THERMAL STABILITY OF ORGANIC COMPOUNDS 1. B. J O H N S , E. A. M c E L H I L L , A N D J. 0 . S M I T H Monsanto Research Gorp., Everett, Mass.
The thermal stability of some simple types of organic structures was investigated to find base stock fluids stable to 550" C. Four types of test apparatus for determination of thermal stability of compounds are described. A comparison is made of stability temperatures found in the liquid phase and in the vapor phase. The data obtained allowed several general conclusions on structural requirements for high thermal stability: all bonds in the molecule have high dissociation energies; no easy paths to decomposition exist; the structure is stabilized b y resonance; and only substituents that enhance resonance stabilization are present. The most stable compounds were found principally in the following very stable classes of compounds: heterocyclics, aromatics, certain substituted aromatic compounds, perfluorinated ring compounds, and aromatic silanes.
SEARCH FOR BASE STOCK FLUIDS
stable to 550' C. for use in
A advanced aircraft and missiles led to a study in this labora-
tory of the thermal stability of a number of compounds representative of simple organic chemical structures. This approach was taken to determine which basic chemical structures have the required thermal stability for use as units in building molecules with the other necessary physical properties: high boiling point and long liquid range. S o systematic study of organic compounds to determine criteria for prediction of thermal stability has been attempted until very recently. Madison and Roberts (7) studied the pyrolysis of many aromatic and heterocyclic compounds at temperatures between 425' and 500' C. and correlated relative stability to molecular structure. Nine compounds tested by Madison and Roberts showed the same relative order of stability in this study. The recent Symposium on Lubrication under Extreme Conditions (9) examined the thermal stability of several types of organic compounds as a measure of their usefulness as lubricants a t elevated temperatures. T h e most extensive study was that of Blake and coworkers (7) who studied the thermal stability of several classes of organic compounds. Their test apparatus provided definite decomposition temperatures for compounds that decompose a t temperatures under their normal boiling points. Earlier thermal studies have principally been on thermal reactions for synthesis or for determination of bond dissociation energies. Comparison of stability from dissociation temperatures in these early studies is difficult because of the large variance in testing conditions affecting decomposition temperature (especially pressure) and the lack of any definition of decomposition temperature. Theoretical
The finite strength of chemical bonds puts a n upper limit on the vibrational energy that molecules may possess without bond rupture. Most organic compounds are limited in stability by having in their structures pathways for rearrangement of the atoms a t temperatures far below the temperatures required for 2
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
straight-forward bond rupture. Furthermore, according to the Boltzman law of energy distribution among molecules, a t any temperature a portion of the molecules will possess energy greater than the bond energy. Stability is not absolute but rather a matter of rate:
where A is the frequency factor related to the vibration frequency of a critical mode of vibration in the molecules and E is the excess energy which must be concentrated in the molecule to cause it to decompose. The rate of decomposition k. should be minimized to increase stability. This can be done by making A as small as possible and E as large as possible. Resonance stabilization has the effect of increasing E. Polybonding, in which an atom is bonded to more than one other atom in the molecule, has the effect of decreasing A . For example, when a collision occurs a t an atom bonded to more than one other atom, the energy of collision can be dissipated over more than one path, thus decreasing the possibility that any one bond will rupture. In complex structures such as the phthalocyanines, graphite, or alumina, even if one bond ruptures, the structure remains intact, and the bond may heal after the collision energy has dissipated. Bond healing is also possible in some simpler molecules. Repair of bonds is possible if the lifetimes of the fragments are long, and if the bond reforms before the fragments rearrange into decomposition products. Simple ionic compounds such as NaCl have this ability to repair bonds. An interesting example of a stable free radical is @ xylylene,
-
C H ? D= C H * ,
which is stable in the gas phase at low
