DE C OM P 0 SITIOS OF BUTYL ACE TAT E
357
(8) S E L ~R., OS A N,D WALTON, H. F . : J. Phys. Cheln. 48, 406 (1911).
(9) SCHUBCRT, J . : J. Phys. Colloid Chem. 52, 340 (1918). (10) T R C ~ D W EF. LL P., , .LSD HALL,T. W.:A n a l y t i c a l C h e m i s t r y , 9th edition, 5-01. 11, p. 403. John Wiley and Sons, Inc., Kew York (1942).
THE THEK3LIL DECOMPOSITIOT\; O F TERTIARY BUTYL ,lCETXTE1 CHARLES E. RUDY, JR., A N D PALL FCGASSI Department of Chemistry, Carnegie I n s t i t u t e of Technology, Pittsburgh, Pennsylvania Receiced A u g u s t 20, 1947
In R flow experiment carried out a t 360"C., Hurd and Blunck (6) showed that lert-butyl acetate pyrolyzed into isobutylene and acetic acid. Their distillation analysis indicated the absence of isomeric butylenes, and qualitative tests upon the products showed the absence of acetaldehyde, ketene, and acetic anhydride. As it appeared from the analytical data that the gaseous decomposition of tertbutyl acetate might have a simple mechanism, it has been investigated in greater detail because organic decompositions of this type are rare and further information upon the less complex reactions is desirable. In this paper the kinetics of the decomposition of tert-butyl acetate will be described. PREPARATIOK O F ESTER
The ester was prepared from tert-butyl alcohol and acetic anhydride, using the procedure of Sorris and Rigby (8). Eastman's "practical" acetic anhydride was distilled and a fraction separated boiling in the range 132"-137°C. The alcohol was an Eastman product and was purified by crystallization and distillation according to the procedure of Schultz and Kistiakomky (10). After the ester was stripped from the reacting mixture, the crude product \vas purified by distillation, using a 5-ft. vacuum-jacketed column with wire-spiral packing. After two distillations the refractive index of the distillate reached a constant value. The purified ester had a standard boiling point of 97.7"C., a specific gravity, d2:: of 0.8604, and a refractive index, n:", of 1.3837. The values cited are in close agreement with those given by Sorris and Rigby (8) and by Bryant and Smith (2). EXPERIMENTAL PHOCEDURE
The decompositiun of the ester n-as follon-ed in a constant-volume system by measuring the pressure at definite time intervals. -4 glass diaphragm gauge with Presented before the Division of Physical and Inorganic Chemistry at the 104th Meeting of the American Chemical Society, which was held in Buffalo, Sex\- Tork, September, 1942. This paper is abstracted from the thesis submitted by Charles E. Rudy, Jr., to the Faculty of the Graduate School of the Carnegie Institute of Technology in partial fulfillment of the requirements f o r the degree of Doctor of Science in Chemistry, April, 1942.
358
CHARLES E . RUDY, J R . , A?iD P.1UL FUGASSI
electrical contact., similar t o the one described by Daniels (A), n-as used for the pressure measurements. The reaction cell had a volume of about 60 cc. and was connected t o the XTacuum line with the shortest possible length of 2-mm. capillary tubing. The cell could be closed off from the vacuum line with a glass stopcock sealed n-ith Picein cement. The stopcock was equipped with an electrical heater so that the Picein cement could be melted n-hen it was necessary to manipulate the stopcock. It was found that, over periods of some hours, small amounts of the reaction products dissolved in the solid cement, so an arrangement was used by which a short column of mercury could be placed in the capillary tube leading t o the reaction cell after the cell had been filled with reactant. The column of mercury effectively shielded the product gases from the lubricant during the course of an experiment. I n the vacuum line mercury "cut-offs" were used throughout, so that the ester came in contact only with glass and mercury. The vacuum line and cell could be evacuated with a two-stage mercury diffusion pump backed b y a Hyvac oil pump. X mercury-vapor thermostat \vas employed for the constant-temperature bath. It was constructed of steel in the form of a Dewar flask. Mercury vapor was confined t o the annular space between the inner and outer cylinders. The inner cylinder was filled with a mixture of fused nitrites. Thermocouple readings taken a t various distances from the bottom of the inner cylinder indicated that the temperature was constant within 0.2" a t 360°C. over a distance of about 4 in., starting a t a point 1 in. above the bottom of the inner cylinder. -It the temperatures where experiments n-ere carried out, the temperature gradient would be still smaller. The reaction cell was located in the constant-temperature zone. calibrated, four-junction,copper-constantan thermocouple was placed alongside the cell and \vas used for measuring the temperature of the cell. The calibration of the thermocouple checked very closely over a range of 120" with the temperatures predicted from the known vapor pressures of mercury. The vapor thermostat was provided with a modified Coffin barostat (31, in which the mercury in the control manometer was replaced by ethylene glycol, and a separate mercury manometer was provided for reading the gas pressure over the boiling mercury HOMOGESEITI- O F THE REACTIOX
