is not observed when the reaction is run in N2 without the CHP. The ln(a/a - x) as a function of the time plot is also shown in Figure 6. The rate of the reaction appears to slow to 0.374 h-1, which is significantly less than the rate in nitrogen without
(9) Chen, K.-Y.; Wu, S.-H.; Kossoy, A. A.; Shu, C.-M. Proceedings of the 34th North American Thermal Analysis Society (NATAS) Annual Conference on Thermal Analysis and Applications, 2006; pp 087.18.962/ 1-087.18.962/12. (10) Grela, M. A.; Colussi, A. J. J. Phys. Chem. 1996, 100, 10150– 10158.
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Figure 5. EHN concentration varies with time during thermolysis.
Figure 6. Thermolysis of ON.
The detailed pathways leading to the formation of the 1632 cm-1 absorption, and the structure of the product will be discussed later. The 1632 cm-1 absorption falls in the range of IR absorptions for carboxylates, chelated ketones, and unsaturated carbonyl compounds (carboxylic acids, ketones, and aldehydes4,5). The thermal reaction of EHN in air or nitrogen with CHP has the potential to produce unsaturated carbonyl compounds (Figure 8). IR analysis of the products of the thermal reaction of ON with CHP under N2 shows that they are similar to what was found for EHN. Impact of Organic Nitrate on Hydroperoxide. The previous discussions show conclusively that the presence of CHP does not alter the rate of the organic nitrate thermal reaction but does impact the products. GC/MS analysis, however, shows that the presence of EHN or ON accelerates the rate of loss of CHP. The rate of loss of CHP by itself at 170 °C in hexadecane under N2 (Figure 9) is a first-order reaction, in which the rate-determining step is homolytic fragmentation of the weak O-O bond to form alkoxy and hydroxyl radicals (Figure 9).
Figure 7. Rate constants for thermolysis of ON.
decreases. Somewhere between 3 and 2 equiv, the concentrations of carbonyl and nitro compounds become similar to what is found in the air case without CHP. The apparent slowing of the rate of thermal loss of EHN and the appearance of the 1632 cm-1 absorption begins to become significant between 3 and 4 equiv of CHP, as discussed. 3834
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Figure 8. Products of EHN thermolysis.
Figure 10. Rates of EHN thermolysis and concentration of CHP.
N2. The formation of carbonyl and nitro compounds is also drastically suppressed. The rate of EHN decomposition is unaffected. Products of the radical decomposition of CHP (R-methylstyrene, cumene, cumyl alcohol, and acetophenone) were found to be significantly less by GC/MS analysis in the reaction of EHN and CHP at 170 °C under N2 when catalytic amounts of Mo(DTC)2 are present. Phenol, a non-radical decomposition product of CHP, was a major product. The rate of CHP decomposition is greatly accelerated. The rate of thermal loss of EHN is no longer apparently slowed by IR and is the same as the thermal reaction of EHN under N2 as if no CHP were present at all (Figure 11). Zero time is the upper most IR line in Figure 11, and 180 min is the lower most IR line in Figure 11. This is because the CHP is rapidly decomposed to stable products by Mo(DTC)2 through the non-radical mechanism and the 1632 cm-1 absorption is not a factor in causing apparent slowing of the rate. The GC/MS analysis of these reactions supports the IR observations (Figure 12). Thermal reaction of CHP generates products such as cumyl alcohol, cumene, acetophenone, and R-methylstyrene (Figure 12). These four compounds are the result of a radical mechanism. They can be found as products in the reaction mixtures of all of the thermal reactions of EHN or ON when CHP is present under N2 in the absence of Mo(DTC)2. GC/ MS spectral data support the presence of these four products. The reaction of CHP with Mo(DTC)2 generates phenol and acetone, which are the results of the non-radical mechanism. Phenol can be found in the reaction mixture of EHN with CHP when Mo(DTC)2 is present. Acetone was not detected. This is probably because the volatility of acetone causes it to escape the vessel during the reaction.
Figure 9. Thermolysis of CHP.
