T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY
Repiatered in U.8.Patent Oflce
0Copyright, 1972, by the American Chemical Society
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VOLUME 76, NUMBER 2
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J A N U A R Y 20,1972
Thermal Decomposition of 3,P-Dimethylhexane, 2,2,3-Trimethylpentane, tert-Butylcyclohexane, and Related Hydrocarbons by Wing Tsang National Bureau of Standards, Washington, D. C. 20234
(Received August 2, 1971)
Publication costs assisted by National Bureau of Standards
3,4-Dimethylhexane, 2,2,3-trimethylpentane, and tert-butylcyclohexane have been decomposed in a singlepulse shock tube. Rate expressions for the main bond-breaking steps are: k(sec-C1Hg-sec-CdHg + 2secC4Hg.) = 1018~a4exp(-37,900/T) sec-1; k(sec-C1Hg-tert-C4Hg~sec-CkH9. tert-CdH9.) = 1016,aoexp(-36,400/ T)sec-l; and k(tert-C4H9-c-CaH11 + tert-C4H9. c-CaH11.) = 10l6J1exp(-37,300/T) sec-l. They lead to = 39 kJ/mol (9.3 kcal/mol), and AHf,aoo(sec-C4Hs.) = 55 kJ/mol (13.2 kcal/mol), AHf.soo(tert-C4H9.) AHr130o(C-C6&1 66 kJ/mol(l5.8 kcal/mol). Rate expressions for the decomposition of d l hydrocarbons formed from sec-butyl or cyclohexyl radicals and methyl, ethyl, isopropyl, tert-butyl, tert-amyl, allyl, 1-methylallyl, propynyl, and benzyl radicals have been calculated. Estimates are given for the rates of decomposition of hydrocarbons formed from n-propyl radicals and any of the compounds listed above. The accumulated singlepulse shock tube data can now give quantitative information on the initial cracking patterns of almost all aliphatic hydrocarbons. The A factors determined in this study provide striking confirmation for the earlier observation that this value, per C-C bond, for alkanes is a constant. On this basis, it appears that D(tert-CSH1l-H) - D(tert-CcHg-H) = 5.4 kJ/mol; D(sec-C4H9-H) - D(i-C3H7-H) = 3-5 kJ/mol; D(n-C3H.i-H) - D(C2H6-H) -1.5 kJ/mol. The absolute magnitude of the A factors are all a t least a factor of 10 lower than expected. It is demonstrated that recent chemical activation and very low pressure pyrolysis studies support the shock tube results.
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Introduction This is the of a series of papers on hydrocarbon decomposition. Previous investigations have determined the rate constants and parameters for the reaction
Ri-Rz +R1* $- R2 * where R1- and R2 * may be any of the following radicals : methyl, ethyl, isopropyl, tert-butyl, tert-amyl, allyl, 1methylallyl, benzyl, and propynyl. This has led to a determination of the heats of formation of these radicals. In addition, it appears that the preexponential factors or entropies of activation for all the alkanes studied are constant (-101a.2 sec-l) and are considerably lower than The present study is concerned with the mechanism and rates of decomposition
of 3,4-dimethylhexane1 2,3,3-trimethylpentane1 and tert-butylcyclohexane. Primary attention will be focused in the reactions that form sec-butyl and cyclohexyl radicals. Their heats of formation as well as rate parameters for bond breaking of all the hydrocarbons formed from either of these two species and any of the radicals listed above can then be determined. Comparisons with the earlier results on 2,3-dimethylbutane and 2,2,3-trimethylbutane decomposition will lead to additional information on the effects of @-methylation on carbon-carbon bond strength. Also of interest will be the relative importance of the various modes of decomposition of sec-butyl and cyclohexyl radicals. Fi(1) W. Tsang, J. Chem. Phys., 43, 352 (1965); 44, 4283 (1966); 46, 2187 (1966). (2) W. Tsang, Int. J . Chem. Kinet., 1 , 245 (1969); 2 , 23 (1970).
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WINGTSANG
144 nally, the heat of formation of tert-butyl radicals will be redetermined. There have been no previous studies on the thermal decomposition of 3,4-dimethylhexaneJ 2,2,3-trimethylpentane, and tertbutylcyclohexane. The heat of formation of sec-butyl radical has been determined by Chekhov, Tsuilingol’s, and 10ffe3 using an iodination technique. Their results have been interpreted by Golden and Benson4 as consistent with AHt,Soo(secCdHs.) = 54.3 lJ/mol (with an uncertainty of 5-10 kJ). For cyclohexyl radicals, ICerr5 gives AHr,300(~C6H11’) = 54 f 13 kJ/mol. This is based on the abstraction of hydrogen atom from cyclohexane by methyl radical6 and a Polanyi plot. Lin and Laidlere have shown that a t temperatures below 600°K the predominant mode of sec-butyl radical decomposition is via the reaction sec-CdHg.
k ----f
CaH6
+ CHs.
where k = lo1* exp(-16,300/T) sec-’. No previous work on the mechanism and rate of decomposition of cyclohexyl radicals can be found. Experimental Section All of the studies have been carried out in a singlepulse shock tube. The experimental equipment and procedure have been described in earlier communications.’ Quantitative data are obtained from the comparative rate technique using the decyclization reaction of cyclohexene8 k(CaH10 .--f 113-C4H6
+ CzH4) =
10l6.O2exp( - 33,60O/T)
as the ~ t a n d a r d . ~This method involves the simultaneous decomposition of two compounds by the same reflected shock wave and the determination of the respective rate constants for decomposition, From a series of experiments, a relationship between the two sets of rate constants can be derived. If the rate parameters of one of these processes are well known, the other set of values can be easily determined. Thus, the usual uncertainty in shock tube temperature is eliminated. I n addition, the extremely short residence time (
