Prepared froni 4-(2,6,6-trimcthylcyclohex-l-cnyl) -but-3-yn-2-01 acetate by epoxidation followed by lithium aluminium hydridc reduction: NMR. (CDCI,) : 1.04(3H, s). 1.20(3H, s ) , 1.27(311, d, J = ~H z ) , 1.34(3H, s), 4,36(1H, q, ,I = 7 Hz), 5,46(1H, d x d , J = 7HZ, J' -5112) ppni; IR. (liq.): 3350, 1955 cm-l.Summary. The literature data on hcteropolar and homopolar 2-center bond dissociation energies in organic molecules in the gas phase and the corrcsponding heats of formation of radicals and ions havc been critically evaluated. Data for more than 500 bonds are represented in tabular form together with the pertinent literature references.Selected electron affinitics and n-bond dissociation energies have also bccn incorporated. The follow-up paper will discuss some cmpirical general aspects of these data particularly regarding the effect of structure on the bond dissociation energies.

The kinetics of the thermal reactions of cyclopropane and cyclobutane behind reflected shock waves have been studied in a single-pulse chemical shock tube. Comparisons of methods for obtaining reflected-shock temperatures and for estimating the extent of unimolecular fall-off are presented.In the temperature range 950-1653 K, the predominant reaction of cyclopropane is the unimolecular isomerization to propene. Fully-corrected first-order rate constants are in good agreement with extrapolated low-temperature literature data up to cu. 1130 K, but at higher temperatures, previously reported curvature in the Arrhenius plot is confirmed.Between 891 and 1400K, cyclobutane decomposes unimolecularly giving ethylene as the only observable product. Fully-corrected first-order rate constants for this reaction are also in agreement with extrapolated low-temperature literature data up to cu. 1080 K but pronounced curvature in the Arrhenius plot is evident above this temperature. Possible explanations for these anomalies above ca. 1100 K are discussed.Recent shock-tube studies of the well-authenticated thermal unimolecular isomerization of cyclopropane to propene 1 * have established that at temperatures greater than ca. 1200 K, the first-order rate constants are substantially lower than those predicted from the data obtained at lower temperatures and severe curvature in the Arrhenius plot is evident. These results suggest that the shock-tube method or current theories of unimolecular reactions may be in error.The present work was undertaken to examine further the kinetics of well-established thermal unimolecular reactions behind reflected shock waves and reports a re-investigation of the cyclopropane isomerization, together with a study of the decomposition of cyclobutane to ethylene.

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Summary. The preceeding paper presents a critical tabular compilation of 2-center homo-and heteropolar bond dissociation energies. This paper deals with some empirically derived general aspects of these data, particularly regarding relationships between structure and reactivity, i.e. substituent effects on bond dissociation energies. 'Extra' stabilization energies generated in electronically delocalized radicals or ions derived from these experimental data are also presented.

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