The rate is measured as 1.98 h-1, and the relative error at the 95% confidence limit is 13.9% (Figure 10). When EHN or ON is present, the rate of loss of CHP is almost doubled. Because there is no change in the rate of loss of EHN or ON when CHP is present, the reaction that accelerates the loss of CHP must be due to the products of the thermal reactions of EHN or ON. NO2 formed in the thermolysis of the organic nitrate could be accelerating the thermal cleavage of the hydroperoxide. The radical-mediated reaction between NO2 and CHP is relatively well-known.6 Impact of Catalysts. The reaction of EHN with 5 equiv of a model hydroperoxide (CHP) under N2 was repeated at 170 °C as in past studies, but this time, a molybdenum (Mo) compound was added. A commercial additive called Mo(DTC)2 or Mo dithiocarbamate was added to the reaction at a catalytic level (5 mol %). Mo(DTC)2 has been studied in our laboratories in the past (unpublished proprietary results) and has been shown to be a very active catalyst for the nonradical conversion of CHP to stable products. When catalytic amounts of Mo(DTC)2 are added to the reaction of EHN and CHP at 170 °C under N2, the rate of EHN loss is not changed from the base case (EHN at 170 °C under N2). This was verified by both IR and GC/MS measurements. The rate of loss of CHP is accelerated significantly. The total CHP charge was completely reacted before the reaction mixture reached 170 °C. Mo(DTC)2 suppresses the formation of the 1632 cm-1 absorption in the reaction of EHN and CHP at 170 °C under 3835
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Figure 11. IR spectra of EHN, CHP, and Mo(DTC)2 at 170 °C.
Figure 12. GC/MS of EHN, CHP, and Mo(DTC)2 at 170 °C.
Figure 13. IR of products of the reaction of EHN, CHP, and Mo(DTC)2.
show the growth products of organic nitrate thermolysis in nitrogen with 5 equiv of CHP. The lower most IR lines in Figure 13 show the growth products of organic nitrate thermolysis in nitrogen and nitrogen with Mo(DTC)2 catalyst. The middle IR lines in Figure 13 show the growth products of organic nitrate thermolysis in air. The addition of 5 mol % of iron(III) acetyl acetonate [Fe(AcAc)3] to EHN and CHP at 170 °C under N2 provides more understanding of the reaction mechanism that gives rise to the 1632 cm-1 absorption. The IR spectra (Figure 14) show that very little if any of the 1632 cm-1 absorption is produced in the reaction. Also, the loss of EHN is accelerated from 0.746 to 1.27 h-1. This rate was verified by GC/ MS analysis and found to be 1.08 h-1. The values 1.27 and
The rate of CHP loss in the reaction with Mo(DTC)2 is very fast. CHP was not found in any of the samples taken from the reaction mixture, including the zero time sample. Apparently, Mo(DTC)2 catalyzes the non-radical reaction of CHP to phenol and acetone as the reaction mixture is heated from ambient temperature to 170 °C. IR analysis of the products of the reaction of Mo(DTC)2 with EHN and CHP at 170 °C (Figure 13) shows that the generation of carbonyl and nitro compounds is drastically suppressed by the presence of Mo(DTC)2. This is consistent with Mo(DTC)2 being a non-radical decomposer of CHP and an efficient radical-trapping molecule. These properties lead to the suppression of the 1632 cm-1, carbonyl, and nitro compound absorptions. The upper most IR lines in Figure 13 3836
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Figure 14. IR of products of the reaction of Fe(AcAc)3 with EHN and CHP.
