Feb., 1961
HEATSO F
COMBUSTION OF hlONOOLEFIX
trations. But in the vicinity of one atmosphere, the solubility of' H2S in water follows Henry's law, and this would seem to be incompatible with the concept of dimerization of part of the H2S. Hence a clearcut explanation of the noted change of the
267
HYDROCARBOSS
apparent ionization constant with H2S concentration is not possible a t this time. Acknowledgment.-This work was supported by the National Science Foundation under grant G5080.
HEATS OF COMBUSTION, ISOI1IERIZATIOS, AND FORMATION OF SELECTED C,, C, A S D C,, MOiYOOLEFIN HYDROCARBONS BY JOHN D. ROCKENFELLER AND FREDERICK D. ROSSINI~ Chemical and Petroleum Research Laboratory, Carnegie Institute of Technology, Pittsburgh IS, Pennsylvania Received Julb 14, 1960
Measurements were made of the heats of combustion, in the liquid state a t 25", of 19 selected monoolefin hydrocarbons, including 10 heptenes, 6 octenes and 3 decenes. From these and appropriate other data wereocalculated values of standard heats of isomerization, formation and hydrogenation, as appropriate, for the liquid state a t 25 For most of the compounds, values are also given for the gaseous state. The relation between energy content and molecular structure for these compounds is discussed.
.
I. Introduction Experimental data leading to values of heats of formation have been reported for substantially all of the monoolefin hydrocarbons through the h e x e n e ~ . ~ -However, ~ very few data leading to values of heats of formation have been available for monoolefins above the hexene~.C-~ It has become apparent that new experimental data on selected monoolefins of the C, to Clo range are needed to provide the basis for testing, within the limits of pmsent-day measurements, any theory relating the energy and molecular structure of the monoolefin hydrocarbons. With a proved theory, one would not only arrive a t a better understanding of the relation between energy and structure for these molecules, but one could calculate the heats of format:ion of an untold number of monoolefin hydrocarl3ons without recourse to further experimental measurement. Accordingly, the present investigation was carried out to obtain experimental data on 19 selected monoolefin hydrocarbons, including 10 heptenes, 6 octenes and 3 decenes. This report also presents a discussion of the relation between the energy and structure of these molecules. II. Apparatus and Experimental Procedures The experimentd values of this investigation are based on the absolute joule :is the unit of energy. Conversion to the defined thermochemical calorie is made using the relation 1
-
calorie = 4.184 (exactly) joules. For internal consistency with other investigations from this Laboratory, the molecular weight of carbon dioxide was taken as 44.010 g./mole. In this investigation, the chemical and calorimetric apparatus and procedures were the same as described by Browne and Rossini.10 The rise of temperature in each experiment was near 2O, with the final temperature being near 30°, the temperature of the jacket of the calorimeter. The amount of the reaction in each hydrocarbon combustion experiment was determined from the mass of carbon dioxide formed in the combustion, as previously described.lO The bomb had an initial volume of 380 ml. One ml. of water was placed in the bomb prior to each combustion experiment. The pressure of the oxygen for combustion was made 30 atmospheres (calculated to 25"). With the exception of the two branched decenes, the compounds measured in the present investigation were API Research hydrocarbons, made available through the API Research Project 44 from materials purified by the API Research Project 6. The API Research samples had the values of purity given in Table I. Description of the purification and determination of purity of these samples has already been given."-l6 As a result of the methods of
TABLE I PURITY OF
THE
API RESEARCH HYDROCARBONS MEASURED
Compound
1-Heptene 3-Methyl-cis-3-hexene 3-Methyl-trans-3-hexene 2,4-Dimethyl-l-pentene 4,4-Dimethyl-l-pentene 2,4-Dimethyl-2-pentene 4,4-Dimethyl-cis-2-pentene 4,4-Dimethyl-trans-2-pentene 3-Methyl-2-ethyl-1-butene
Purity, mole %
