I Determining.
W. R. DEASON, W. E. KOERNER, and R. H. MUNCH Organic Chemicals Division, Monsanto Chemical Co., St. Louis 77, Mo.
..
Maximum Heat Load in Equipment Design How to Find Heat Evolution Rates Here is a simple method for comparing thermal stability of various materials. When properly used, it can save time and trouble in scale-up
H i S T O R I C L L Y efforts to minimize the safety hazards accompanying the translation of a chemical process to a larger scale have involved scaling u p gradually and watching closely for rapid rates of self-heating which would imply the lack of sufficient heat-removal capacity. A small scale differential thermal analysis technique provides a simple method for comparing the thermal stabilities of various substances and measuring maximum heat evolution rates that can be used for engineering design. Previous differential thermal analysis apparatus used to study phase transitions (2, 3) and the recently described apparatus of Borchardt and Daniels (7) have usually involved facile heat interchange between the sample and the furnace. The low sensitivity is usually not important, because a sufficiently large temperature differential is still obtained when the entire sample is allolved to undergo the phase change or the chemical reaction in solution ( 7 ) goes to completion. Many samples of interest were liquids which had appreciable vapor pressures a t the temperatures where thermal decomposition was expected. To avoid sample loss and misleading thermal effects due to evaporation, the use of sealed cells was desirable. A sensitive apparatus was required to detect small rates of heat evolution before pressure build-up could rupture the sample tube. High sensitivity can be achieved by minimizing heat interchange between
sample and furnace through the use of a large air gap or solid insulation. With solid insulation, heat transfer between the furnace and sample is directly proportional to the first power of the temperature difference between them. Hence, the data obtained might be more amenable to unambiguous mathematical treatment. However, if small samples are dictated by other considerations, end effects would be large and require calibration or a more involved mathematical treatment. Secondly, the large heat capacity of solid insulation would make rapid cooling of samples at the end of the experiment impossible unless a complicated cooling system were used. A wide air gap for insulation permits the use of a simple furnace, and the sample can be cooled easily a t the end of the experiment. The furnace can be readily cleaned if the sample tube does rupture, a n advantage when toxic and noxious substances are studied. Minimal confinement of the sealed sample tube also lessens the hazards when a sample tube ruptures. Because heat transfer can take place by conduction, convection, and radiation, a simple mathematical treatment is not possible, but calibration of the apparatus provides a means for readily converting the data obtained to heat-evolution rates.
Apparatus The furnace consisted of a 15-inch
section of 3-inch steel pipe, A , with welded steel flanges, parallel 33- and 50-ohm heating elements, B, and magnesia insulation, C. A '/r-inch removable steel partition, D, was supported and
\
b/
, TO RECORDERS
i_
T2 VARIAC
A sensitive apparatus for thermal stability measurements gives data for designing large scale equipment VOL. 51,
NO. 9
SEPTEMBER 1959
997
2W V
POWER OFF A I R COOLING
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REFERENCE T U B E T E M P E RATURE(OC) I
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T I M E (HOURS)
Figure 1. Typical temperature differential data obtained for 2,4-dinitrotoluene were used as a basis for comparing samples
positioned by guide pins welded on opposite sides of the furnace tube. Circular wire clamps supported the sample tube, E, and reference tube, F, approximately '/4 inch away from the partition. Oversize asbestos paper disks, G, and 0.006inch aluminum disks, H , of the same diameter as the furnace minimized heat loss without preventing rapid escape of gases if sample tube rupture occurred. The furnace flanges rested on two firebricks to allow rapid venting if a sample tube burst. No hazardous results have been experienced in cases where the sample tube burst. An air jet, J , located below the sample tube cooled the sample rapidly a t the end of an experiment. .4ir cooling stopped many exothermic reactions before the sample tube burst. The expendable sample tubes were made from 22-mm. borosilicate glass tubing and were approximately 5 5 mm. long. A 6-mm. re-entrant thermocouple well extended to within approximately 8 mm. of the bottom. The 8-mm. diameter side a r m permitted rapid filling of the tube and was sealed off after the sample was added. The voltage applied to the furnace heater was increased linearly with time by rotating a variable transformer shaft 0.001 r.p.m. with a suitable motor drive. The heating rate varied from about 0.5' per minute a t 100' C. to about 1' a t 300" C. The rate of temperature rise was always essentially linear over the narrow range of interest for calculations. T h e temperature difference between the furnace wall and the sample tube wall over this temperature range was essentially constant and approximately 11.5' C. Under these conditions the temperature difference between the sample-
998
tube 3 O
\\
c.
all and the thermocouple well was
The thermocouples used \vere made of Leeds Br Sorthrup No. 30 B Br S gage iron and Constantan wire. The cloth insulation on that portion of the thermocouple wires inside the furnace was replaced with small tubing made of Teflon (tetrafluoro-ethylene polymer, Surprenant hlanufacturing Co.). Any recorder having a suitable range may be used for recording the sample temperature. A strip chart recorder of 1-mv. range having an offset zero to record -0.5 to +0.5 mv. is recommended for recording the differential temperature. -4 fast response time is not required.