pressure (70) and has been stored in solid @-xyleneat low temperature (44). Since E cannot be greater than the energy of dissociation of the weakest bond in the molecule, an estimate was made of the minimum value for E required for a thermally stable 550' C. fluid. Compounds containing alkyl chains have been found stable to about 350' C. (7), and the C-C bond dissociation energy is about 75 kcal. (8). T h e bond dissociation energy
required for a compound to be as stable a t 550' C. as a n alkyl chain is a t 350' C., assuming no change in the frequency factor A , can be found by solving this equation for E: 75,000
E
E = 99 kcal. I t may be possible to achieve thermal stability in a molecule containing a bond of lower dissociation energy only if the products of dissociation are thermally stable and of relatively low volatility so bond healing can occur, as in the phthalocyanines. Measurement of Thermal Stability
Four different methods were employed for the determination of decomposition temperatures reported here. They are designated by V, G, H, and D. Method V was used for measurements in the vapor phase a t 1 atm. on compounds having normal boiling points under 250' C. The other methods were used for measurement of decomposition temperatures of liquids and solids. Method G , which employs the microisoteniscope, was especially useful for very small samples (0.3 gram) of liquids or solids whose vapor pressures did not exceed about 1.5 atm. a t the temperature of decomposition. When applied to solids, the method gave the vapor pressure curves for the solid and liquid states, as well as the decomposition rates a t fixed temperatures. From these data the heats of sublimation and vaporization, the melting point and boiling point, and the rate constants for decomposition a t chosen temperatures could be calculated in the usual manner. Methods H and D, identical in principle, utilize high pressure isoteniscopes and are especially useful for compounds having high vapor pressures a t their decomposition temperatures. Method V. Measurements in the vapor phase were made in the recycle apparatus (Figure 1). The sample (5 to 10 grams) was placed in the 50-ml. pointed three-necked flask equipped with ground joints. This flask was attached to the tubular reactor of 967, silica glass or Monel, 1 inch in diameter and 18 inches long. A thermocouple well extended the entire length of the reactor. A return tube was connected by a ball joint to the top of the reactor. This tube carried a condenser, connections for the manometer and the gas buret, and then
passed through a side neck of the sample flask to return the condensate below the liquid level in the flask. T h e system was made in parts and was connected by ball and socket joints for ease in use and cleaning. When the Monel tube was used, a hypodermic syringe tube (with a cup in which a column of mercury was placed for a seal) was put in the lower part of the return tube to allow for the greater expansion of the Monel tube on heating. To the third neck of the flask was attached a small, closed funnel with a stopcock and a side arm connected to the gas buret tee (for Toepler pump) by capillary tubing. The apparatus was purged with nitrogen before each run. The sample was boiled into the reactor tube, which was kept a t the test temperature, condensed in the return tube, and recycled. During the run, the funnel stopcock was closed, and the gas buret stopcock was open to the return tube. The amount of fixed gas formed at a given temperature in a given time was measured. The gas evolved was measured by reading the change in volume in the buret after a run of about 1 hour. The volume readings were made before and after a run, with the sample in the flask cooled to a temperature where its vapor pressure was less than 2 mm. of Hg. A dry ice-cooled bath was used for the more volatile samples. At the end of the run, it was necessary to use Toepler pump action with the mercury buret to return all the sample to the flask. The reactor temperature was controlled by a IYheelco Model 402 regulator from a thermocouple a t the wall of the reactor. The temperature profile. read by a thermocouple in the reactor well, was flat within 25' C. for the middle 6 inches, and the temperature a t a given spot stayed within 2' C. during a n hour run. T h e 967, silica reactor was encased in copper tubing to obtain good heat distribution. Gas samples were taken from the buret a t the end to confirm decomposition by spectrometry or gas chromatography. The decomposition temperature was taken as the temperature a t which gas evolution was detectable. Method G. T h e microisoteniscope (Figure 2) consisted of a manometer system attached to a modified Bodenstein gage with
w 28MM. DIA.-
n MANOMETER RETURN TUBE
32CM.
I
GAS BURET
VoL.15-16 CC.+ -.