99.84 f 0.10 (99.85 f (99.85 f . l o ) " 99.88 f .09 99.85 f .OS 99.95 f .04 99.85 f . l l 99.81 f .03 (99.85 i .lo)" 99.95 f .04 99.77 f . I 3 9 9 . 8 6 f .12 (99.85 zk . l o ) " 99.81 f .08 99.95 f .03 99.93 f .05 99.91 f .OT
(1) This investigation was supported in part by a grant from the National Science Foundation. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy i n Chemistry a t the Carnegie Institute of Technology. 2,3,3-Trimethyl-l-butene (2) Univers.ity of Notre Dame, Notre Dame, Indiana. 1-Octene (3) F. D. Rossini, K. S. Piteer, R. L. Arnett, R. M. Braun and 2,2-Dimethyl-cis-3-hexene G. C. Pimentel, "Selected Values of Physical and Thermodynamic 2,2-Dimethyl-trans-3-hexene Properties of Hydrocarbons and Related Compounds," A P I Research Project 44, Csrnegie Press, Pittsburgh, Pa., 1953. 2-Methyl-3-ethyl-1-pentene (4) E. J. Prosen and F. D. Rossini, J . Research iVatZ. Bur. Standards, 2,4,4-Trimethyl-l-pentene S6, 269 (1946). 2,4,4-Trimethyl-2-pentcne (5) E. F. Bartolo and F. D. Rossini, THISJ O U R N A64, L , 1685 (1960). 1-Decene (6) F. M. Fraser an -5.69 i .IO - 1 . 1 7 =I= .06 -4.92 i .43 - 0 . 7 6 I .OS
Heat of isomerization for the gas a t 25 , kcal./mole 0.00
-1.95IO.57 -G.34 i . 2 9 -4.58 i ,18 -6.SC zk .10 -5.68 i: . 4 3
THE
Standard heat of hydrogenation, AHho, of l-octene less that of the isoiner a t 25O, keal./mole Liquid Gas 0.00 0.00 +1.78&0.70 i-1.94f0.70 -2.G4 i .55 - 2 . 4 5 I .55 -3.89 i: .50 - 3 94 f .RO - 3 . 4 0 =I= .lj3 - 3 . 1 1 =k . 6 3 -2.09 i .50 - 1 , 9 3 1 .50
The uncertainties in this table are twice the standard deviation.
of the stmdartl heat of combustion of the given isomer to that of the l-alkene, for the liquid state a t 25", taken the same as the corresponding ratio for the bomb process a t 25", which is not significantly different from the corresponding ratio for the bomb process a t 30°, which was determined experimentally in the present investigation ; the standard heat of isomerization of the l-alkene to the given isomer, for the liquid state a t 25", calculated frorn the foregoing data, with the values for l-heptene and l-octene recommended by Loeffler and Rossini,' and taking the difference between AH and AE to be the same for the several isomers in each group; the standard heat of vaporization of the l-alkene less that of the given isomer, a t 25O, from the data of Camin and Rossini,19 with the difference between this and the heat of vaporization a t saturation pressure being considered not significant; the standard heat of isomerization of the 1-alkene l,o the given isomer, for the gaseous state a t 25", calculated from the corresponding values for the liquid state and the differences in the heats of vaporization; and the standard heat of hydro(19) D L. C u r i n and E'. D. Rossini, J . Chen. Enar. D a h . 6 , 368 (1960).
genation of the l-alkene less that of the given isomer, for both the liquid and gaseous states a t 25", using our standard heats of isomerization for the appropriate paraffin hydrocarbons. Table VI1 gives similar information for l-decene and its two isomers for the liquid state only, as no data are available for the heats of vaporization of the two isomers. From the values in Tables TT, T'I, and VII, one can easily calculate the values of the heats of formation of the corresponding substances using the following recommended values of the standard heats of formation a t 25", in kcal./mole3J: 1heptene (liq), -23.41 ; l-heptene (g), -14.89; l-octene (liq), -29.52; l-octene (g), -19.82; 1deceiie (liq), -41.73. V. Data of Other Investigations No data appear t o be reported in the literature on the heats of combustion of the compounds measured in the present investigation. However, data on heats of hydrogenation in the gaseous state near. 82" have been reporred by Doliver, Gresham, Kistiakowsky and Vaughan'* for the following compounds also measured in the present investigation :
JOHN D. ROCKEXFELLER ASI) FHXDERICK D. Rossmr
270
HEATS5 O F ISOMERIZATION, AND THE
TABLE VI1 DIFFERENCE I N THE HEATS O F HYDROGENATION, FOR THE LIQUID STATE, CERTAIN DECENES Ratio of the heats of combustion a t 25'
Compound
1-Decene 2,2,5,5-Tetramethyl-cis-3-hexene 2,2,5,5-Tetramethyl-trans-3-hexene The uncertainties in this table are twice the
SUMMARY OF THE
1.OOOOO 1.00155 f 0.00026 0.99489 f 0.00037 standard deviation.
