Question

Why do the most stable alkenes have the smallest heat of hydrogenation?

Answer 1

Hint : Heat of hydrogenation of an alkene is defined as the standard enthalpy of the catalytic hydrogenation of that alkene. Note that the catalytic hydrogenation of an alkene is an exothermic reaction always. We can measure the stability of carbon-carbon double bonds from the heat of hydrogenation of alkenes.

Complete Step By Step Answer:
Alkenes are unsaturated hydrocarbons having a double bond. Hydrocarbons are the compounds which contain carbon and hydrogen in their compounds. Alkenes have a reactivity higher than that of alkanes and they are less stable than alkanes. The heat of hydrogenation of alkenes depends on the stability of alkene. Heat of hydrogenation is inversely proportional to stability of the alkene which means, if the numerical value of heat is high, then more energy is released, which means double bond is broken down very easily. The most stable alkene of the trio,$(E) - 2 - hexene$ Hydrogenation liberates the smallest amount of heat. The difference in the heat of hydrogenation is equal to the difference in heat of formation for these three straight chain alkenes. Therefore, the most stable alkenes have the smallest heat of hydrogenation.

Note :
The measure of the stability of carbon-carbon double bonds is given by the heat of hydrogenation of alkenes. The enthalpy of hydrogenation is defined as the change in enthalpy which takes place when one mole of an unsaturated compound reacts with excess hydrogen to become completely saturated at room temperature and atmospheric pressure.