Some thermodynamic measurements on organo-boron compounds

The symmetrical thioboranes (RS)3B , where R= methyl, n-propyl, n-butyl, n-amyl and phenyl, have been synthesized. Standard heats of formation and the energy (E) of the boron-sulphur bond were evaluated for each compound in the series. E(B-S) was found to be nearly constant except for trimethylthioborane.The synthesis and thermochemistry of tricyclohexylboron and compounds of the series 3-nBXn where X-C1, Br and n= 0,1,2, have been carried out. Standard heats of formation, enthalpies of transition and boron-carbon bond energies of these compounds are reported, the strength of the boron-phenyl bond was found to increase with successive replacement of phenyl by halogen, and this was interpreted in terms of structural features. The results of a single crystal X-ray crystallographic examination of triphenylboron are given.The synthesis and thermochemistry of the substituted derivatives are reported.The conjunctive effect of substituents in the aromatic ring on the adjacent dioxaboron ring was found to be negligible. Comparison of the experimental heats of formation with those calculated from a standard set of bond energies indicated that these molecules were strained to the extent. This was interpreted in terms of two opposing effects, (i) strain in the five-membered ring and (ii) resonance energy. A simple geometric method for estimating strain energies in boron hetero-cycles is derived and discussed.