RICHARD K. LYON
1486 Acknowledgments, The assistance of NIr, James Terrier and afr. John c. SYnnott with 80me of the experimental work, and helpful discussions with Dr, Mark Salomon, are gratefully acknowledged. This work was supported in part by the Petroleum Research Fund of the American Chemical Society and the Air Force Cambridge Research Laboratories, Office of Aerospace Research, but does not neCeSsarilY constitute the opinion of these agencies.
(40) NOTEADDED IN PROOF. Hydration constants of ions and undissociated salts in acetonitrile have been determined by I. M. Kolthoff and coworkers [ J . Amer. Chem. SOC.,89, 1682, 2621 (1967)l. Values of KI for the association of water with ions in acetonitrile [4 1 (Li+), 2.6 (Na+), 1.3 (K+), 11 (Cl-), and 1.0 (Cloa-)] closely parallel our values for PC given in Table IV above. Recent reevaluation of the free energy of transfer for anions between PC and water by A. J. Parker and coworkers [“Solvation of Ions XVI,” in press] gives A G ~ ,= +8.6 kcal for c i - and 10.0 kcal for BPh4-. This implies 6.4 kcal for Li+, considerably diminishingthe difference between Li+ and C1-, and producing a better correlation with KI for these two ions but giving a value for Li+ in disagreement with both the correlations of Figure3 and the extrapolation method of ref 39.
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Equilibrium Distributions of Some Octane and Nonane Isomers by Richard K. Lyon Corpopate Research Laboratories, Esso Research and Engineering Company, Linden, New Jeraey 07036 (Recezved September 83,1970) Publication coat8 assisted by the Esao Research and Engineering Company
All 18 of the isomeric octanes and four of the nonanes were treated with an isomerization catalyst, AlBr3. Relative reaction rates were measured. For three sets of isomers equilibrium distributions were obtained and compared with the API Project 44 data.
Introduction Equilibrium distributions among isomeric compounds are in general calculated from heat of combustion and heat capacity measurements, a procedure involving small differences in large numbers. I n some favorable cases, however, catalyst,s can bring about isomeric interconversion. If perturbations due to side reactions are negligible, the observed distribution is the equilibrium distribution within the accuracy of chemical analysis. Aluminum halide catalyzed isomerizations have been used to obtain the equilibrium distribution of the butane, pentane, hexane, and heptane isomer sets. Extension of these measurements to the higher hydrocarbons would be of interest since these equilibrium distributions would show the effect of interaction between remote groups. I n the present st,udy all 18 octane isomers and four nonane isomers were treated with AIBra. Complete equilibrium among all isomer forms of eit,herthe octanes or nonanes could not be achieved, since some of t,he isomerizations necessary for complete equilibrium are slow compared with the concurrent degradation reaction. Three subsets of isomers, the singly branched octanes (SBO), the doubly branched nonquaternary octanes (DBNO), and the triply branched nonanes (TBN), l I 2
The Journal of Physical Chemistry, Vol. 76,No. 10, 1971
were found to achieve steady-state distributions. Relative rate measurements for all observable reactions were made and the steady-state distributions are proven to be the equilibrium distributions.
Experimental Section The hydrocarbons used in these experiments were Chem. Samp. Co. of 99% purity and were used without further purification. The catalyst was AIBra from Matheson Coleman and Bell. As initially received it had a catalytic activity suitable for these measurements and was used as received. Using a drybox, 0.1 g of AIBra was weighed into a glass vial which was then sealed with a rubber septum. The vial was then placed in a constant temperature bath and 5 cc of the reactant mixture was then injected. The reactant mixture was always 50 vol % isomeric octane or nonane and 50% methylcyclopent,ane (MCP). At various times after reactant injection, samples were withdrawn via syringe, injected into water to deactivate the catalyst, and analyzed by a P&E F-11 gas chromatograph with a 300-ft squalane (1) F . E. Condon, Catalysis, 6, 43 (1958). (2) H. Pines and N. E . Hoffman, “Friedel-Crafts and Related Reactions,” Vol. 11, G. A. Olah, Ed., Interscience, New York, N. Y., 1964.
