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 gas-phase kinetics and the mechanistics of the unimolecular elimination of isobutylene from triisobutylaluminum have been studied in the presence of excess ethylene or 1-butene for temperatures ranging from 107 to 173°. The 1-alkenes reacted very fast with the diisobutylaluminum hydride formed in the elimination process, thus avoiding complications from back and side reactions. The reaction is apparently homogeneous, when carried out in an all-Teflon reaction vessel. The computed least-squares analysis of the measured rate constants for the unimolecular elimination process yields (with standard errors) log k (sec-1) = (11.2 ± 0.4) -(26.6 ± 0.7)/#, where equals 4.58 X 10-3 (° ). Experiments with /3-D-triisobutylaluminum show that the deuterium is transferred to the aluminum atom in the elimination process. It can be concluded that the reaction involves a relatively tight polar four-center transition state. The observed preexponential factor indicates a loss of entropy of ~12 cal/(deg mole) in forming the cyclic transition state. The general applicability of the concept of four-center reaction mechanisms in the chemistry of aluminum alkyls and their derivatives is outlined. The activation energy of the back-reaction, the addition of isobutylene to the diisobutylaluminum hydride, is estimated at 6 ± 3 kcal/mole. The pertinent thermodynamic data have been reviewed.It is generally accepted that reactions involving aluminum alkyls and their derivatives are largely controlled by molecular mechanisms.1 Radical reactions only occur at elevated temperatures, concurrent with the molecular reaction paths.16The preference for molecular reactions (in contrast to most other metal alkyls) originates from the relatively high Al-C and Al-H bond strength involved and from the electron deficiency of the aluminum atom with the valence coordination of three. This leads to the ready formation of electron-deficient or "half" bonds.The majority of the reactions (both intraand intermolecular) involving aluminum alkyls and their derivatives can be rationalized with a four-center transition state. In a few exceptional cases six-membered transition states are operative2-3 as in the addition of butadiene to diisobutylaluminum hydride, where both 1,2 and 1,4 addition have been observed.23 Six-center transition states are possible with a 1,3diene, allowing for six "half" bonds. With 1,4-or 1,5-dienes, two consecutive four-center additions take place.16 With longer chains, the steric effects inhibit the intramolecular addition of the second olefinic bond to the Al-C bond already formed.4 A general concept of four-center and six-center transition states in chemical reactions has been outlined in view of the predictability of the kinetic parameters of such reactions.5 (1) (a) K. Ziegler in "Organometallic Chemistry," . H. Zeiss, Ed.,

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