Bond dissociation energies—a continuing story

Abstract

Most modern bond dissociation energies are obtained from kinetic rather than thermodynamic studies. This requires assumptions sometimes explicit and sometimes implicit which always require close scrutiny. Three case histories are examined spanning the period from 1934 to the present which illustrate the importance of understanding the chemistry involved in even very simple reactions. The oldest involves the reactions of alkyl iodides with HI, which yielded the C—H bond strengths in CH₄, C₂H₆ and PrⁿH. The second explores the pyrolysis of alkyl sulphides and mercaptans which exposed an unexpected isomerization of alkylthiyl radicals. The last reviews the reaction Br atoms with isobutane which can establish the tertiary C—H bond strength. Most recent experimental results from VLPR studies are puzzling, showing an unexpected and unexplained variation of the second-order rate constant with initial (Br) atom concentration. Extrapolation gives ΔH^°_f298(Bu^t)= 9.4 ± 0.5 kcal mol^–1 and DH^°₂₉₈(Bu^t—H)= 93.7 ± 0.5 kcal mol^–1, in agreement with relative bromination and iodination studies.