I n clean glass vessels the decomposition of the ester is heterogeneous and not reproducible. Hon-ever, if the products of the decomposition are allowed to remain in the cell for some time a t the temperature of the reaction, the glass surface becomes inert and measurements are reproducible. The period of time necessary t o inactivate the surface decreases markedly as the temperature is increased. -It 360°C. the surface is inert after being in contact with the reaction products overnight. Alt300°C. much longer times are required for surface deactivation. The surfaces of all cells used for taking the data given here werp deactivated a t 360°C. Similar effects have been reported for the decomposition of other tert-butyl compounds (10). The nature of the deactivation process is not known, but it appears probable that isobutylene forms a polymeric film on the surface. Inspection of a deactivated cell gave no evidence of a film, but
DECOMPOSITION O F BUTYL hCETATE
359
some evidence in favor of this assumption was obtained by means of the follon-ing experiment : T o a reaction cell 3.2 g. of 200-mesh Pyrex pori-der x-as added and a series of rate measurements Jvas made at 291°C. I n the first experiment the rate of decomposition n-as immeasurably fast. The reaction products n-ere allon-ed t o remain in the flask overnight, and the experiment was repeated the follon-ing day. With successive experiments the rate of decomposition decreased, until a t the seventh experiment the rate n-as approximately ten times faster than the normal rate. When the glass pon-der T T - ~ Sremoved from the cell, it was found t o be light bron-n in color indicating that some type of film had been formed. S o film as visible on the walls of the cell. In carrying out the experiments reported here, the surface was deactivated a t 360°C. and a series of experiments made a t lower temperatures. The surface was treated again a t 3GO"C., and check experiments were made at the lower temperature. This procedure n-as repeated several times, and excellent checks Tvere obtained. The fact that experimental ~-aluescould be closely reproduced, together n-ith the fact that a plot of log k us. l / T is a straight line over a wide temperature range, indicates that the reaction as studied is essentially homogeneous. ORDER O F THE REACTIOK
Within the accuracy of measurement the final pressure, p,, is twice the initial pressure, po, as obtained by extrapolating pressure readings back to zero time. The decomposition of the ester is first order, as indicated by the fact that the usual log ( p , - p t ) vs. time plot gave straight lines out t o 95 per cent decomposition. I n addition, as tabulated in table 1, the time of half-life is independent of the initial pressure, and the ratio of the time of three-quarters life t o the time of half-life is 2, as required by a first-order process. VELOCITT COSSTASTS . i S D E S E R G T O F ACTIVATIOX
Velocity constants were calculated graphically by preparing plots of log ( p , - p , ) us. time and measuring the slope of the best straight line through the experimental points. The values of k determined in this manner are tabulated in table 2. If the logarithms of the velocity constants listed in table 2 are plotted against
the reciprocal of the absolute temperature, a straight line is obtained whose equation is log k (see.-') = 13.342 -
-10,500 ___ 2.3RT
The energy of activation for the decomposition of tert-butyl acetate is accordingly 40,500 Gal. LOK-PRESSCRE E XPERIJIESTS
-1series of experiments were made in the vicinity of 290°C. in which low initial pressures of e3ter were used. The experimental data are shown in table 3. The
360
CHARLES E. RUDY, JR., hlUD P.IUL FUGASSI
TABLE 1 Times of fractional l i f e TEMPERATW E
PO
tl/2
"C.
mm. Hg
min.
303.1 303.3 303.5 291.2 292.1 279.6 278.3 269.0 256.2 243.6
127.2 102.5 117.7 23.9 158.1 236.8 79.5 195.3 208.3 274.1
1.18 1.27 1.23 2.32 2.33 4.63 5.72 11.57 26.30 74.50
1
,
t3l4
I
min.
I
2.40 2.42 2.43 4.97 4.83 9.67 11.52 23.60 54.20 155.30
2.04 1.91 1.98 2.14 2.07 2.09 2.02 2.04 2.06 2.08
TABLE 2 Velocity constants TEXPERATUXE O C
.
