Heats of Dissociation of Carbon-Hydrogen Bonds in Methane and Its Radicals

Abstract

The evidence pertaining to the heat of sublimation of graphite is summarized, with the conclusion that the value of 170.6 kcal. per mole is the most probable. This figure is consistent with a value of 393 kcal. for the heat of removing the four hydrogen atoms from methane, the assignment of the heats of the successive dissociations being as follows: $$\begin{array}{c}{\mathrm{CH}}_4={\mathrm{CH}}_3+\mathrm{H}\text{−101\hspace{0.17em}}\mathrm{kcal}.\\ {\mathrm{CH}}_3={\mathrm{CH}}_2+\mathrm{H}\text{−<87\hspace{0.17em}}\mathrm{kcal}.\\ {\mathrm{CH}}_2=\mathrm{CH}+\mathrm{H}\text{−>125\hspace{0.17em}}\mathrm{kcal}.\\ \mathrm{CH}=\mathrm{C}{(}^3\mathrm{P)+}\mathrm{H}\text{−80\hspace{0.17em}}\mathrm{kcal}.\end{array}$$ The limits to the values for the second and third processes are determined on the basis of the kinetics of the reaction between sodium and methylene chloride. The methylene appearing above is in its lowest state, a singlet state in which the bonding to the hydrogen atoms is through p‐orbitals of atomic carbon, the bond containing a certain amount of ionic character. It is suggested that the formation of CH₂ in a triplet excited state, in which the bonding to the hydrogen atoms is through sp hybrid orbitals of atomic carbon, corresponds to the following thermochemical values: $$\begin{array}{c}{\mathrm{CH}}_3={\mathrm{CH}}_2^{*}+\mathrm{H}\text{−106\hspace{0.17em}}\mathrm{kcal}.\\ {\mathrm{CH}}_2^{*}=\mathrm{CH}+\mathrm{H}\text{−106\hspace{0.17em}}\mathrm{kcal}.\end{array}$$ From this suggestion it follows that the energy of excitation of the methylene ``molecule,'' the lowest state (singlet), to the methylene ``diradical,'' the triplet excited state, is at least 19 kcal. The evidence for the existence of the two states is discussed.