1.08 h-1 are within 8% of each other. The rate constant for EHN at 170 °C under N2 averages 0.746 h-1. This is a 70% increase in the rate. Zero time is the upper most IR line in Figure 14, and 150 min is the lower most IR line in Figure 14. Catalytic amounts of Fe(AcAc)3 accelerate the rate even more (1.49 h-1) when CHP is not present. This is a 100% increase over 0.746 h-1. The mechanism of the EHN reaction with Fe(AcAc)3 is thought to be non-radical. The high amounts of carbonyl and low amounts of organic nitro compounds generated in this reaction are consistent with a non-radical reaction of EHN. The rate of loss of CHP is also greatly accelerated from 1.98 to 15.5 h-1 (Figure 14). This is an 8-fold acceleration in the rate of CHP decomposition by Fe(AcAc)3. The products of CHP radical decomposition were identified by GC/MS. There was no phenol found to indicate any participation of the non-radical decomposition mechanism for CHP when Fe(AcAc)3 is present. There are two possible explanations for the rate of Fe(AcAc)3 loss of 1.27 h-1 instead of 1.49 h-1. One is that the production of small amounts of the R,β-unsaturated carbonyls (IR, 1632 cm-1 absorption) may be enough to dampen the rate acceleration and limit it to 1.27 h-1 instead of the fully accelerated rate of 1.49 h-1. Second is some of Fe(AcAc)3 is consumed in the reaction with CHP. CHP and EHN compete for the reaction with Fe(AcAc)3. The low production of the 1632 cm-1 absorption is even more interesting. Catalytic amounts of Fe(AcAc)3 accelerate the loss of EHN through the non-radical mechanism, and the loss of CHP is accelerated through a radical mechanism. The results with EHN, CHP, Mo(DTC)2, and Fe(AcAc)3 suggest that, when EHN and CHP are reacting through radical mechanisms, their chemistries can interact to form R,β-unsaturated carbonyls and the growth of the 1632 cm-1 absorption in the IR. If either EHN, CHP, or both are reacting through non-radical mechanisms, R,β-unsaturated carbonyls are not
generated in significant amounts and the 1632 cm-1 absorption will not become a factor in the rate. Both EHN and CHP have to react via radical mechanisms for the 1632 cm-1 absorbance to appear. Copper(II) oleate [Cu(oleate)2] has been shown in our past research1 to rapidly and catalytically decompose CHP by the non-radical mechanism. The IR spectra of the reaction with catalytic amounts of Cu(oleate)2 added to EHN and CHP at 170 °C under N2 are shown in Figure 15. Zero time is the upper most IR line in Figure 15, and 120 min is the lower most IR line in Figure 15. The 1632 cm-1 absorption is not present. It is as if the CHP were not in the reaction. This is the same result observed with catalytic amounts of Mo(DTC)2 and Fe(AcAc)3, and the reasoning behind it is the same. The Cu(oleate)2 differs from the other two catalysts in that it catalyzes the non-radical decomposition of both CHP and EHN. The rate of the EHN reaction by IR is 1.50 h-1 (Figure 15), which is a little higher than the rate (1.07 h-1) of the reaction without CHP. That may be because less copper was used (5 versus 2.9 mol %). The rate of loss of EHN by GC/MS (Figure 15) is 1.58 h-1, which is well within experimental error of the IR measurement. The rate of CHP loss was too fast to measure in the presence of 5 mol % Cu(oleate)2 at 170 °C. Cumyl alcohol, from past research, is known to be the main product, which results from catalytic, non-radical decomposition of the CHP by Cu(oleate)2. IR shows that the high concentration of carbonyl generation and the low concentration of organic nitro compound generation are similar to what was found in past reactions of Cu(oleate)2 with EHN under N2 in the absence of CHP. The three different catalysts demonstrate the need for both EHN and CHP to decompose by radical mechanisms to generate R,βunsaturated carbonyls and the 1632 cm-1 IR absorbance. The mechanistic scheme in Figure 16 summarizes the various reaction pathways in the mechanism and their 3837
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Figure 15. Reaction of Cu(oleate)2 with EHN and CHP.