Figure 1. Recycle apparatus (method V) used for determination of thermal stability of volatile compounds in the vapor phase
u
Figure 2. Bodenstein gage used for detection of pressure change in microisoteniscope (method G) VOL. 1
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,
a n exceedingly thin glass Bourdon tube. A pressure difference of 2 mm. of Hg caused a 1-mm. pointer deflection. T h e position of the pointer with respect to the fixed index pointer could be read to less than 0.1 mm. with a cathetometer. The vapor pressure of the sample in the lower bulb was determined by adjusting the air pressure in the jacket until the movable pointer was brought to its zero position. The pressure in the jacket was read on a mercury manometer by means of a cathetometer to 0.1 mm. The probable instrumental error in measuring the vapor pressure of a compound did not exceed 0.2 mm. T h e sample was introduced through the side arm on the lower bulb. This side arm was then sealed to a glass tube connected to the vacuum system. Air was pumped simultaneously from both sides of the gage, and when a vacuum of 0.001 to 0.0001 mm. of H g had been obtained, the molten metal bath was raised with a screw jack until the gage was covered to about 1 inch above the top of the Bourdon tube. T h e temperature of the bath was increased until noticeable distillation or sublimation of the sample into the side arm began. The metal bath was then lowered and the side arm sealed off. The bath was then removed completely, the sample cooled to 25’ C., and the zero point of the gage determined while the vacuum in the jacket was held below 0,0001 mm. of Hg. The heating bath was raised again and the pressure in the jacket was adjusted to keep the pointer near the index as the temperature was raised. At intervals, accurate temperature and pressure readings were made. Pyrolysis rates were determined a t two or more temperatures for each compound by observing the increase of pressure a t constant temperature over intervals of time. T h e method used and the assumptions made in calculation of the rate constants are described later. Method H. The first high pressure isoteniscope designed (Figure 3) was used for most of the tests under pressure. I t has the advantage of being very simple to assemble. The apparatus consisted of a stainless steel pipe (capacity about 12 ml.) capped at one end and connected to a Heise precision pressure gage, reading zero to 2000 p s i . , by stainless steel capillary tubing filled with mercury. The gage was graduated to 2 p.s.i. and had a maximum hysteresis of 2 p.s.i. There was a rupture disk on a tee in the line.
The connecting tubing was filled with mercury by evacuating the system a t both ends to 0.0001 mm. of Hg and filling to the bottom of the sample pipe. The mercury-filled line kept the samples from distilling into the gage and from plugging the connecting line (in the case of solid samples). The sample pipe was heated by an oven made from an insulated brass block containing six heating elements, two of which were connected through a Wheelco Model 402 temperature regulator for temperature control. Temperatures were read by thermocouples in the oven and a t the bottom of the reactor. T h e sample (3 to 5 grams) was charged to the pipe, the pipe flushed with nitrogen, and the cap screwed on. T h e oven was lowered on the pipe to just above the mercury level, and the pipe was heated to the desired temperature. Thirty to 45 minutes were required for equilibration, after which the change in pressure with time was recorded to obtain the rate of decomposition a t the given temperature. The isoteniscope was calibrated to determine the true temperature by measurement of the vapor pressure of mercury a t several temperatures. Temperature profiles taken a t several temperatures were found to have a small temperature range for the bulk of the reactor (9’ C. for the middle six inches a t 530’ (2.). Method D. The high pressure isoteniscope was a n improved design (Figure 4) that eliminated the use of mercury for a containing fluid and gave a closer temperature profile of the sample chamber. The over-all dimensions were approximately 5 5 / 8 X 2.5 inches. The sample chamber had a capacity of about 10 ml., and sample weights of 2 to 4 grams were used. The metal diaphragm was designed to deform elastically with pressure up to 100 p.s.i.g. One apparatus was fabricated of Monel with a 0.005-inch-thick disk. The isoteniscope has also been fabricated of soft steel. The threads on the plugs were 20 per inch to ensure a tight fit and to avoid leaks. The sample was put in the test chamber under nitrogen. One side of the chamber was closed by a metal diaphragm. A pressure increase in the chamber caused the diaphragm to extend until it touched a needle on the other side, thus closing an electric circuit (indicated by a microammeter). Kitrogen was introduced into the upper part of the bomb to return the diaphragm to its original position. The pressure of the nitrogen was read on a Heise gage when the electrical contact opened. The temperature of the sample was measured by a thermocouple in a well extending into the test chamber. The decomposition point was obtained, as with the other high
rQUARTZ
r
-CERAMIC
UNE AND GAUGE FILLED WITH MERCURY
1
SOAPSTONE
I
I
DISK
1r SAMPLE SPACE
/ i
MERMOCWPLE
7 __-
I RUPTURE DISK
METAL DIAPHRAGM
LaoGEN INLET
Figure 3. Simplest type of high pressure isoteniscope (method H) consists of closed pipe attached to pressure gage by Hg-filled line 4
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
N GP R l!! ‘
ELECTRIC CONTACT
Figure 4. Diaphragm-type high pressure isoteniscope (method D) eliminates necessity for mercury in apparatus
pressure isoteniscope in method H, from the rate of pressure increase a t a fixed temperature.
