The vapour pressure of aluminium triethyl is measured in the range 60-120°C, and found to obey the relationship loglop -10-85-3613/7', which gives values of 186"C, 16.5 kcal/mole of vapour and 35.9 cal/mole deg. for the boiling point, heat of vaporization and Trouton's constant. These values are anomalous and are attributed to the presence of association,The heat of association is -10.2 kcal/mole of A.l,Et,. This value is used to evaluate the Arrhenius parameters of the reaction of aluminium triethyl monomer and a-alkenes.Alkyl aluminium compounds are associated species, but the extent of association depends strongly on the nature of the alkyl group. Aluminium n-alkyls are dimeric,l while aluminium tri-(iso-propyl) and tri-(j so-butyl) are monomeric. 1-3 Aluminium di(alky1) hydrides are trimeric.2Electron diffraction studies have established that the electron deficient aluminium tri-n-alkyl dimers have a boron hydride structure with bridging alkyl groups, i.e.,
The kinetics of the addition reactions of aluminium triethyl with several linear, branched and cyclic alkenes have been re-studied, using dilatometry and g.1.c. product analysis. The concentration dependence of rate is consistent with the monomeric form of aluminium triethyl alone being reactive. The Arrhenius parameters are analyzed in terms of four-centre transition states for the addition react ions.Conflicting reaction mechanisms 1-6 have been proposed for the addition of aluminium triethyl to n-alkenes. In previous publications in this series l 9 it was established that the reaction was inhibited by dimerization of aluminium triethyl and that monomer alone was active in adding to the alkene. This confirmed the findings of Zeigler et aL39 Natta et aZ.,5 however, concluded that dimer was the active species while Allison et aL6 considered, from a comparison of Arrhenius parameters, that dimer participated either exclusively or in conjunction with monomer in the rate-determining step of the addition reaction. The similarity in structure between aluminium trialkyl dimers and the soluble co-ordinated anionic polymerization initiator complexes, obtained from reacting bis-cyclopentadienyl titanium dichloride and aluminium triethyl, i.e., C5H5 X C2H5\ / \ / Ti A1 / \ / \ C5H5 X C2H5 where X r C 2 H 5 or C1, has been used to infer that the addition reaction involved dimer participation alone.7A further study of this system has been made using dilatometry since initial studies with n-hexene-1 indicated that the volume change which accompanied the reaction was directly related to the reaction extent, and that the technique was less restricted than that of a constant pressure manostat, used previously with more volatile alkenes. EXPERIMENTALThe alkanes and alkenes, listed in table 1, were employed as reagents or calibrants for g.1.c. analysis. The purities listed are minimum values as determined by g.1.c. analysis. The reagents were used after drying under vacuum over activated molecular sieves (type 4A), and vacuum distillation.
The kinetics of the addition reaction of aluminium trimethyl to n-octene-1 have been studied for comparison with those of aluminium triethyl and in order to confirm the details of the mechanism. In both, the reactions are inhibited by association of the alkyl compounds into dimers since only the unassociated species are reactive. The higher degree of association of aluminium trimethyl alone accounts for its lower reactivity and the maximum difference in activation energy between the two reactions (8&6 kJ mol-l) is similar to that in the strengths of the methyl and ethyl bonds.While the addition of aluminium triethyl to alkene-1 has been of considerable research interest,1-8 there is no reported study with aluminium trimethyl. This is surprising since it is potentially capable of resolving the controversy over the mechanism of the addition reaction,1-8 and of deciding the importance of association of the aluminium alkyls on their reactivity to addition. Accordingly the kinetics of the addition of aluminium trimethyl to n-octene-1 have been studied for comparison with those of aluminium triethyl.EXPERIMENTAL n-ocTmE-1 (Koch-Light Ltd.) was dried over activated molecular sieves, type 4A, degassed and twice distilled in vacuo. Purity, measured by g.1.c. analysis, was >98 %.ALUMINIUM TRIMETHYL (Ethyl Corporation Ltd.) was supplied by Dr. A. D. Caunt, I.C.I. Ltd., Plastics Division, to whom we are indebted for advice on handling and storing. It was handled in a glove box under an atmosphere of dry air-free nitrogen. It was purified by repeated distillation in vacuo in all-glass apparatus.ALUMINIUM TRIETHYL (L. Light and Co. Ltd.) was used unpurified. It was 90-95 % pure as measured from analysis of the hydrolysis products.Reaction rates were measured dilatometrically in decahydronaphthalene solutions by procedures developed previously. 2~ RESULTS PRELIMINARY STUDIESInitial experiments with aluminium trimethyl and n-octene-1 in decalin indicated that a definite reaction occurred with measurable rates in the temperature range 400-440 K. G.1.c. analysis of the reaction hydrolysis products also revealed the presence of unsaturated and saturated c8 and C9 hydrocarbons. Dilatometry was selected to monitor the reaction as it was followed by considerable volume contraction.
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