Stability Of Alkenes

Stability Of Alkenes

The relative stability of alkenes can be determined by comparing their enthalpies of hydrogenation. Enthalpy of hydrogenation is the heat evolved when one mole of alkene reacts with hydrogen. For example, both the geometrical isomers of bur-2-ene consume one mole of H₂ and produce the same product, butane upon catalytic hydrogenation in the presence of a catalyst such as platinum or palladium.

It has been observed that enthalpy of hydrogenation of cis-isomer is 120.0 kJ mol^–1 and that of trans-isomer is 115.0 kJ mol^–1. Since the product has the same energy in both the reactions, any difference in the heat produced must be due to the difference in the energies of the starting alkene. The trans-isomer evolves about 5 kJ mol^–1 less heat than the cis-isomer, it means that it contains 5 kJ mol^–1 less energy. In other words, the trans-isomer is 5 kJ mol^–1 more stable than the cis-isomer.