1314
EARL \TARRICK AND PAUL FUGASSI
RCFERCSCES (1) BRCNaCER, EMMETT, AND T E L L E R : J. Am. Chem. SoC. 60, 309 (1938). (2) BRUSAUER: S d s o r p t i o n of Gases a n d Vapors, Vol. I, p. 399. Princeton University Press, Princeton, Nery Jersey (1943). (3) COHAN:J. Am. Chem. SOC.60, 433 (1938). (4) COHAN:J. 4 m . Chem. Soc. 66, 98 (1944). (5) CONN.4ND CONTCLLY: Division of Petroleum Chemistry, 110th hfeeting of the American Chemical Society, Chicago, Illinois, Scptember, 1946, Preprint, p. 73. (6) EMYETT:Advances zn Colloid Sczence, Vol. I, p. 1. Interscience Publishers, Inc., Kew York (1912). ( 7 ) GREEXSFLLDER A N D VOGE:Ind. Eng. Chem. 37, 514 (1945). (S) HARKIKS AND J G R A :J. Am. Cheni. SOC.66, 919 (1944). (9) HARKINS A N D J U R A :Division of Petroleum Chemistry, September 1945 Meeting in Print, p. 115. (10) HARKINS A N D JERA: J. Am. Chem. Soc. 66, 1366 (1944). (11) HARVEY: J. Am. Chem. Soc. 65, 2343 (1943). (12) KISTLER,FISHER, A N D FREEMAS: J . Am. Chem. SOC65, 1909 (1943). (13) hlcBAm: J. Am. Chem. Soc. 67, 699, (1935). (14) RIES, \-AN NORDSTRAND, JOHSSOS, A N D B.XI.ERJIEISTER: J. -Am. Chem. SOC. 67, 1242 (1945). (15) SULLIVASASD HERTEL:A d v a n c e s it! Collozcl Science, Vol. I, p. 37. Interscience Publishers, Inc., Kew York (19421, (16) THOMPSOX: Phil. X a g . [41 42,148 (1871). (17) ZSIGMONDY:Z. anorg. Chem. 71, 356 (1911).
T H E KINETICS OF THE THERMAL DECOMPOSITION OF ferf-BUT\-L PROPIOXATE' EARL W'ARRICK Department
0.f
ASD
PAUL FUGASSI
C h e m i s t r y , C a m e g i e I n s t i t u t e of Technology, Pittsburgh, Pennsylvania Receieed 1Varch 10, 1948
The gaseous decomposition of tert-butyl acetate ( G ) , previously studied in this laboratory, proved to be a simple reaction which, on the basis of all known evidence, is a unimolecular process. As such a process represents the decomposition of isolated molecules similar studies on related molecules, if their decomposition proves t o be unimolecular, offers an empirical method for the evaluation of the effects of substituent groups. It ~ x t planned s originally to study the effects of substituting a chlorine atom for a hydrogen atom and a methyl radical for a hydrogen atom in the acetate portion of the molecule. However, the effect of chlorine substitution as exemplified in tert-butyl monochloroaceiate is quite Pre3ented before the Division of Physical and Inorganic Chemistry a t the 105th Meeting of the American Chemical Society, Detroit, Michigan, April 12, 1943. This paper is abstracted from the the& submitted by Carl Warrick t o the Faculty of the Graduate School of the Carnegie Institute of Technology in partial fulfiilment of the requirements for the degree of Doctor of Science in Chemistry, March 22, 1943.