Method The sample, either powder or liquid, was introduced into a previously weighed sample tube with the aid of a smallstemmed funnel. The tube was filled about three fourths full. This provided air space for pressure build-up, if gases were evolved on heating, and permitted the side a r m to be sealed off without subjecting the sample to high temperatures. The thermocouple inserted in a thin glass electrical insulation sheath and one junction of the differential thermocouple were placed in the thermocouple well of the sample tube and wedged in place with a small piece of asbestos. The other junction of the differential thermocouple was placed similarly in the reference tube. The reference material was chosen to match the sample fairly closely in specific heat and thermal conductivity. Polychlorinated biphenyls (produced by Monsanto Chemical Co.) were used for most samples which were
INDUSTRIAL AND ENGINEERING CHEMISTRY
liquid at the decomposition temperature, Ivhile po\vdered alumina was used for solid samples. Other liquid references were water, sulfuric acid, and dioctyl phthalate. A reference material should be thermally stable and must not undergo phase changes over the temperature range being studied. The sample and reference tubes were placed in the wire clamps on the steel partition, which was then inserted in the furnace, and the opening was closed with the asbestos and aluminum disks. An initial period of 45 seconds in which 110 volts were applied to the furnace created a temperature gradient between the furnace and the samples. The furnace was then switched to the motordriven variable transformer which was always preset to 15 volts. Experiments up to 350' C. required about 7 hours. Even \\ hen no phase changes or chemical reactions were occurring, the differential temperature was rarely obxrved to be zero, because of differences in the total heat capacities and thermal conductivities of the samples and sample rubes. In most cases the differential temperature remained constant or changed only gradually, except when endothermic or exothermic reactions were occurring. \\'hen the differential temperature was increasing so rapidly that the error in measuring the slope of the curve was large, the furnace was turned off and the sample was air-cooled. Occasionally sample tubes burst when they were heated to temperatures considerably above the boiling point of the sample or when large amounts of gaseous reaction products were produced.
Theory From the temperature differential data (Figure 1) the temperature a t which heat evolution from the sample was detected could be chosen fairly well and used as a basis for comparison of samples, or sample stability could be characterized by the temperature required to produce a given temperature differential between the sample and the reference. Both methods give so-called decomposition temperatures which are obviously a function of the apparatus used to measure them, but they will permit samples to be arranged in order of stability, subject to the limitation that the specific heat of the samples being compared is similar. This limitation is of no consequence if batch-to-batch variations in a single product are being studied, and the similarity in heat capacity on a volume basis of many organic materials permits wider comparisons to be made with a good degree of confidence. The basic equation presented by Borchardt and Daniels ( 7 ) can be utilized as a first approximation to describe the relationships existing in this apparatus,
HEAT L O A D DETERMINATION Table 1.
Point KO.
1 2 3 4 5 6
7 8 9
Calculation of Heat Evolution Values for 2,4-Dinitrotoluene (Detailed experimental data from a typical experiment) k h T,
Temp., O
d(AT)/dt, hlv./Min. 0.00050 0.00077 0.00118 0.00165 0.00232 0.00343 0.00473 0.00651 0.01046
c.
208.5 213.0 217.6 221.8 226.0 230.8 235.7 240.7 246.1
Cp d(AT)/dt, Cal./Mm./Kg. 6.3
9.7 14.5 21.5 29.2 43.2 59.5 81.9 131.6
recognizing that a temperature gradient does exist in the sample and reference cells, and heat transfer in the system probably is not proportional exactly to the first power of the temperature differential as implied in the khT term. In considering this equation dH - = Cp dt
'G)+
khT, Cal./Min. 0.15 0.17 0.25 0.31 0.47 0.62 0.88 1.21 1.69
Cal./Min./-
Cal./Mm./Kg.