Heat of isomerization for the liauid a t 25', kcal.'/mole
0.00 +2.45 f 0.41 -8.06 i 0.59
TABLE VI11 RESULTS OF TURNER, KETTLETON AND PERELM.4NZ0 (TNP) AND RESULTS FROM THISLABORATORY~ I
Conipounds
A
Val. 65
I1
AT
25",
FOR
Standard heat of hydrogenation AHho of 1-decene less that of the isomer, for the liquid a t 2 5 O , kcal./mole
0.00 +9.04 f 0.76 - 1 . 4 7 f 0.86
COMPARISON WITH CORRESPONDING
I
- I1
TNP This laboratory Difference between the heat of hydrogenation in acetic acid Standard heat of Heat of hydrogenation soln. and in the pure liquid hydrogenation of the in the liquid state hydrocarbon a t 2 5 O , in acetic acid s o h . pure liquid a t 25'. kcal./mole a t 25', kcal./mole kcal./mole
-26.67 -27.99 +1.32 -25.15 -26.53 +1.38 ( - 1.52)b ( - 1.46)b (-0.06 f 0.65)" -25.52 -26.76 1.24 2,4,4-Trimethyl-1-pentene -26.79 -27.53 0.74 2,4,4-Trimethyl-2-pentene (1.27)b (o.77)b (0.50 f 0.65)' -27.32 -28.02 0.70 A 4-Methyl-cis-2-pentene -26.38 -26.94 0.56 B 4-Methyl-trans-Ppentene A-B ( - 0.94)b ( - 1.08)b (0.14 f 0.38)' A4 4,4-Dimethyl-cis-2-pentene -30.80 -31.66 0.86 -26.51 -27.74 1.23 B 4,4-Dimethyl-truns-2-pentene A-B ( - 4.29)b ( - 3.92)b (-0.37 f 0.5Y)' -36.24 -39.26 3.02 A 2,2,5,5-Tetramethyl-ci~-3-hexene -26.87 -28.75 1.88 B 2,2,5,5-Tetramethyl-trans-3-hexene A-B ( - 9.3Vb ( -10.51)b (1.14 f 0.Y8)c All the foregoing compounds are reported in this investigation except the cis and trans isomers of 4methy1-2-pentenel which were reported by Bartolo and Rossini.6 Ir Heats of isomerization for the given pair of isomers. Measure of the accord of the values of the heats of isomerization from the two sources. B A-B A B A-B
2,4-Dimethyl-l-pentene 2,4-Dimethyl-2-pentene
4,4-dimethyl-l-pentene, 2,4,4-trimethyl-l-penterie 0.20. It is seen that, within the respective limits and 2,4,4-trimethyl-2-pentene. Also, data on heats of uncertainty, the values are in substantial acof hydrogenation in the liquid phase in solution in cord. acetic acid were reported by Turner, Kettleton and Table VI11 summarizes the results obtained by Perelmang on the following compounds : 2,4-di- Turner, Nettleton and Perelman*O and gives a methyl-1-pentene, 2,4-dimethyl-2-pentene, 2.4,4- comparison with corresponding results from this trimethyl-1-pentene, 2,4,4-trimethyl-2-pentene, 4- Laboratory, calculated from the recommended methyl-cis-2-pentene, 4-methyl-trans-?-pentene, heats of formation for the 1-alkenes and the paraf4,4-dimet hyl-cis-2-pentene, 4,4-dimethyl-trans-2- fins formed as products in the hydrogenation, pentene, 2,2,5,5-tetramethyl-cis-3-hexene and 2,2,- together with the measured heats of isomeriza5,5-tetramethyl-trans-3-hexene. The cis and trans tion of the given olefins. The values in the tables isomers of 4-methyl-2-pentene are included here are fully identified. The following points may be because they were also measured in our labora- noted: (a) The values of the heat of hydrogenatory.6 tion in the liquid state a t 25" for the material in From the data of the present investigation, and solution in acetic acid and for the pure hydrocarbon, the recommended values for the standard heats of excluding the decenes, differ by about 1.0 kcal./ formation of the l-alkenes3J5and the correspond- mole, on the average, which difference may posing paraffin hydrocarbons3 which are the products sibly be accounted for by a difference of this amount of hydrogenation, one obtains values for the in the heat of solution of the olefin and the correvtandard heat of hydrogenation for the gaseous sponding paraffin in acetic acid; (b) For the indistate at 25" for the following three monoolefins in cated pairs of compounds, the values of the heats of kcal./mole : 4,4-dimethyl-l-pentene, - 30.09 i isomerization, marked in parentheses with a foot0.58; 2,4,4-trimethyl-l-pentene, -26.89 i 0.63; note "b," from the two sets of data are in sub2,4,4-trimethyl-2-pentene, -28.07 f 0.50. Con- stantial accord, except for the two decenes, where version to 25" of the experimental data of Dolliver, the difference is somewhat beyond the limits of the Gresham, Kistiakowsky and Vaughan18on the heat (20) F. D. Rossini, Chapter on "Chemical Thermodynamics of of hydrogenation of these same three monoolefins Hydrocarbons" in "The Chemical Background for Engine Research," yields the following values, respectively, in kcal./ R . E. Burk and 0. Grummit, Editors, Interscience Publishers, New mole: -29.31 i 0.20; -27.02 =t0.20; -28.17 f York, N. Y., 1943.
