Thermochemistry of Cyanocarbons

Abstract

The heats of combustion of several cyanocarbons have been measured as well as auxiliary heats of vaporization (from vapor pressures) and solution in order to determine heats of formation. The cyano‐olefins, tricyano and tetracyano ethylene have negative stabilization energies indicating electrostatic repulsion between the highly polar cyano groups predominates over electronic delocalization stabilization (—15 and —27 kcal/mole, respectively). In 7,7,8,8‐tetracyanoquinodimethan the net effect is stabilization (20 kcal/mole). The electron affinity of 7,7,8,8‐tetracyanoquinodimethan has been estimated at 86 kcal from the heats of formation of its aqueous ion‐radical salt solutions and an estimate of the hydration energy of the ion radical. Heats of solution of ion‐radical salts indicate that the binding energy of ion‐radical pairs in the crystal is of the order of 14 kcal/mole‐ion radical. The heats of formation of aqueous solutions of the strong organic acids pentacyanopropene and hexacyanoisobutylene compared with calculated values for the unionized forms indicate enthalpies of ionization (acid strength) comparable to mineral acids.