Alkanes - Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals - Hydrocarbons
Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals
Homolytic bond dissociation energies provide a convenient way to estimate the relative stabilities of radicals. The energy required to break covalent bonds homolytically is called homolytic bond dissociation energy and abbreviated by the symbol . We find the following values of for the primary and secondary C-H bonds of propane.
This that for the reaction in which the designated C-H bonds are broken homolytically, the values of are those given below:
These reaction differ in the amount of energy required and in the type of carbon radical being produced. More energy must be supplied to produce a primary alkyl radical from propane than is required to produce a secondary carbon radical from the same compound. This must mean than the primary radical has absorbed more energy and thus has greater potential energy. As the relative stability of a chemical species is inversely related to its potential energy, the secondary radical must be more stable than the primary radical by 3.5 k cal . mol-1.
The tertiary radical is more stable than the primary radical by 7 k cal mol-1. The kind of pattern that we find in these examples is found with alkyl radicals generally; overall their relative stabilities are the following:
Tertiary > Secondary > Primary > Methyl