Feb., 1961
HE.4Ts O F
coMsvsTIos
OF ~ ~ O S O O L E F IHYDROCARBOXS K
I, H
respective uncertainties. In Table VIII, the values marked in parentheses with a footnote “c” indicate the measure of accord of the values of heats of isomerization.
VI. Discussion
271
H
where R1 is a normal alkyl radical or any alkyl radical wit,h no branching on the carbon atoms once or twice removed from the doubly bonded carbon atom, AHhn = -30.0 kcal./mole; 11. H H \ /
The data presented in this investigation, and earlier by Turner, Nettleton and per el ma^^,^ for the difference in the energy content of the cis and trans isomers of 2,2,5,5-tetramethyl-3-hexene (symmetrical di-t-butylethylene) show how extremely large can be the effect of steric hindrance or molecuH R2 lar constraint in some molecules having bulky where Rz is an alkyl radical with branching on one radicals adjacent to one another in space. I n this or both of the carbon atoms once or twice recase, the trans isomer is nearly free of molecular moved from the doubly bonded carbon atom, AHhO constraint whereas the cis isomer has an energy = -29.4 kcal./mole; content near 10 kcal./mole greater than that of the 111, R trans isomer. ‘The cis isomer can be put together \ /R by nature through synthesis, but the model of the , AHho = -28.0 kcal./mole; C=C molecule cannot be made intact with the ordinary H \E rigid, solid atomic models scaled to size. About 20 years ago, one of us (FDR) summarized the status then of our knowledge of the relation of energy content and molecular structure for the aliphatic monoolefin hydrocarbons, someH ‘R what as follows: I n the case of the isomers of the aliphatic monoolefins, it appears that, with regard to energy content, increase in the stability of the molecule (toward lower energy content) is H R produced by (a) having the double bond a t the middle of the longest chain of carbon atoms, (b) VI, R having the maximum number of alkyl groups at\ C=C , AHho = -26.4 kcal./mole; tached to each of the doubly bonded carbon atoms, and (c) further increasing the compactness of the H \R molecule by branching on the side chains, up to VII, R the point where close proximity in space of hydro\ gen atoms on different carbon atoms begins to , AHho = -25.8 kcal./mole. C=C produce steric hindrance or constraint in the molecule and a consequent increase in the energy conR R‘ tent. A11 of the data subsequently reported on the foregoing types 111, IV, V, VI and VII, the C6, C7,Cs and Clomonoolefins are in substantial R In represents any alkyl radical. To the foregoing accord with the foregoing statement. values of AHhO, selected appropriately according Careful analysis of the data now available for to the structure of the monoolefin, the following nearly 50 aliphatic monoolefin hydrocarbons indi- structural quantities are to be added algebraically: cates that the xdue of the heat of hydrogenation (a) for each neopeiityl group, or equivalent longer of any such given olefin may be estimated within group, attached to the doubly bonded carbon 0.5 kcal./mole by a simple set of rules, which take atoms, -0.4 kcal./mole; (b) for each t-butyl into accouiit quantitatively the structural factors group, or equivalent longer group, attached to the mentioned above. Sinre values of the heat of doubly bonded carbon atoms, -0.6 kcal./mole; formation are 1 1 0 5 ~available for substantially all (c) for a “cis” combination of two t-butyl groups, the paraff in hydrocarbons,s*21it thus becomes pos- - 10.0 kcal./mole (in addition to the structural sible to make a reasonably reliable estimate of the quantity for each t-butyl group itself); (d) for a heat of formation of any given aliphatic monoolefin “cis” combination of one t-butyl group with an efhydrocarhon. The simple rules, pertaining to the fectively smaller alkyl group, of lesser steric heat of hydrogenation, AHhO, a t 25”, for the hindrance, -3.2 kcal./mole (in addition to the gaseous state, according to the reaction structural quantity for the t-butyl group itself). With the foregoing simple rules, one can reproCnHdd Hz(g.1 = CnHzwdg) duce the values of the heats of hydrogenation of the approximately 40 Cg to Cln monoolefin hydrocarare as follows. bons measured in this Laboratory with an average Extending the correlation presented in the preceding report 011 the hexenes,6 one may distinguish deviation of ~ 0 . 4 0kcal./mole, the maximum deviation being 1.2 kcal./mole. It appears then that seven simple types of olefin hydrocarbons the above simple rules may be used to calculate reliable values of the heat of hydrogenation of any (21) J. B. Greenshields and F. D. Rossini, THISJOURNAL, 68, 271 monoiilefin hydrocarbon, with an average uncer(1958).
/=\
\
/”
’ /“ ’
+
272
w.8.GILKERSONAND I
