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The Journal of Physical Chemistty, Vol. 83, No. 18, 1979
Additions and Corrections
ADDITIONS AND CORRECTIONS 1974, Volume 78
Donald W. Rogers* and Somchitra Skanupong: Heats of Hydrogenation of Sixteen Terminal Monoolefins. The Alternating Effect. Page 2569. Some time ago we published a series of heats of hydrogenation, AHh,of 1-alkenes relative to 1-hexene which was taken as a thermochemical standard.l The value we assigned to 1-hexene was in error, as we have pointed out in subsequent publications,2 necessitating an upward revision of the heats of hydrogenation of the 1-alkenes of about 0.7 kcal mol-' so that they conform to the equation -AHh = 29-96 0.039n
+
where 5 5 n 5 20 is the number of carbon atoms in the alkene. Although the value we used in the initial publication is wrong, we have hesitated to publish a formal correction in this journal because it is not at all clear from the literature what the correct value for the heat of hydrogenation of 1-hexene really is. Reliable sources, using both combustion and hydrogenation data, report values from -30.00 kcal mol-' in the ideal gas state3to -30.25 kcal mol-' for the liquid-phase hydrogenation of 1-hexene in glacial acetic acid4 corrected for solvent effects attendant to this process. Kistiakowsky's heats of hydrogenation are frequently taken as the paradigm in this field and, although he did not measure AHh for 1-hexene, he did obtain -30.34 for 1-butene and -30.14 for 1-heptene? the latter value having been verified by Williams.6 Arguments can be made for other values not very different from the ones cited but the magnitude of experimental error makes it difficult to choose among them. Recently we determined the heats of hydrogenation of several linear alkynes,2 among them the 1-alkynes from 1-hexyne to 1-decyne. Because heats of hydrogenation of terminal linear alkynes are more than twice as large as those for the corresponding alkene, any inaccuracies in the standard heat of hydrogenation are magnified in the results and a more critical comparison with independently de-
termined values is possible. Taking Kistiakowsky's value of -69.70 f 0.03 kcal mol-' for the heat of hydrogenation of 1-propyne' as a benchmark for comparison and Skin,~ ner's value of -30.25 f 0.2 kcal mol-' as the ~ t a n d a r dwe obtained good agreement with our values for the higher alkynes at their point of contiguity with the literature.2 Our mean m h for terminal alkynes is -69.43 f 0.42 kcal mol-'. Kistiakowsky's value is within the f0.42 kcal mol-' 95% confidence limits expressed in this result. (There are theoretical reasons for anticipating a slight decrease in the heats of hydrogenation of 1-alkynes as the chain length increases.2) There remains a discrepancy, however, in that the heat of hydrogenation observed by Skinner4is for the liquid to liquid conversion while the value needed is for the gasphase hydrogenation. The difference in heats of vaporization between 1-hexene and n-hexane8is 0.20 kcal mol-l which makes the gas-phase value less exothermic by that amount. Nevertheless, the goodness of fit resulting from calculations with -30.25 kcal mol-' as though it were a gas-phase value leads us to suppose that it is very nearly correct and that the true value for Skinner's liquid phase LiHh is a little higher than his reported value (but still within his experimental uncertainty). Our conclusion is that Skinner's value of AHh(l-hexene) = -30.25 kcal mol-' is the best standard value for hydrogen calorimetry available at this time.-D. W. Rogers
References and Notes (1) D. W. Rogers and S.Skanupong, J. Phys. Chem., 78, 2569 (1974). (2) D. W. Rogers, 0. A. Dagdagan, and N. L. Allinger, J . Am. Chem. SOC.,101,671 (1979). (31 F. D. Rossini et al.. "Selected Values of Phvsical and Thermodynamic Properties of Hydrocarbonsa d Related Compounds", Carnegie Press, Pittsburgh, Pa., 1953. (41 H.A. Skinner and A. Snelson, Trans. faraday Soc., 55, 404 (1959). (5) G. 8. Kistiakowsky, J. R. Ruhoff, H. A. Smith, and W. E. Vaughan, J. Am. Chem. Soc., 58, 137 (1936). (6) R. B. Williams, J . Am. Chem. Soc., 64, 1395 (1942). (7) J. B. Conn, G. B. Kistiakowsky, and E. A. Smith, J. Am. Chem. Soc., 61, 1868 (1939). (8) J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds", Academic Press, New York, 1970.
0022-3654/79/2083-2430$01.00/00 1979 American Chemical Society