303.8 303.3 303.5 303.4 303.5 303.1 303.1 303.1 303.2 291.4 291.4 291.8 291.5 292.1 291.9 292.1 292.1 291.5 291.2 291.4 290.9 291.4 278.4 279.1 279.3
PO
mm. Hg
132.9 102.5 117.7 13.6 111.9 202.7 59.4 127.2 84.3 116.8 128.6 130.8 176.6 122.3 167.4 71.4 158.1 136.8 23.9 51.3 28.7 55.5 141.8 107.4 189.1
kX
103
sec.-l
9.33 9.40 9.60 9.56 9.76 9.31 9.44 9.43 9.34 4.40 4.39 4.61 4.47 4.40 4.69 4.80 4.59 4.59 4.44 4.50 4.43 4.70 1.960 1.953 1.989
TEMPERATURE
PO
"C.
mm. Hg
279.6 278.3 278.7 278.3 268.6 268.7 268.8 269.0 268.1 254.9 255.1 255.2 255.2 255.4 256.2 256.3 255.7 255.8 255.6 242.2 242.6 243.4 243.6 242.7
236.8 160.0 51.8 79.5 143.0 113.6 263.1 195.3 88.1 205.9 65.3 126.6 163.4 140.0 208.3 126.5 101.3 224.5 50.0 181.1 150 .o 146.6 274.1 172.6
kX
103
set;'
2.054 2.042 1.935 1.918 0.980 0.968 1.Ooo 0.975 0.962 0.357 0.371 0.386 0.392 0.391 0.413 0.415 0.412 0.412 0.383 0.145 0.138 0.148 0.147 0.151
velocity constants given in the last column of this table are the values givan in the third column corrected t o 291°C. The average value of the constants given compared t o the value 4.38 X obtained in the last column is 4.46 X
36 1
DECOJIPOSITIOS O F BUTYL .iCET.ITE
from the preceding equation. It is apparent that the value of h. is not decreasing with the pressure and that the pressure a t which falling off in li will be obtained is less than 3.8 mm., as would be expected for a molecule as complicated a s tert-butyl acetate. DISCUSSIOS
The analytical data of Hurcl and Hlunck (6'1, iogcthrr ivith thc fact that the pressure doubles, indicate that the decomposition of tert-butyl acetate occurs according to the equation
CH&OOC(CH,), and that whether producth pressure'
$ (('H3)d1=C"2
+ C&COOH
the equilibrium lies far t o the right. It is of inter& to detcrmine thermodynamic data would predict an equilibrium favoring the and insignificant dimerization of acetic acid at the temperatures and used in our experiments. TABLE 3 Low-pressure experimeiits k X 103
TEMPERATURE
"C.
291.40 291.45 290.20 290.75 290.80 291.20 290.80 291.25 291.30
mm.
sec.-l
set.-'
17.0 16.0 12.8 10.4 9.9 6.3 5.8 5.5 3.8
4.32 4.99 3.80 1.14 4.27 4.80 4.42 3.84 5.35
1.21 -1.86 4.03 4.51 4.33 4.72 4.48
3.77 5.27
Since the decomposition of the ester is an endothermic reaction, the equilibrium constant was calculated a t 525"K., which is approximately the lon-est temperature a t which rate measurements were made. Unfortunately, the free energy of formation of fert-butyl acetate is not known, so that approximations must be made. .In approximation based on the methods of Parks and IIuffman (9) yielded AFo at 525°K. = - 10,GOO cal. for the reaction as a-ritten above. - h o t h e r approximation, based in part on the free-energy equations of Bruins and Czarnecki (11, gives a t 525°K. A F o = -12,600 cal. -1s these values yield an equilibrium constant of about lo5. it is obvious that the reaction is essentially complete as written. For the dimerization of acetic acid there are available the data of MacDougall ( 7 ) and Fenton and Garner (5). -It 525°K. both sets of data predict approximately an equilibrium constant around 5 X lo4 for the dissociation of the dimer. Assuming that acetic acid vapor ia present at a partial pressure of 300 mm., which is higher than any pressure encountered in our data, the degree of dissociation of the dimer is around 0.98, a value which is sufficiently high t o indicate that dimerization would not affect the pressure readings.
