Gaseous Heats of Hydrogenation of Alkenes
The Journal of Physical Chemistry, Vol. 83, No. 15, 1979
electron transfer from adduct to free cation radical.
References and Notes (1) For part 7, see J. F. Evans and H. N. Blount, J . Am. Chem. Soc., 100, 4191 (1976). (2) J. F. Evans and H. N. Blount, J . Org. Chem., 41, 516 (1976). (3) H. N. Blount and J. F. Evans in "Characterization of Solutes in Nonaqueous Solvents", G. Mamantov, Ed., Plenum Press, New York, 1976, and references therein. (4) H. N. Blount, J . Nectroanal. Chem., 42, 271 (1973). (5) T. Kuwana and N. Winograd in "Electroanalytical Chemistry", Vol. 7, A. J. Bard, Ed., Marcel Dekker, New York, 1974. (6) J. F. Evans, H. N. Blount, and C. R. Ginnard, J . Electroanal. Chem., 59, 169 (1975). (7) J. F. Evans, J. R. Lenhard, and H. N. Blount, J . Org. Chem., 42, 963 (19771. (8) J. S. 'Fritz, "Acid-Base Titrations in Nonaqueous Solvents", Allyn and Bacon, Boston, 1973. (9) A. A. Pilla, J . Nectrochem. Soc., 118, 702 (1971).
1975
(10) H. N. Blount, N. Winograd, and T. Kuwana, J. Phys. Chem., 74, 3231 (1970). (11) W. von Benken and T. Kuwana, Anal. Chem. 42, 1114 (1970). (12) G. Grant and T. Kuwana, J . Electroanal. Chem., 24, 11 (1970). (13) J. F. Evans and H. N. Blount, J . Phys. Chem., 80, 1011 (1976). (14) S.Feldberg in "ElectroanalyticalChemistry", Vol. 3, A. J. Bard, Ed., Marcel Dekker, New York, 1969. (15) D. T. Shang and H. N. Blount, J. Hectroanal. Chem., 54, 305 (1974). (16) M. D. Hawley and S. W. Feldberg, J . Phys. Chem., 70, 3459 (1966). (17) R. E. Sioda, J. Phys. Chem., 72, 2322 (1968). (18) R. Dietz and B. E. Larcombe, J . Chem. SOC. B , 1369 (1970). (19) H. N. Blount and T. Kuwana, J. Nectroanal. Chem., 27, 464 (1970). (20) J. F. Evans and H. N. Blount, J . Electroanal. Chem., in press. (21) The limit of solubility of DPA in acetonitrile is ca. 1.1 mM. Hence, preparation of millimolar DPA'. dictates essentially exhaustive electrolysis of the DPA solution, and such a process requires the better part of an hour. (22) I n the OCR/SE experiment, as in the case of SPSISE, the spatial distribution of DPA'. in the diffusion/reaction layer adjacent to the electrode is nonhomogeneous.20
Gaseous Heats of Hydrogenation of Some Cyclic and Open Chain Alkenes Richard Fuchs" and L. Alan Peacock Department of Chemistly, University of Houston, Houston, Texas 77004 (Received February 20, 1979)
Gaseous heats of hydrogenation at 25 " C [",(g)] have been calculated for cyclohexene, 1-methylcyclopentene, cycloheptatriene, 1-methylcyclohexene,methylenecyclohexane, 1-ethylcyclopentene, ethylidenecyclopentane, 1,3,5,7-~yclooctatetraene, ethylidenecyclohexane, vinylcyclohexane, allylcyclopentane, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-dodecene, 1-hexadecene, and trans-2,2,5,5-tetramethyl-3-hexene, based on previously measured solution values of A",, and experimental heats of vaporization and heats of solution of the alkenes and the hydrogenation products. Calculated MH,(g) values are from 0.4 kcal/mol less exothermic to 2.5 kcal/mol more exothermic than experimental values in solution, and 0.4-1.2 kcal/mol less exothermic than experimental gaseous values measured at 82 "C. Gaseous heats of formation of 2,2,5,5tetramethylhexane and cyclooctatetraene have been calculated from AHH,(g)values. m&) values from various sources are compared.
AH(v+ s ) A"
I
-AH(v+S)
(soh)
0022-3654/79/2083-1975$01 .OO/O
where AH(v+s), the enthalpy of transfer from vapor to solvent, is measured by the relationship AH(v+s) = AHB- AH, AHBis the heat of solution of the liquid hydrocarbon in the solvent, and AHvis the hydrocarbon heat of vaporization. The experimental value of AHH,(soln)may or may not include the alkene heat of solution. In Turner's work3 the alkenes were dissolved and thermally equilibrated before measurement of the heat of hydrogenation, and AHH,(g) is given by AH&) = AHH,(soln) + MJalkane) - AHJalkene) AH,(alkane) + AH,(alkene) In other s t ~ d i e s l , ~where , ~ , ~ AHs(alkene) and AHHz are measured simultaneously, the last term of above equation is omitted. Note that if a substance is formed or destroyed in solution, a AHs or -AHs term becomes part of the overall thermal effect. A complete analysis of the hydrogenation of 1-heptene in acetic acid a t 29 "C was carried out by william^,^ who used AHB,AHv,and heat capacity corrections to calculate mH,(g)j which was within 0.1 kcal/mol of Kistiakowsky's experimental gaseous value a t 82 0C.2 A rather widely accepted view1 has been that the heat of solution terms for nonpolar compounds introduce errors of