THERMAL DECOMPOSITIOS O F BUTPL PROPIONATE
1315
large, and the particular static method being used for rate measurements did not yield satisfactory results. Accordingly the gaseous decomposition of tert-butyl propionate was investigated, and the kinetics of this reaction will be described. PRCPARATIOX O F ESTER
The sample of ester used in this investigation had been prepared by Mr. Paul Cohen, using the method of Norris and Rigby (4). The propionic anhydride was obtained from the Eastman Kokak Company and was redistilled. Eastman tert-butyl alcohol was purified by fractional crystallization and distillation. After preparation and stripping, the crude ester fraction :vas distilled through a 5-ft. vacuum-jacketed column with n-ire-spiral packing. The purified ester boils at 116.4-116.5"C. a t 738 mm., has a specific gravity, di' = 0.8517, and a refractive index, n;,"= 1.39320. The refractive index of the sample n-as identical with the value reported by Palomaa ( 5 ) for the same compound. E X P C R I X E S T I L PROCEDURE
The apparatus used for the experimental rate measureInents was similar to that employed by Rudy and Fugassi (6). Monoamylnaphthalene was emplcyed in place of mercury as the liquid in the vapor thermostat, and the stopcock previously used t o seal the reaction cell from the vacuum line was replaced by a porous glass disk mercury cut-off (3). The null-point gauge was of the spoon type and was equipped with electrical contacts. A description of this gauge will be published later. The reaction cell, n-hich had a volume of about GO cc., was located in the previously determined constant-temperature zone of the thermostat. A twojunction copper-constantan thermocouple was located alongside the cell and was used for determining the temperature. The thermocouple was constructed from calibrated wire, and its calibration was checked against the boiling points of naphthalene, biphenyl, and benzophenone. HObIOGESEITY O F THE REACTION
As with the decomposition of tert-butyl acetate (6) and other ter2-butyl compounds ( 7 ) , the decomposition of tert-butyl propionate is heterogeneous and not reproducible in clean glass vessels. However, if the reaction products are allowed to remain in the flask overnight, the catalytic activity of the surface is decreased. At 300°C. the period of timenecessaryto deactivate the surfaceis quite long, but a t 3GO"C. complete deactivation is obtained in a few hours. The following cycle of experiments was used to demonstrate that the measurements Tvere reproducible. After deactivation of the surface overnight at 360°C. a series of experiments nere made on successive days a t some lower temperature such as 280°C. and it as observed that consistent results xere obtained, although after each experiment the reaction products nere allom-ed to ieinain in the flask :it the temperature of the experiment until the nest experiment ~ v a bc x i 7 ied out. After the last experiment of the series, the cell containing the miction products was raiied to 3GO"C. and kept at that temperature for a d:iy The tempeiature
1316
EA4RL ITARRICIi .LSD P.'iCL FUGASSI
of the thermostat was then lowered to 260°C. and additional experiments made. The velocity constants obtained after the second treatment at 360°C. checked those obtained before, and it was concluded on the basis of this reproducibility that the reaction in the presence of treated surfaces was homogeneous. The fact that the usual log X; us. 1/T plot is a straight line over a temperature range of 55°C. also indicates the homogeneity of the reaction. ORDER O F THE REACTION
Experimentally the final pressure, p,, is twice the initial pressure, po. The initial pressure is obtained by extrapolating pressure readings back to zero time. The ester decomposition is first order, as a plot of log p0/(p,-pt) against timegives straight lines out to 90 per cent decomposition. In addition, as listed in table 1, the time of half-life is independent of the initial pressure and the ratio of the time of three-quarters life to the time of half-life is 2, as required by a first-order reaction. TABLE 1 Times o j fractzonal lafe at 250°C. _ _ I _
PO
mm. Hg
___
18 45 75 90 120
I
_ I1 2
_. -
- -__
I
mzn.
I
52 43 42
I
_
_
I __- - - I
.
___ ~ __ _ _ ~
-
I
108 84 88 81
I
42
-
- _-
-
I
-
__ l3/4/11/2
I
84
I
__
-I-
mtn.