11.7 13.3 19.5 24.2 36.7 48.5 68.8 94.6 132.0
18.0 23.0 34.3 45.7 65.9 91.7 128.3 176.3 263.6
k1T term reveals that it is the amount of heat that fails to be transferred from the furnace wall to the sample as a consequence of the decreasing temperature differential between the furnace wall and the sample with increasing AT. Thus if the heating rate is linear with time, the following relation exists
kAT
it was originally hoped that the heat transfer constant, k , would be so small that the k A T term (where A T is the temperature difference between the sample and reference) could be disregarded. By drawing tangents to the differential temperature curve shown in Figure 1 the data shown in the third column of Table I were obtained. The factor for converting the slope of the differential curve, d(l T)/dt, from millivolts per minute to Cp d(AT)dt in calories per minute per kilogram of sample as shown in column four of Table I was calcul3ted from the follolving formula. Using a specific heat of 0.2 for the 15.5gram sample tube, a specific heat of 0.45 for the 12.8-gram sample, and thermocouple e.m.f. relationship of 18.2' C./mv., a factor, f. of 12.575 cal./mv./kg. was calculated. The logarithms of the heatevolution rates given in the fourth column (Table I) are plotted as open circles iw. the reciprocals of the absolute temperatures in Figure 2. If a single decomposition mechanism predominates, the rate of heat evolution will be directly proportional to the rate of decomposition. Use of the D T A apparatus described to detect a small amount of decomposition, particularly when the heat of decomposition is high-e.g., aromatic nitro compounds-results in very little decomposition during the period when the d ( l T ) dt points are measured. With the concentration of decomposable material essentially constant, the reaction rate constant is directly proportional to the rate of decomposition, and thus a n Arrhenius-type plot of the data is indicated. Consideration of the source of the
where dq./dt is the heat received by the sample per unit time from the furnace prior to the onset of a detectable rate of decomposition, dqJdt is the heat received by the sample per unit time a t each point chosen for calculation, A T is the temperature difference between the sample and the reference a t the point of calculation, and 2, - Tois the temperature difference between the furnace wall and the sample wall before the decomposition rate is detectable. The values for k A T shown in the fifth column (Table I) were calculated from a value of 11.78' C. for T , - Toobtained from a study of the temperature differences existing in the apparatus and a value of 7.75 calories per minute for dqo calculated by multiplying the heat capacity of the sample tube and contents by the average heating rate during the decomposition period (0.857' C. per minute). Converting these values to calories per minute per kilogram yields the numbers shown in the sixth column. Adding the values in the fourth and sixth columns yields values for dH/dt shown in the seventh column and plotted as the solid circles in Figure 2. Comparing the values in the fourth and sixth columns shows that the original hope that the k A T term would be small in comparison with the Cp d(1T)dt term was not realized. Columns four and six show further that excluding point KO.1, which was measured a t a point where there was the largest uncertainty in the value of d(AT)/ dt, the average of the ratios of k A T to Cp d(dT)/dt is 1.2 with a standard deviation of 0.12. Thus data for 2,4-dini-
(ht. cap. of sample -I- tube in cai./" C.) (g./kg.)( wt. of sample, g. = X (mv./min.)
cal./min./kg. =
dH/dt,
Kg.
O
trotoluene can be expressed approximately by
While it was certain that the inclusion of the k A T term yielded results more correct than those obtained using the Cp d(AT), dt term alone, actual calibration was desirable to check the adequacy of the assumption that the heat transfer was proportional to the first power of the temperature difference between the furnace wall and the sample tube.
