Thermodynamic properties of selected boron compounds with particular reference to the peroxyboranes and their role in hydrocarbon oxidation

Abstract

The standard enthalpy of formation of tri(tertiary-butylperoxy)horane has been determined from a thermochemical study of its hydrolysis in an isoperibol reaction calorimeter and the enthalpy of vaporisation has been determined using tensimetry. The strength of the peroxide bond, E(BO-OR), and the boron-to-peroxide bond, E(B-OOR), were derived and the corresponding values for tri(normal-butylperoxy)borane estimated. The role of boron containing coreactants in hydrocarbon autoxidations in directing the reaction towards increased alcohol:ketone ratios was explained in terms of the formation of organoperoxyboranes as intermediates and their subsequent decomposition under oxidation conditions.

Mesityldichloroborane and ortho-, meta-, and para-tolyldichloroboranes have been studied thermochemically and their standard enthalpies of formation derived. The enthalpies of vaporisation of meta-tolyldichloro-borane and mesityldichloroborane have been determined tensimetrically. Derived boron-to-carbon bond energies were interpreted in terms of steric and electronic effects of the methyl substituents. The steric effect of ortho methyl substitution caused a decrease in the boron-to-carbon bond energy whilst meta-methyl substitution caused a similar decrease through an electronic effect.

Pure samples of rubidium and caesium tetrachloroborates have been prepared using a modified synthetic procedure. Hydrolyses were studied thermochemically from which the standard enthalpies of the two salts were derived. Using a semi-empirical equation, the 'thermochemical radius' of the tetrachloroborate anion and the lattice energies of the prepared tetrachloroborates were estimated. The thermodynamic instability of thealkali-metal tetrachloroborates relative to the corresponding tetrafluoro-borates was demonstrated.

A list of standard enthalpies of reaction of organoboron compounds was compiled for inclusion in 'CATCH' tables (Computer Analysis of ThermoCHemical data) processed by J.B. Pedley et al of the University of Sussex. The intent is that these tables should provide a widely accessible and readily updated source of thermodynamic data.