potential interaction with each other. The figure emphasizes the need for both EHN and CHP to decompose by radical mechanisms to generate R,β-unsaturated carbonyls. It is possible that carbonyls, such as ethyl hexyl aldehyde, created in the radical mechanism of the EHN reaction could be dehydrogenated by the radicals and/or products of the radical mechanism from the CHP reaction to form the R,β-unsaturated carbonyls. The left bars in Figure 16 refer to thermolysis performed in air; the middle bars in Figure 16 refer to thermolysis performed in nitrogen; and the right bars in Figure 16 refer to thermolysis performed in nitrogen with 5 equiv of CHP. The air reactions of the two catalysts that accelerate the loss of EHN in N2 and N2 with CHP present have been investigated in our laboratories. These catalysts still accelerate the loss of EHN in air just as they do in N2 in the presence of CHP, but air is much more effective at apparent slowing of the rates than CHP. It could be that CHP does not account for all of the generation of the 1632 cm-1 or, as mentioned previously, the natural hydroperoxides generated in air are more efficient at generation of R,β-unsaturated carbonyls and the 1632 cm-1 absorption than CHP. The effectiveness of CHP relative to air also appears to depend upon the catalyst. In the case of no catalyst, CHP apparently slows the rate almost as much as air but, with Fe(AcAc)3, CHP is much less effective than air. This is because Fe(AcAc)3 accelerates EHN loss by the non-radical mechanism. The radical mechanisms for both EHN and CHP are necessary to produce a significant generation of unsaturated carbonyls and the 1632 cm-1 absorption. The Cu(oleate)2 reaction has a higher rate in the presence of CHP under N2 than it does in air or N2 without CHP. This could be because the reaction with CHP present was performed with 5 mol % Cu(oleate)2 and the other was performed with 2.9 mol % Cu(oleate)2. The point is that CHP is completely ineffective in causing apparent slowing of the loss of EHN. CHP is completely reacted by the non-radical mechanism before the reaction mixture reaches 170 °C.
Figure 16. Mechanism of decomposition of EHN and CHP.
Summary and Conclusions The main conclusion of this paper is that hydroperoxides are responsible for the formation of the 1632 cm-1 peak seen in the IR during the thermal decomposition of organic nitrates in air. This IR absorption is due to the formation of unsaturated carbonyl compounds formed by oxidation reactions mediated by hydroperoxides. The IR absorption of organic 3838
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Francisco and Krylowski
-1
nitrates is 1638 cm , which is close to the IR absorption of the unsaturated carbonyls. Past work using IR gave lower rates of unimolecular thermal decomposition of organic nitrates when thermolysis was conducted in air, and the rates were measured by following the disappearance of organic nitrate by IR. The 1638 cm-1 absorption disappears over time during thermolysis in air but the 1632 cm-1 peak increases. Rates of the Reaction of EHN and with CHP. (1) EHN (170 °C, N2) generates an IR absorption at 1632 cm-1 when CHP is added (1-5 equiv). (2) IR shows that the rate of the thermal reaction of EHN (170 °C, N2) is apparently slowed when the concentration of CHP is g4 equiv. This is similar to the EHN reactions in air without CHP. (3) The rate of the EHN reaction (IR) is not changed at CHP concentrations ranging from 1-3 equiv. (4) GC/MS shows that the rate of the thermal reaction of EHN (170 °C, N2) is not slowed by the addition of CHP. (5) IR shows that the thermal reaction of ON (170 °C, N2) with 5 equiv of CHP is similar to EHN. The 1632 cm-1 product appears, and the rate is apparently slowed, similar to the reaction run in air. (6) GC/MS analysis, however, shows that the rate of the thermal reaction of ON at 170 °C under N2 is not slowed at any concentration of CHP (at 5 equiv CHP, k = 0.582 h-1). Products of the Reaction of EHN and with CHP. (1) IR shows that the reaction of EHN (170 °C, air) generates higher concentrations of carbonyl and organic nitro compounds than the reaction in N2. (2) The reaction of EHN (170 °C, N2) with CHP generates higher concentrations of carbonyl and nitro compounds than the reaction in air without CHP. When the CHP concentration is 