Calibration Calibration of the apparatus involved simulation of exothermic reactions by increasing the rate of heat output from a n electrically heated sample tube as the furnace temperature was increased. Any factors overlooked in the previous treatment would automatically be taken into account by a method which closely simulated a n actual thermal decomposition determination. The calibration tube was of the same size and shape as a sample tube. T h e bifilar winding of No. 30 B 8r S gage Nichrome wire was of such diameter (16 mm.) that the heat capacities of the tube and contents were equally divided between the inside and the outside of the heater. Synthetic exothermic reactions were carried out as follows: A graph of the logarithm of heat evolution us. the reciprocal of the absolute temperature
lo'LI8
2 0 10-?iTOK
24
Figure 2. Thermal stability data for 2,4-dinitrotoluene
C./mv.) (mv./min.) -Empirical
VOL. 51, NO. 9
correction factor X Cp
SEPTEMBER 1959
d(AT) ~
dt
999
was drawn for the desired reaction. From this straight-line plot the calories of heat evolved a t various temperatures were recorded in tabular form. Knowing the specific heats and weights of the calibration tube and inert contents along with the resistance (5.5 ohms) of the heating coil, it was possible to calculate the amount of electrical current required to produce the desired number of calories of heat per minute a t the given temperatures. -4graph of current us. sample temperature was constructed from the calculated data and used as a sample heating schedule. -4s the temperature of the furnace rose, rheostats were manually adjusted to supply the designated amount of current to the heater in the calibration tube. The current supplied to the heater was increased a t 1O intervals of rise in sample temperature. These small increases in current made a t relatively short-time intervals compared to the thermal lag in our apparatus produced smooth, not discontinuous differential temperature curves. Values of Cp d ( A T ) / d t were calculated for each calibration experiment and plotted against the reciprocals of the absolute temperatures. I n most cases the Cpd(AT)/dt data plotted as a straight line parallel to the theoretical line, as in Figure 3. T h e three lines represent three different theoretical heat evolution curves from which sample heating schedules were determined. The solid line
t. q J
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TCB
6
AROCLOA
moClOR
3300 ~
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6500
I I-
=D3jTOKPr-
-
29
Figure 3. Calibration data for DTA apparatus -Typical
experiment
- - - Variations at lower slopes
. . .. . . h a i l
variation caused by changes in heating rate of furnace
A
Experimental values of Cp
d( AT)
-
df Data obtained with polychlorinated biphenyl 0 Data obtained with trichlarobenzenes 0 Variable transformer drive speed 33% faster than that normally used, AROCLOR is a registered trade-mark of Monsanto Chemical Co.
W
0
1000
is repremitative of a typical experiment. The experimental values of Cp d ( A T ) / d t were obtained using polychlorinated biphenyl in the electrically heated and reference tubes. An average correction factor of 1.8 was calculated for these data by dividing the theoretical heat evolution values by the experimental values a t the various temperatures. Similar experiments with the tubes filled with a n 80% 1,2,4-20% 1,2,3-trichlorobenzene blend gave correction factors of 2. At higher slopes duplicate results were obtained using either liquid. The correction factors obtained using polychlorinated biphenyl varied from 2.4 to 3.8 because the experimental points were not on a line parallel to the theoretical line. With the trichloro-benzenes the factor varied from 3.5 to 4.2. In general, when the lines were not parallel the average value was used as the experimentally determined correction factor. The correction factors calculated for these results were 1.8 and 2.0. The experimentally determined correction factors varied with temperature as well as with the slopes of the logarithm of heat evolution us. reciprocal temperature plots. Several series of experiments involving different slopes were made a t various temperature ranges. The correction factors were compared by taking the slopesof the experimental Cp d ( A T ) / d t us. l / T o K curves and plotting them against the correction factor. The results obtained over the 80° to 130' C., 130' to 190' C., and 240' to 300' C. temperature ranges are indicative of the family of curves shown in Figure 4. The correction factors vary from about 2 for the higher slopes to about 4 or 5 a t the lower slopes. depending on the temperature range. The lowest slopes encountered in practice were about 3500, while the highest were about 12,000 cal.1' C./'min./kg. The use of the correction factors may be illustrated for the experiment previously calculated. The slope of the line drawn through the experimental points in Figure 2 (broken line) is 8500 cal.,'°C./min./kg. From Figure 4 using the 190' to 240' C. curve, we find a correction factor of 2.3 is applicable. The solid line on Figure 2 shows the experimental line corrected by this factor. Interpolation is used when the range of measurable decomposition rates falls intermediate between the temperature ranges used in Calibration. A k A T value was also calculated assuming as before that heat transfer was proportional to the first power of the temperature difference involved. The sum of the k A T term and Cp d ( A T ) / d t did not always equal the known heat input, probably because of variation in heat transfer mechanisms and heating rate in different temperature ranges. An empirical multiplicative correction factor to be applied to the calculated heat evolution rate would still be needed, although
INDUSTRIAL AND ENGINEERING CHEMISTRY
4+-
I
CORRECTION 2 3 FACTOR 4
#
l
i
5
Figure 4. Correction factors from calibration of DTA apparatus - _ -Interpolation for use in 190' to 240' range
smaller in absolute magnitude than the factors shown in Figure 3. T h e absolute accuracy of the two methods of calculation would not be greatly different, as they are both equally dependent on the exactness with which the homogeneous heat evolution produced by a chemical reaction was simulated by the use of a resistance kvire heater in the calibration cell. The procedure involving only calculation of the Cp d ( A T ) / d t term and application of the empirical multiplicative correction factor is more convenient for routine use.