Results and Discussion Decomposition Temperature and Effect of Pressure. I n this work, decomposition temperature is defined as that temperature a t which the compound decomposes a t the rate of 1 mole % per hour, except for values obtained by measurement in the vapor phase (method V) where the decomposition temperature is the temperature a t which gas evolution is first detected. Decomposition is observed by the increase in pressure a t constant temperature. Its rate is calculated from this pressure increase, assuming that each molecule of gas is formed by the decomposition of one molecule of the initial compound. Secondary decomposition of the primary fragments becomes important as the extent of decomposition increases, but it is not considered important in the early stages of decomposition. Blake and coworkers (7) have discussed in detail the measurement of thermal decomposition points and estimated the magnitude of possible error arising from use of the above assumption (under 10' C.). T h e rate for the unimolecular reaction can be expressed by: - dn/dt = kn
(3)
where n is the number of moles of sample and dn is the number of moles of gaseous products formed in time dt, which is calculated from values of dp/dt. T h e rate of 1 mole % per hour corresponds to k = 1.66 X min.-' Decomposition temperatures of compounds tested by method G are in close agreement with values obtained by Blake. T h e decomposition temperature of triphenyl-s-triazine found to be 467' C. by method G compares well with 464' C. found by Blake ( I ) , although method G employed only I / ~ Oas large a sample. Decomposition temperatures of samples tested in the liquid phase a t low pressure (method G and Blake) and in the liquid phase a t high pressure (methods H and D) show some difference (Table I). T h e effect of pressure over the liquid o n decomposition point has not been studied. I t is not known whether pressure over the liquid sample would exert a detectable inhibiting action on formation of volatile decomposition products. Also, the materials of the apparatus, glass or metal, may affect the decomposition point of certain compounds. Since the rate of decomposition increases exponentially with temperature, the definition of decomposition point, T,, as the temperature a t which the rate of decomposition is 1 mole % per hour gives a practical measure of stability. It has been pointed out, however (5), that the relative order of stability of two compounds may not be the same if a different rate is chosen as the basis of comparison. T h e slopes of the curves for log d p / d T us. 1 / T depend on the activation energies for decomposition. T h e curves for two compounds being compared may cross in the temperature region of interest. Decomposition generally occurs a t a lower temperature in the liquid phase than in the vapor and a t a lower temperature in high pressure vapor than in low pressure vapor. This is probably due to the greater frequency of collision in the liquid phase and in the high pressure vapor than in the low pressure vapor. This effect may be considerable. If the decomposition rate depends directly on concentration of molecules, the rate for naphthalene a t 425' C. in the liquid phase should be about 200 times greater than in the vapor phase. Conversely, for the same rate of decomposition in the vapor phase as in the
Table 1.