362
CHARLES E. RUDY, JR., AKD PAUL FUGASSI
It appears that the polymerization or decomposition of isobutylene does not proceed at appreciable velocities under the conditions of our experiments. The thermal polymerization of isobutylene has been investigated b y Steacie and Shane ( l l ) ,and their data indicate that the rate of polymerization Tvould be very small a t the temperatures used here. The pyrolysis of isobutylene is knon-n t o occur a t temperatures higher than the temperature a t which polymerization is rapid. The decomposition of tert-butyl acetate is very likely a unimolecular reaction. The simplicity of the products an8 the low temperatures required for decomposition n-ould indicate that a chain mechanism involving free radicals is not an important part of the reaction mechanism. -1ssuming that the decomposition process is unimolecular, the conventional picture would visualize the approach of the carbonyl oxygen of the ester t o a hydrogen atom on one of the methyl groups, leading t o the formation of a cyclic six-membered ring Tvhich by electron shift splits into isobutylene and acetic acid. It is assumed that the ring is closed b y the formation of a hydrogen bond between carbon and oxygen. This type of hydrogen bond has not been recognized at room temperatures, and it appears unlikely that it u-ould exist at higher temperatures. d n equally valid visualization of the decomposition would have the ether oxygen approach a hydrogen atom on the methyl group, forming a cyclic four-membered ring which then decomposes. Because of strain energy the four-membered ring is apparently a less popular concept than the six-membered ring. On the basis of the transitionstate theory the formation of a cyclic ring should result in an abnormal frequency factor, but reference t o equation 1 shows that the frequency factor is normal for the decomposition of tert-butyl acetate. It seems unlikely that a cyclic intermediate is the explanation of the reaction mechanism. Ethyl acetate is reasonably stable at 290°C., while fert-butyl acetate decomposes rapidly at this temperature. At 550°C. Hurd and Blunck (6) found that the main products of the decomposition of ethyl acetate were ethylene and acetic acid, but that small amounts of acetaldehyde, formaldehyde, methane, hydrogen, carbon monoxide, and ketene were present. The complexity of the products suggests that a complicated mechanism is operative in the decomposition of ethyl acetate. The difference in the behavior of ethyl acetate and tertbutyl acetate is paralleled by ethyl bromide and tert-butyl bromide. Ethyl bromide decomposes, entirely or in part, by a bromine atom chain, Tvhile tert'butyl bromide decomposes in a unimolecular process. The substitution of tn-o hydrogen atoms for two methyl groups in tert-butyl acetate or tert-butyl bromide gives a molecule of greater stability. One feels intuitively that the difference in inductive effects between tn-o methyl groups and tT1-o hydrogen atoms is not sufficiently great t o account for the observed difference in stability. TThile the lack of a chain mechanism in the decomposition of tert-butyl compounds might be explained by the assumption that isobutylene reacts readily with free radicals and inhibits the chain, such an assumption does not explain why the unimolecular process in the decomposition of tert-butyl compounds should have a lower energy of activation than the chain process in the decomposition of thc corresponding
363
DISPERSIOSS OF LE.1D MOSOXIDE IS S T L E S E
ethyl compound. It appears that iome peculiarity accompanies the tert-butyl grouping, hut the nature of this peculiarity is not apparent. 5lX\l.\RY
1 . The thermal decompo4tion of tert-butyl acetate into isobutylene and acetic acid has been studied by a static method in the temperature range 243-303°C. and at pressures from 5 t o 2T5 mm. 2 . The reaction in treated glass flask.; i q hoinogeneou. and has an eneYgy of activation of 40,500 cal. 3 . The variation of the velocity constant, k . ivith absolute temperature, I',is given by the equation:
log k (set.-')
=
13.342 -
40,500 2.3RT ~
4. It is suggested that the decomposition is a unimolecular process. REFERESCES ( l j BRCISYASL)CZARXECKI: Ind. Eng. Chciii. 33, 201 (1941). (2) BRYAXT .*XI)SMITH:J. Ani. Chem. S O C .58, 1014 (19361. J. Am. Chem. SOC.55, 3646 (1933). (3) COFFIK: (4) DANIELS:J. Am. Cheni. SOC.50, 1115 (1928). (5) FESTOSASD GARXER:J . Ani. Cheni. SOC.60, 771 (1'338). (6) HCRDA K D BLUXCK:. J. .Im. Chem. SOC.60, 2419 (1938). (7) MACDOCGALL: J. Am. Cheni. SOP.58, 2585 (1936). ASD RIGBY:J. Ani. Chem. Soc. 54, 2088 (1932). (8) SORRIS (9) PARKS . ~ N DHUFFMAN : The Free Energies of Some Orgcoaic C o m p o u n d s . Catalog Company, In?., S e n York (1932). (10)SCHULTZ ASD KISTIAKOWSKY: J. r\m. Chem. SOC.56, 395 (1934).
The ChemicaI
(11) STEACIEA N D S H . ~ N ECan. : J. Research 16B, 210 (1938).
THE EFFECT OF SURFACE-ACTIVEAIGESTSUPON DISPERSIOKS OF LEAD MOXOXIDE I S XYLENE V. R. DAMERELL
AND
11. J. I'OGT
Department of C h e m i s t r y , W e s t e r n Reserve U n i v e r s i t y , Cleveland, Ohio Received J u l y
$4,1947
The work herein described is a continuation of the study of the effect of surfaceactive agents upon the particle-size distribution of solids dispersed in xylene. I n this paper dispersions of finely divided lead monoxide (litharge) have been so investigated. CHEMICALS
The xylene was a 3Ierck reagent-quality product which was further purified by a fractional distillation over barium oxide, t o remove water. The fraction