Conclusions The heat evolution data obtained by this method give the maximum heat load to be considered in the designing of equipment. The calibration factors are sufficiently reliable for all practical applications of heat evolution data. When the results d o not yield a straightline plot, rerunning of the same sample will often show whether the original results were caused by trace impurities or whether a complex decomposition mechanism is indicated. Autocatalysis is readily detected by rerunning the same sample. Increased heat evolution rates at comparable temperatures are evidence of autocatalysis. The method cannot be used when phase changes occur a t temperatures close to the decomposition temperature. T h e slopes of the heat evolution curves are characteristic of the reactions taking place. Heats of reaction could be calculated if additional knowledge concerning the amount of material reacted could be obtained. A systematic study of the thermal stabilities of nitro compounds has been completed, in which attempts were made to correlate the effects of ring substituents on the thermal stabilities of substituted nitrobenzenes.
HEAT L O A D DETERMINATION
Effects of Ring Substituents and Added Contaminants on Nitrobenzenes
THE
present work is concerned with the factors that influence the stability of nitro compounds and the nature of the decomposition reactions. The study was restricted to pure compounds, because they offered the greatest hope for simple interpretation. The compounds selected afforded an opportunity for studying the isomeric effect of ring substituents on the thermal stabilities of substituted nitrobenzenes. The procedure described above was rigidly adhered to in obtaining the results reported. The inert reference used in all experiments were the polychlorinated biphenyls. KO difference was observed when these were interchanged. Thermal Stabilities of Pure Nitro Compounds
The nitro compounds studied were chosen to include representative substituted aromatic compounds. Semilogarithmic plots of heat evolution rates us. the reciprocal of the absolute temperature are shown in the accompanying figures. These data are summarized in Table 11. The third column gives the heat of activation for each decomposition reaction as calculated from the slope of the heat evolution rate plots. The last column gives the temperature at which each compound evolves heat a t the rate of 0.2 calorie per minute per kilogram of sample. This rate of heat evolution i s the lower limit of detectability of the apparatus described and provides a convenient, arbitrary comparison temperature. The simplest compound studied, nitrobenzene, was stable to a t least 300' C. No attempt was made to obtain data a t higher temperatures because of its high vapor pressure. (The sample tube was sealed under vacuum to minimize the total pressure in the sample tube.) The heat of activation for m-dinitrobenzene, 107,500 cal. per mole, was the highest observed in this study. The data show the meta isomer to be the least stable of the three mononitroanilines and the para isomer most stable. A comparison of these data with those given in Figure 5,A, indicates the nitroaniline isomers are even less stable than 1,3,5-trinitrobenzene. The nitrochloro-
benzenes \+'ere the most stable of the nitro compounds studied. The mononitrochlorobenzenes are more stable than m-dinitrobenzene and approach the stability of nitrobenzene (for which no data were obtained). The amino group decreases the stability of the nitrobenzene nucleus, whereas the substituted chlorine causes no added instability. A second substituent group which decreases the stability of the nitrobenzene nucleus is the hydroxyl group. The stabilities of the mononitrophenol isomers vary widely, as shown in Figure 5,D, and in contrast to the nitroanilines, the most stable of the isomers is m-nitrophenol while p-nitrophenol is the least stable. Another surprising fact is the similarity of the stabilities of 2,4-dinitrophenol and 2,4,6-trinitrophenol. I t was expected that the trinitrophenol would be much less stable. (The heat evolution data plotted for 2,4,6-trinitrophenol were obtained for a sample which con-
tained 6% water. A sample dried to 0.257, water gave similar results.) The last group of pure compounds studicd were the nitrotoluenes (Figure 5 . E ) . Considering the decomposition temperature ranges, their stabilities are comparable to the nitrophenols; here again rnnitrotoluene is the most stable, but the p-nitrotoluene is more stable than the o - nitrotoluene. 2.4.6 -Trinitrotoluene was not studied. Effect of Contaminants on Thermal Stability of Nitro Compounds
Thermal stability problems frequently involve the stability of still residues. All contaminants with low volatility in the still feed are concentrated in the still residue along with any high boiling decomposition products from the distillation. Contaminants which might reasonably be expected in still residues from nitro compounds are more highly nitrated
Table 11. Thermal Stabilities of Pure Nitro Compounds (No uniform pattern of isomeric stability exists)
Sample
Fig. No.
Nitrobenzene
Heat of Activation
Temp. for Heat Evolution Rate of
Cal./Mole
0.2 Cal./Min.jKg.
...
Monoa m-Dib 1,3,5-TriC
5, A
5, A
c.
