The kinetics of the gas phase intramolecular elimination of isobutene from 2-d1-triisobutylaluminum and the deuterium isotope effect in reactions involving cyclic transition states

✍ Scribed by Kurt W. Egger

• Publisher: John Wiley and Sons
• Year: 1969
• Tongue: English
• Weight: 654 KB
• Volume: 1
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550010505

✦ Synopsis

The intramolecular elimination of isobutene from 2-d,-triisobutylaluminum has been studied in the gas phase for temperatures ranging between 102.4 and 184.6'C. The reaction is apparently homogeneous and obeys the first order rate law, yielding the following Arrhenius relationship: log kelim (sec-') = 11.1 -27.218 where 0 equals 4.58 x T('K) in units of kcal/mole. Excess ethylene was added to the starting material in order to avoid complications from the backreaction. The cyclic 4-center nature of the transition state proposed earlier has been unequivocally demonstrated by deuterium labelling. Mass-spectral analyses show that the isobutene formed contains no deuterium. The hydrolyses products of the mixed trialkylaluminum formed during the reaction consist of monodeuteroethane and 2-d,isobutane. The observed negative entropy of activation of -12 cal/'-mole agrees with prediction and implies a reasonably tight transition state structure. Combined with the corresponding data for the non deuterized Al(i-bu), reported earlier, these data result in a primay kinetic deuterium isotope effect of k,/k, = 1.3 x corresponding to a ratio of the isotopic rate constants of 3.7 at 25OC. This result is in excellent agreement with a predicted value of 1.4 x and it is in line with literature data on similar reactions involving cyclic transition state complexes.