Comparison of Decomposition Temperatures Decomposition Temp., C. Blake Compound (7) H D m-Bis(m-phenoxyphen0xy)benzene 456 47 4 466 p-Quaterphenyl 455 482 ... Tetraphenylsilane ... 482 482 Triphenylphosphine oxide 407 454 ...
liquid phase, the temperature of the vapor must be held considerably higher than that of the liquid. Table I1 shows the relative values for five compounds Correlation of Structure with Thermal Stability. Thermal stabilities measured by decomposition temperature of organic compounds in the following groups were obtained : aromatic and substituted aromatics; heterocyclic compounds containing oxygen, nitrogen, and/or sulfur ; compounds containing silicon, boron, and phosphorus; and ff uorinated compounds. Detailed data, including vapor pressure constants obtained on some of the compounds, are given in another article (6). Products of decomposition detected are summarized, although no attempt is made for complete analysis of products since they are numerous. A comparison of stabilities of individual compounds together with a n attempt to correlate relative thermal stabilities with chemical structure is detailed there. T h e general conclusions drawn from these data are given here. STABLECOMPOUNDS. I n general, when organic molecules decompose. a small fragment breaks off the molecule. Hvdrogen is the most common decomposition product. The most stable compounds were found to have certain characteristics : All bonds in the stable molecule have high dissociation energies. Perfluorination and perchlorination of aromatic rings increase stability. Partial fluorination, on the other hand, or addition of perfluoroalkyl side groups markedly decreases stability. This may, in part, be related to the ease of splitting-out of HF, thermodynamically favored because of its high heat of formation. This may occur by either a n inter- or intramolecular process. No easy paths to decomposition are present in stable compounds. A large proportion of organic molecules have "easy" paths to decomposition-e.g.. compounds where the loss of a mole of HP or HX intermolecularly leads to stable ring formation. Also, alkanes, polyphenyls, and polyphenyl ethers decompose by radical mechanisms a t activation energies lower than the bond energy of the weakest bond. T h e structure of stable compounds is stabilized by resonance. Resonance-stabilized rings usually remain intact when bonds to ring substituents rupture. T h e difference in resonance stabilization of hexaphenylborazole and hexaphenylphosphorazole is reflected in their relative stabilities. T h e stable molecule has substituents that enhance resonance stabilization of the basic structure without causing bond
Table II. Comparison of Decomposition Temperatures in the Vapor and liquid Phase Decomposition Temp.,. C. L7apor at Compound I atm. Liquid Quinoline 650 510-535 2,2 '-Bipyridine 620-650 482 2-Phenylimidazole 600 477 Naphthalene 62G-650 570b Biphenyl 510-540 537b a First gas evolution detected by volume increase in vapor phase tests and decomposition rate of 7 mole yoper hour in all other tests. b Temperatures are above critical temperatures for naphthalene, 480' C., and biphenyl, 528' C.
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weakening from electron-repelling effects or steric crowding. T h e difference in stability of triphenylamine and tri-2-pyridylamine may be due to electron repulsion weakening the NamineCDyridyl bond. Strained ring systems in general have low thermal stability.
ORDER OF STABILITY.T h e approximate order of stability of groups placed as connecting linkages between benzene rings, where R is phenyl, is: R
I
-0-,-N-,
R
1 -Si--, I R
CN
OR
I -Si--, I
-CH2--,
-C=C-,
I I
CN
OR
OR
--CF2--CF*-,
-C-,
I -Si-0-Si-, I OR
OR
I I
R -B-,
I
-CF2
OR