No decompn. at 300' C. 107,500 56,300
275 232
41,700 22,450 59,500
190
Nitroanilines 0-d
m-= P-d
108 218
Nitrochlorobenzenes 5,
0-d
m-c
c
P-d 2,4 DLd
80,400 80,400 80,400 83,400
278
56,100 63,900 45,900 60,600 62,000
184 220 166 158 156
49,800 75,400 52,800 38,900 59,200
197 239 220 162 189
300 300 263
Nitrophenols 0-b
m-< P-b 2,4 DLC 2,4,6
Nitrotoluenes 0-b
5, E
m -c
5, E 59 E
Pf 2,4
5,
Mixed DLd
5, E
0
Redistilled.
Fisher.
E
Eastman.
Monsanto.
e
Merck.
VOL. 51, NO. 9
f
Du Pont.
SEPTEMBER 1959
1001
1
1
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7
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1.9
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21
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,250,
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2.3
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T'C ,200,
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io3 ,?,
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1
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0 3 1 ~ 0 ~
Pc. , ,
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T°C IS
, 200
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io3,
* With C,H,,CHO
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1.9
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2.3
17
I03/ToK
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2.1
19
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23
-I
IO L 19
1O3/T0K
~
il
213 IO3/ToK
*
\
Figure 5. Thermal stabilities of pure nitro compounds A. 6.
Nitrobenzenes Nitroanilines
C. D.
Nitrochlorobenzenes Nitrophenols
compounds, carbonaceous materials from decomposition reactions, and inorganic materials including metals, salts, acids, and bases. A mixture of monoand dinitrated material would be expected to have a thermal stability in between that of the pure compounds. Thus, excluding other contaminants, safety considerations should be based on the more highly nitrated compound. Organic contaminants, including carbon, would be expected to decrease the thermal stability of nitro compounds, if they are more readily oxidized than the compound itself. A study was made of the effect of carbon on the thermal stability of nitro compounds (Table 111). The heat evolution data obtained for samples to which 2070 powdered carbon
1002
E. F.
Nitrotoluenes p-Nitrophenol plus reducing compounds or inorganic contaminants
gas black (G. Elf, Fisher Scientific Co.) had been added are given in Figure 6 , A and B. In the case of o-nitrotoluene and p-nitrophenol no decrease in the thermal stability of samples containing 20% carbon was observed. These observations lead to the conclusion that these materials themselves are more readily oxidized than the carbon, which in this case served only as a n inert diluent. T h e pronounced effect of carbon on the thermal stability of p-nitrochlorobenzene is illustrated by the heat evolution data in Figure 6,C. The presence of 1% carbon in p-nitrochlorobenzene gives the same results as added 57, carbon. In Figure 6,C, the heat evolution values shown as solid lines were calculated on the total weight of sample-i.e.,
INDUSTRIAL AND ENGINEERING CHEMISTRY
weight of nitro compound plus the weight of the carbon-because this is the same calculation that must be made for carbonaceous still residues. The data were also calculated on the basis of the weight of the nitro compound for the samples of p-nitrochlorobenzene containing 20 and 507, carbon (dotted lines in Figure 6,C). p-Yitrophenol, which was shown to be more readily oxidized than carbon, was chosen for the tests concerned with the effects of easily oxidized organic materials and inorganic contaminants (Table 111). Two samples of Monsanto technical p-nitrophenol and one of Fisher C.P. material were tested (Figure 5 3 ) . The data for Monsanto sample B lay between those shown for Monsanto sample A and the Fisher material. The varia-
HEAT L O A D D E T E R M I N A T I O N T3 C
16'
I
21
19
17
I
1
23
IO3/ToK
Figure 6.
Effect of carbon on thermal stability
A. Nitrobenzenes, m-nitrophenol, and 2,A-dinitrochlorobenzene.
tion shown for these two samples is considered to be real and outside the limits of experimental error. Sample B was used for the remainder of the experiments listed in Table I11 for p-nitrophenol. Two readily oxidizable materials added to p-nitrophenol were 2-ethylhexanol and 2-ethylhexanal. The results (Figure 5,F) indicate that the aldehyde is more readily oxidized than the alcohol and, as predicted, the thermal stability of the p-nitrophenol is lowered correspondingly. The inorganic materials added to p-nitrophenol included powered aluminum, sodium nitrate, ferric oxide, powdered copper, and magnesium oxide. I n concentrations of 5Oj,, these materials had no effect on the thermal stability. Similar results were obtained for a sample to which 1% silver filings and 1% silver chloride were added. The data for samples containing 5Ye ferric chloride and 5% aluminum chloride are shown in Figure 5,F. These inorganic materials were selected as the ones which might possibly be found as contaminants originating from either the raw materials or the equipment used in the processes. The effect of strong acids on the thermal stability of p-nitrophenol was also determined. The presence of 1% sulfuric acid caused a pronounced decrease in stability (Figure 5 , F ) , whereas 1% nitric acid of 1% phosphoric acid had no detectable effect on the thermal stability.