1
40 __
I
1314
__
-
2.0s 1 95 2 10 2 02 2 00
__
VELOCITY CONST-iSTS 4ND E S E R G Y O F ACTIVATION
Velocity constants were calculated graphically by preparing plots of log p o / ( p , - p t ) agzinst time and measuring the slope of the best straight line through the experimental points. The values of k are listed in table 2. A plot of the logarithms of these constants against the reciprocal of the absolute temperature gives a straight line whose equation is logk (set.-')
=
12.794 -
39,160 2.3RT ~
The energy of activation for the decomposition of tert-butyl propionate is 39,160 cal. .INALYSIS O F PRODUCTS
The kinetic experiments cn the decomposition of tert-butyl propionate showed that the final pressure, p,, \vas twice the initial pressure, PO,of the ester. By analogy with the decomposition of terf-butyl acetate one would expect isobutylene and propionic acid to be formed in equiinolecular quantities. To determine whether these products were present in these amounts, a special reaction cell was employed. This consisted of a cylindrical cell of about 200-cc. capacity fur-
1317
THERJIAL DEC03IPOSITIOS O F BUTYL PROPIOXA4TE
nished n-ith two connections at the top and one at the bottom. One of the top connections was sealed through a mercury cut-off to the vacuum line. The other top connection nas macle of capillary tubing sufficiently long so that when the end of the tubing vas immersed in a beaker of mercury and the cell evacuated, a barometric height of liquid mercury x-as present in the capillary tube. The bottom connection went to a mercury reservoir and by use of pressure or vacuum gas could be drawn into the reaction cell or expelled. Over the reaction cell a small furnace was fitted and the temperature of the cell \vas controlled manually. In operation, v-ith the cell evacuated, ester was admitted to the cell and the cell heated for a definite time interval a t 315°C. Following the decomposition TABLE 2 ---
T-eloczty constants
_ _
TEMPERATURE
-
‘C
296 295.8 294.0 293 7 283.9 282 35 281.45 281.45 281 2 281.1 281 0 280 95 280.9 280.7 250.65 280.0 2iO 9 270.9 270 85 270 45
k X 103
PO
-
?nm Hg
i4 45 59 58 75 30 59 76 i6
I
TENPERATURE
PO
‘C. 270.35 269.62 260.82 260.4 260.35 260.28 260.2 260.05 260.0 250.5 250.3 250.3 25Q.3 250.1 250 0 240.75 210 2 240.1 240.0 239.9
mm. Hg
-
25 25 5 8 5
5
0 0 0 so 0 71 0 125 0 36 5 71.5 T6 0 59 8 23 0 88 0 81 5 T3 5
k X 103
-_
~
see
-1
5 933 5 182 4 088 3 858 2.821 2 114 1 996 2 386 2 245 2 231 2 201 2 280 1 911 2 057 1 958 2 245 1 363 1 350 1 232 1 076
._..
sec
46.5 65.5 66.0 31.0 24.0 55.0 84.0 111.0 17.0 90.0 18.0 45.0 165.0 120.0 i5.0 65.0 78.0 13.0 i2.0 45.0 -
-1
1.113 1 051 0 5803 0 5596 0 4836 0 5389 0 5297 0 4005 0 5527 0 2731 0 2264 0 2557 0 2570 0 2685 0 2464 0 1289 0 1357 0 1444 0 1458 0 1289 _ _ _
the cell n-as allon-ed to cool t o room temperature and by suitable manipu1a:ion of the mercury a sample of boiled distilled n-ater Jvas dran-n into the cell. The gas sample n-as next expelled into a eudiometer tube and the volume of gas subsequently measured. The n-ater sample WE next expelled and the cell flushed several times with distilled n-ater. The \\-ashes n-ere added to the original sample and titrated with standard base. In cine experiment gas uas expelled into a eudiometer tube using only meriury tis the confining liquid. Xfter the volume of gas had been measured, it \vas transferred t o another eudiometer tube, using water as the confining liquid. AIeasurenient of the volume of the gas over water gave the same number of moles present as when the gas was confined over mercury. This indicates that no appreciable portion of the gas was water soluble.
1318
EARL WARRICH A N D PAUL FUGASSI
A11 saniples of gas dissolved completely (better than 99 per cent) in Jleiiigks' reagent, causing a yellow turbidity in the reagent. When the yellon- Denigks reagent was boiled, a heavy orange precipitate was obtained. This spec>ifictest indicates that the gas vas isobutylene. The sodium salt solutions obtained by basic titration of the water washes mere pooled and concentrated. From the concentrated solution a derivative of bromophenacyl bromide was prepared The melting point of the derivative 11-asexactly the same as that obtained from a preparation made from pule propionic arid and bromophenacyl bromide. Some of the analytical results obtained are tahulatetl in table 3 . From thew results it can be seen that for a 30-min. reaction tinip the ratio of propionic. acid t o isobutylene is unity. For longer reavtion times isobutylene seems t o bc disappearing, as the ratio is greater than unity. It is believed t h a t polymerizatjnn of isobutylene is ocmirring, and under contlitim- of our experiments v here the surface is exposed t o liquid water and liquid iiiercury, it does not seem possible to eliminate the polyinwization reaction completely.