2-3 equiv, the concentrations of carbonyl and nitro compounds are similar to the air reaction without CHP. (3) Major products were identified by GC/MS that are common to the thermal reaction of EHN at 170 °C in both air and N2. The concentration of major products appears to be higher in the air reactions. The complexity of both reactions prohibited the identification of many of the minor products. (4) Hydroperoxides were identified by GC/MS in the EHN reaction in air at 170 °C but not in N2. (5) Much higher concentrations of R,β-unsaturated carbonyls were identified by GC/MS analysis in the air reaction of EHN at 170 °C. The major IR absorbance of this class of carbonyls lies in the 1630 cm-1 region. R,β-Unsaturated carbonyls are the cause of the 1632 cm-1 product that appears in the EHN reaction at 170 °C in air or nitrogen when CHP is present. (6) Products of the radical decomposition of CHP (R-methylstyrene, cumene, cumyl alcohol, and acetophenone) were found by GC/MS analysis in the reaction of EHN and CHP at 170 °C under N2. Rates of the Reaction of EHN and with CHP and Catalysts. (1) When catalytic amounts of Mo(DTC)2 are added to the reaction of EHN and CHP at 170 °C under N2, the rate of EHN loss is not changed from the base case (EHN at 170 °C under N2). This was verified by both IR and GC/MS measurements. The rate of loss of CHP is accelerated signi-
ficantly. The total CHP charge was completely reacted before the reaction mixture reached 170 °C. (2) Catalytic amounts of Fe(AcAc)3 accelerate the non-radical reaction of EHN with CHP at 170 °C under N2 to 1.27 h-1 by IR measurement [70% increase over the reaction without Fe(AcAc)3] and 1.08 h-1 by GC/MS (IR and GC/MS within 8% of each other). (3) Catalytic Fe(AcAc)3 without CHP accelerates the reaction of EHN (170 °C, N2) by IR measurement. EHN and CHP compete for Fe(AcAc)3, reducing the impact on rate acceleration for the EHN reaction. Slight production of the 1632 cm-1 absorbance may apparently lower the IR measurement of the rate. (4) The rate of CHP loss by the radical mechanism (EHN at 170 °C, N2) is accelerated by Fe(AcAc)3. (5) Cu(oleate)2 accelerates the non-radical reactions of organic nitrates. The rate increases in the following way: under air < under N2 < under N2 in the presence of hydroperoxides. Hydroperoxide under N2 apparently slows the rate of loss of organic nitrate less efficiently than air when Cu(oleate)2 is present. The nonradical loss of CHP is accelerated to the point where the rate was not measurable under these conditions. Products of the Reaction of EHN and with CHP and Catalysts. (1) Mo(DTC)2 suppresses the formation of the 1632 cm-1 absorption in the reaction of EHN and CHP at 170 °C under N2. The formation of carbonyl and nitro compounds is also drastically suppressed. The rate of EHN decomposition is unaffected. (2) Products of the radical decomposition of CHP were found to be significantly less by GC/MS analysis in the reaction of EHN and CHP at 170 °C under N2 when catalytic amounts of Mo(DTC)2 are present. Phenol, a non-radical decomposition product of CHP, was a major product. The rate of CHP decomposition is greatly accelerated. (3) The growth of the 1632 cm-1 absorbance in the reaction of EHN and CHP at 170 °C under N2 is significantly less when catalytic amounts of Fe(AcAc)3 are present. (4) Carbonyl and nitro compounds are products of the reaction of EHN and CHP at 170 °C under N2 with catalytic amounts of Fe(AcAc)3. High concentrations of carbonyl compounds and low concentrations of nitro compounds are consistent with non-radical decomposition of EHN. (5) GC/MS analysis of the EHN and CHP reaction at 170 °C under N2 with catalytic amounts of Fe(AcAc)3 shows that only R-methylstyrene, acetophenone, cumene, and cumyl alcohol are the products. No phenol was detected. This is consistent with radical decomposition of CHP. (6) There is significant generation of carbonyl compounds and cumyl alcohol in the CHP reaction and little if any organic nitro compounds. This is an indication that Cu(oleate)2 decomposes CHP by the non-radical mechanism as expected. Acknowledgment. The authors extend a special thank you to Prof. A. R. Katritzky of the University of Florida in Gainesville for synthesis of some of the organic nitrates.
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