PERFLUORINATED RING COMPOUNDS.Stability of perfluorinated ring compounds was found significantly higher than that of the parent aromatic or acyclic compounds tested under the same conditions. Both C6F6 and crude cyclic C6F12 were found to be stable in the vapor phase a t 670’ C., the temperature limit of the test apparatus; C6F6CF3is reported (2) to be formed in the pyrolysis of the former near 835’ C. Partially fluorinated compounds are generally of lower stability, with HF as a frequent decomposition product. The high stability of cyclic ( 2 3 8 compared with its hydrocarbon analog is remarkable. Maximum stability of hydrocarbons is achieved in a c6 ring, but with fluorocarbons C 4 and C Srings show surprising stability, even without the factor of resonance stabilization. CONFIGURATION. Configuration is important in determining stability of aromatic and heterocyclic compounds. Molecules with the naphthalene and fluorene structures are exceptionally stable. The anthracene structure is less stable. Polyaromatics or heterocyclics decrease in stability with increase in ring linkages, but there is indication of a leveling off in stability a t three or four linkages. Position of substitution was found important in some cases; 1,l ’-binaphthalene is significantly less stable than 2,2 ’-binaphthalene because of the ease with which the 1,l ’-isomer decomposes to perylene. SILICONCOMPOUNDS. Compounds with the Si-Caromatio and Si-O-C,,,,,i, linkages are stable to the 465’ to 480’ C. range. Compounds containing Si-0-Si linkages attached to phenyl are less stable. BORON COMPOUNDS. The stability of boron compounds tested varied over a wide range. T h e only promising structure (495’ to 505’ C.) is represented by the ring-stabilized
6
I L E C PRODUCT RESEARCH A N D DEVELOPMENT
compound o-phenylene phenylboronate. Compounds containing B-0-B linkages are significantly less stable. PHOSPHORUS COMPOUNDS. No compounds tested containing phosphorus have been in the range of stability sought. T h e most stable compound was triphenylphosphine oxide. STABILITYABOVE 475’ C. Individual compounds found stable a t 475’ C. or higher in the condensed phase included naphthalene, biphenyl, dibenzothiophene, triphenylamine, diphenyl ether, 1,3,5-triphenylbenzene, triphenylene, 2,2 ’binaphthalene, hexachlorobenzene, decachlorobiphenyl, dibenzofuran, quinoline, 2-phenylnaphthalene, o-phenylene phenylboronate, diphenyldiphenoxysilane, tetraphenylsilane, 2-phenylimidazole, p-quaterphenyl, 6-phenylquinoline, 2,2 ’biquinoline, 2,2 ’-bipyridine, and 1,2,6-tripyridinobenzene. STABILITYABOVE 600’ C. Individual compounds found stable a t 600” C. or higher in the vapor phase included hexafluorobenzene, quinoline, 2,2 ’-bipyridine, pyridine, imidazole, perfluorocyclohexane, pyrimidine, naphthalene, thiophene, benzene, and 2-phenylimidazole. Acknowledgment
The authors gratefully acknowledge the assistance of J. W. Dale, who directed the work with fluorinated compounds; of H. R. DiPietro, who prepared many of the heterocyclic and phosphorus compounds; of G. J. O’Neill for preparation of many of the fluorinated compounds; of G. R. Wilson for the data on 2-phenyl-l,3-benzodiazaborole; and of H. F. Martin, Jeanette C. Alm, Shirley Liebman, and W. R. Smith for analysis and identification of decomposition products. literature Cited (1) Blake, E. S., Hammann, W. C., Edwards, J. W., Reichard, T. E., Ort, M. R., J . Chem. Eng. Data 6 , 87 (1961). ( 2 ) Desirant, Y., Bull. c l a m xi.:acad. roy. Belg. 41, 759 (1955). (3) Errede, L. A,, Hoyt, J. M., J . A m . Chem. SOC.82,436 (1960). 4) Errede, L. A,, Landram, B. F., Zbid., 79,4952 (1957).
5) Fisch, K. R., Wright Patterson Air Force Base, Ohio, private communication, 1961. ( 6 ) Johns, I. B., McElhill, E. A., Smith, J. O., “Thermal Stability of Some Organic Compounds,” J . Chem. Eng. Data 7, No. 2, in press. (7) Madison, J. J., Roberts, R. M., IND.ENC. CHEM.50, 237 (1958). (8) Steacie, E. W. R., “Atomic and Free Radical Reactions,” Reinhold, New York, 1954. (9) “Symposium on Lubrication under Extreme Conditions,” J . Chem. Eng. Data 6 , 76-160 (1961). (10) Szwarc, M., J . Polymer Sci. 6 , 319 (1951). RECEIVED November 18, 1960 ACCEPTEDJuly 13, 1961 Work supported in part by U. S. Air Force, Wright Air Development Division, under contract AF 33(616)-5553.