-
TABLE 3 Analytical results
ESTINATED UECOlIPOSITIOS
iCIU
min
p r renl
moles X 104
B 30 30 30 45 45 60 120
50 100 100 100 100 100 100 100
0.56 1.44 1.12 1.36 1 .J3 1.35 1.50 1.80
TINE OF HE &PING
-
MOLES ACID
G4S
YOLES G A S
__ __ _ _ moles X 10L
0.519 1.413 1.083 1.33 1.27 1.19 1.23 1.J0
I
I
'
I I
I
1.078 1.019 1.033 1.017 1.12 1.12 1.21 1.29
DISCCSSIOS
The analytical anti kinetic (lata indicate that fert-butyl propionate decomposes according t80the reaction:
CzHsCOOC(CH,)3
+
(CH3)2C=CH,
+
CzHsCOOH
Using the empirical htandard free-energg equations of Bruins and Czarnecki (1) and the data of &sex and Clarke (2) for ethyl acetate, it is estimated that K , for the above reaction at 313"K., the lowest temperature used in our experiments, is about 3 'x IO'. This indicates that the reaction iq essentially complete in the direction indicated. Interpolation of the data of Trautz and Xoeschel (8) for the dissociation of piopionic acid dimer shon-s that at 510°K. and 200 mm. pressure practically all the. propionic acid is present as the monomer. The mechanism for the decomposition of fert-butyl acetate has been previously discussed ( 6 ) arid thc conclusions dravn there apply also to the decomposition of tert-hutyl propionate because of the clo-e similarity of the tn-o reactions. The effect of replacing a hydrogen atom by a methyl group in the acetate por-
1319
d G I T G O F DRY SILVER BROMIDE
tion of terl-butyl acetate is small, as would be expected from the known small inductive effect of the methyl group. The frequency factor is lowered slightly and the energy of activation decreases by only about 1300 cal., from 40,500 cal. to 39,160 cal. SUMMA R T
1. The thermal decomposition of terf-butyl propionate follows the reaction:
C,H&OOC(CH3)3
(CH3)2C=CH?
-+
+
C2HbCOOH
2. The kinetics of the decomposition have been studied in the temperature range from 239.9' to 296°C. and at pressures from 17 to 125 mm. of mercury. 3. The reaction is first order. The relationship between velocity constant ( k ) and absolute temperature ( T ) is given by the equation: log k
=
12.794 -
39,160 2.303R T
~
4. Like the thermal decomposition of tert-butyl acetate, the decomposition of tert-butyl propionate appears t o be a unimolecular process. REFERENCES BRUINSAND CZARNECKI: Ind. Eng. Chem. 33, 201 (1941). ESSEXA N D CLARK:J. Am. Chem. SOC.64, 1290 (1932). FUQASSI A N D WARRICK: Ind. Eng. Chem., Anal. Ed. 16, 713 (1943). NORRISA N D RIGBY:J. Am. Chem. SOC.54, 2088 (1932). PALOMAA: Ber. 68B,303 (1935). RUDYA N D FUOASSI: J. Phys. Colloid Chem. 62,387 (1948). SCHULTZ A N D KISTIAKOWSKY: J. Am. Chem. SOC. 66, 395 (1934). TRAUTZ A N D MOESCHEL: Z. anorg. Chem. 166, 13 (1926).
STUDIES Or\; AGIn'G OF PRECIPITATES AKD COPRECIPITL4TION. XLII
AGINGOF SILVERBROMIDE IS
THE
DRYSTATE'
I. SHAPIROZAND I . 11. KOLTHOFF School of Chemistry, Unieersity of Xinnesota, M i n n e a p o l i s , Aiinnesota Receiked K a r c h 10, 1948
From previous work carried out in this laboratory it appeared that freshly precipitated silver bromide has a large surface development and is subject to thermal aging at room temperature in the air-dried state (2). The degree of 1 This paper is based on a thesis submitted by Isadore Shapiro t o the Graduate Faculty i f the University of Llinnesuta in partial fulfillment of the requirements for the degree of Doctor of Philosophy, August, 1944. * Present address: U. S. Naval Ordnance Test Station, Pasadena, California.