Discussion The parent compound chosen as a comparison base was nitrobenzene. The -C1, -OH, -CHs, and -KHs groups were considered as substituents on the
Table 111.
B . 0-Nitrophenol and o-nitroaniline. C. p-Nitrochlorobenzene
Effects of Contaminants on Thermal Stabilities of Nitro Compounds Addition of carbon has most effect on most stable compounds
Sample
%
Cal./Mole
Temp. for Heat Evolution Rate of 0.2 Cal./Min/Kg., C.
20 20
22,900 38,200
157 176
C C C
20 20 20 20 20
C C
5
50,900 40,400 45,900 49,800 39,200 80, 400 36,200 36,200 35,600 25,950 45,150 46,200 47,200 38,900 44,400 65,000 55,050 55,050 46,550 46,200 46,200 46,200 46,200 46,200 46,200 46,200 46,200
171 178 166 197 186 300 207 207 200 157 168 167 166 150 117 154 138 138 138 167 167 167 167 167 167 167 167
Contaminant Compound C
Nitrobenzene o-Nitroaniline Nitrophenols
n
L
,.
0-
c;
m-
c
P o-Nitrotoluene 2,4-Dinitrochlorobenzene p-Nitroehlorobenzene
...
C
... ...
2-ethylhexan01 2-ethylhexanal AlCla FeClz FeClP ,6HzO
H
a
..
1
20 50
C p-Nitrophenol" p-Nitrophenolb p-NitrophenoP p-Nitrophenolb
Heat of Activation,
so4 (coned.)
.. .. ..
20 20 5 5
5
"Os (concd.) Hap04 (concd.)
1 1 1
A1 powder
5
Cu powder NaNOa FezOa MgO Ag powder AgCl
5 5 5 5
Monsanto technical, Sample A.
1
O
1
* Monsanto technical, Sample B.
benzene nucleus. The data are arranged in Table IV in a manner to facilitate the comparison of the various isomers of the substituted nitro compounds. The temperatures a t which 0.2 calorie per minute per kilogram was evolved, along
Fisher C.P.
with the heats of activation, are given as the bases for comparison. The compounds are arranged in the order of decreasing stability from left to right as based on the heats of activation of the ortho isomers. Any order chosen for the VOL. 51, NO. 9
SEPTEMBER 1959
1003
Table IV.
Effects of Substitution o n Thermal Stability of Nitrobenzene
Temperatures at which 0.2 col./min /kg. is evolved and heot of activation (cal /mole)
Iaomer Ortho
Temp. Cal./mole Meta Temp. Cal./mole Para
Temp. Cal./mole
Benzene
...
... ...
... ...
Aniline
278 80,400
184 56,100
197 49,800
190 41,700
300
220 63,900
239 75,400
22,450 59,500
80,400 300
166 45,900
220 52,800
107,500
263 83,400
158 60,600
162 38,900
232 56,300
... ...
156 62,000
...
275
nitrophenols, nitrotoluenes, and nitroanilines would show exceptions when the various isomers were considered. Nitrobenzene, which gave no evidence of decomposition when heated to 300’ C., was considered the most stable of the compounds studied. Comparing the temperatures for the ortho isomers, onitrochlorobenzene approaches nitrobenzene instability, but only small differences exist between the much less stable o-nitrophenol, o-nitrotoluene, and o-nitroaniline. The heats of activation decrease from left to right, indicating a decrease in thermal stability. Comparing both the temperatures and the heats of activation for the other isomers, we find an inversion with respect to rn-nitrophenol and m-nitrotoluene and to p nitrophenol and p-nitroaniline. .L\ possible explanation for the very low stability of rn-nitroaniline may be found on examination of the resonance structures of the nitrobenzene nucleus. The nitro group is an electron-attracting group and decreases the availability of electrons on the benzene ring. The availability of electrons is decreased most a t the ortho and para positions, leaving the metd position, by comparison, with a slight excess of electrons. In this respect, an m-amino group would be expected to be as active as a free amine. Suppoiedly the nitro group of one molecule reacts with the amino group of another, splitting out water to form the corresponding azoxy compound, which then could undergo further thermal decomposition. The low stability observed for p-nitrophenol might be expected if a stepwise decomposition of nitrophenol is considered. The most likely intermediate product of oxidation would be quinone. The quinone most readily formed is the p-quinone and secondly the o-quinone. The meta isomer is not formed. If a quinone-type structure is involved in the thermal decomposition of the nitrophenols, it would explain the low stability of the p-nitrophenol and the increased stability of the m-nitrophenol. The similarity of the data obtained for the
1 004
--
Toluene
80,400
I . .