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EQUILIBRIUM DISTRIBUTIONS OF SOME OCTANEAND NONANEISOMERS
The observed steady-state distributions of the singly branched octane isomers are given in Table I. A sample was judged to have reached steady state not from the data to be averaged but from the approach of the methylcyclopentane-cyclohexane ratio to its equilibrium value. The methylcyclopentane-cyclohexane ratio is a good index, since the singly branched octane isomers interconvert faster than do methylcyclopentane-cyclohexane. When this happened the catalyst was still active as proven by the continued slow interconversion of the singly branched octane and doubly branched nonquaternary octane isomers. During the isomerization of the doubly branched nonquaternary octane isomers a t 13") the distribution of the singly branched octane isomers formed was ob0.002 and (3served to be 4-ibfC7/2-h!tC~ = 0.266 MC, EtHex)/2-R4C7 = 0.771 0,001. During the isomerization of the singly branched octane isomers a t 13" the distribution of the doubly branched nonquaternary octane isomers was observed to be (2,3-Di\IH 2-M-3-EtP)/2,5-Dn.IH = 0.230, 2,4-DMH/2,5DMH = 0.807, and 3,4-DMH/2,5-DMH = 0.0564. In Table 11, the relative rates at which each singly branched octane isomer converts to the others and at which the equilibrating mixture of singly branched octane isomers form other products are given. The rate of formation of the other products was observed t o be independent of the starting singly branched octane isomer. Since the rates of mutual interconversion are high compared with the rates forming other products, this is to be expected. Table I1 gives rate constants for the isomerization of singly branched octane ta both doubly branched nonquaternary octane and 2,2-dimethylhexane. The latter reaction is a true direct reaction rather than a secondary isomerization of doubly branched nonquaternary octane, since the ratio of 2,2-dimethylhexane t o doubly branched nonquaternary octane is 0.05 independent of reaction time. Further using the value of ~ D B N ~ + ~ , ~ - Dgiven M H in Table IV, one calculates that the amount of 2,2-dimethylhexane observed early in the reaction is 25 times as great as that which could be formed by secondary reaction. Table I1 does not give rate constants for the direct interconversion of 2-methylheptane with either 4methylheptane or ethylhexane, only the interconversion of these three with 3-methylheptane. Based on product ratios extrapolated to zero conversion, these direct interconversions do not occur to an observable extent, 5% of the total isomerization. The steady-state distributions of the doubly branched nonquaternary octane isomers are given in Table 111. The amount of 2-methyl-3-ethylpentane present in these distributions could not be determined, since it was always less than 15% of the 2,3-dimethylhexane. I n Table IV the relative rate constants are given for the total rate a t which each doubly branched non-
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Figure 1. Typical relative rate plot.
capillary column. Peak areas were measured by an Infotronics CRS-100 integrator. Analysis of isomer mixtures of known weight composition showed that within experimental error all the octane isomers had the same sensitivity. The isomeric analysis was complete (3-MC7) and with two exceptions-3-methylheptane ethylhexane (EtHex) could not be separated. 2Methyl-3-ethylpentane (2-M-3-EtP) could be detected in the presence of 2,3-dimethylhexane (2,3-DMH) only if the former were more than 15% of the latter. Xumerous rate measurements, all relative t o the rate of the methylcyclopentene-cyclohexane (CH) equilibration, were made. Figure 1is typical of the better results. The vertical ordinate (conversion of doubly branched nonquaternary octane isomers to singly branched octane isomers) is the expression appropriate to a first-order irreversible reaction. This assumption of irreversibility is a valid approximation only a t low conversion. Due to the kinetic complexity of the isomer interconversion this approximation was necessary, hence only low conversion measurements were used to calculate the relative rate constants reported here. The horizontal ordinate (conversion of methylcyclopentane to the equilibrium methylcyclopentanecyclohexane mixture) is the expression appropriate to a first-order reversible reaction, where K is the methylcyclopentane- cyclohexane equilibrium constant.
Results Normal octane was observed t o isomerize slowly to the methylheptanes, the relative rate constant at 59.1" being 0 . 1 3 l ~ ~ ~Dimethylhexanes ~ = ~ ~ . were also observed, but the ratio of their yield to the methylheptane yield extrapolated to zero a t zero conversion of the noctane.
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The Journal of Physical Chemistry, Vol. 76, No. 10, 1971
RICHARD E