Dinitro
Temp. Cal./mole Trinitro Temp. Cal./mole
Nitro Compound Chlorobenzene Phenol
...
108 218
...
... ... ...
dinitrophenol and the trinitrophenol is very surprising. It was expected that the 2:4,6-trinitrophenol would be much less stable than 2,4-dinitrophenol. The experiments performed on samples to which carbon had been added were an attempt to simulate still residues of the various materials. The instability of still residues becomes a serious problem when all the impurities are concentrated in a small volume and subjected to higher than usual temperatures in attempts to increase yields. The results of this study indicate that the most stable of the compounds are most highly affected by- the addition of carbon-the addition of carbon lowers the thermal stability of a compound only when the carbon itself is more readily oxidized than the compound. The rather unstable p-nitrophenol is a n example of the lack of this effect on the thermal stability. An example showing a marked decrease in stability is that of p-nitrochlorobenzene as shown in Figure 6,C. In this case? identical results were obtained for the addition of either 1 or 5yo carbon. The marked decrease in the heats of activation for the samples containing carbon is a n indication that a different type of decomposition reaction was taking place. Too few experiments were performed to give a clear picture of the effect of small carbon concentrations. The results obtained in the study of the effects of various contaminants on the stability of p-nitrophenol further substantiate the previous conclusions. The easily oxidized 2-ethylhexanol and the more easily oxidized 2-ethylhexanal have corresponding effects on the stability of the p-nitrophenol to which they were added. Both the aldehyde and the alcohol were demonstrated to be stable to temperatures above those for which their mixtures with p-nitrophenol were found to be unstable. The aluminum chloride and ferric chloride were expected to decrease the stability of p-nitrophenol because of their catalytic nature. Although the decomposition temperature was lowered,
INDUSTRIAL AND ENGINEERING CHEMISTRY
the heGitsof activation increased in these two cases. This is in itself proof that the reaction of the p-nitrophenol in the presence of these materials is different from the reaction of materials in the presence of carbon. The remainder of the inorganic contarninanfi listed previously had no measurable effect on the thermal stability of p-nitrophenol. The only one of the three strong acids which affected the thermal stability of p-nitrophenol was sulfuric acid. Temperaturewise, this lowering of stability was comparable to that objerved with ferric chloride. However, with the acid there was only a slight change in the heat of activation. The use of DTA for determining thermal stabilities still leaves much to be desired. The greatest need is for a method of determining the amount of material reacted a t a given time. Another is for a measure of the rate of pressure buildup. This additional information would facilitate the applicability of DTA to thermodynamic studies, as well as make it more valuable in the safety field. Conclusions
The nitro group acts as an oxidizing agent in the thermal decomposition of aromatic nitro compounds. The presence of easily oxidized substituents such as amino, hydroxyl, or methyl groups on the aromatic nucleus decreases the thermal stability of aromatic nitro compounds. Mixing an aromatic nitro compound with an easily oxidizable substance lowers the thermal stability of the nitro compound, provided the added material is more easily oxidized than the substituent groups on the benzene ring. The multiplicity of factors affecting the thermal stability of any given sample precludes accurate quantitative predictions of heat evolution rates. Acknowledgment
The authors are indebted to G. B. Kistiakowsky, Harvard University, for stimulating discussion during the development of the heat evolution method for determining thermal stabilities. Helpful discussions with L. B. Barkley, M. C. Freerks, and Q. E. Thompson concerning theoretical aspects of the relative stabilities of isomers are gratefully acknowledged. Literature Cited (1) Borchardt, H. J., Daniels, F., J . Am. Chem. SOC.79,.41 (1957). (2) Smothers, V V. J., Chiang, Y., “Differential Therrnal Analysis- Theory and Practice,” Chemical Publishing Co., New York, 1958. g, Y., Wilson, A,, “Bibliography of Uirt’erential Therm d Andy&,’’ Univ. of Arkansas, Research Series Publ. 2 i, November 1951.
RECEIVED for
review September 26, 1957
ACCEPTED May 4, 1959
