Question

What is the order of heat of hydrogenation of the following compounds:

A. 2 > 3 > 5 > 1 > 4

B. 2 > 3 > 5 > 4 > 1

C. 1 > 2 > 3 > 5 > 4

D. 5 > 3 > 2 > 4 > 1

Answer 1

Hint: Heat of hydrogenation (i.e. $$\mathrm{\Delta }{H}^o$$) of an alkene is basically the standard enthalpy of the catalytic hydrogenation of an alkene and catalytic hydrogenation of an alkene is an exothermic process. This indicates that the heat of hydrogenation of an alkene is always negative.

Complete answer:

Heat of hydrogenation of an alkene is actually considered to be the measure of stability of the carbon-carbon double bonds (i.e. $$C=C$$). It should be noted that smaller the value of heat of hydrogenation i.e. $$\mathrm{\Delta }{H}^o$$, more is the stability of the double bond. So depending upon the heat of hydrogenation of alkenes, the stability trend of carbon-carbon double bonds will be as follows: $\text{ tetra-substituted > tri-substituted > di-substituted > mono-substituted > unsubstituted}$. 

In the given question, the structure (1) is tetra-substituted and therefore it is the most stable among all given structures and structure (5) being 1,1 di-substituted is least stable among the given structures. 

Thus, the order of stability among the given structures is: 

$\text{1 (tetra-substituted)>4 (tri-substituted)>2 (Z-di-substituted)>3 (E-di-substituted)>5 (1,1 di-substituted)}$. 

The most stable alkene possesses the smallest value of heat of hydrogenation. On the basis of the order of stability, Heat of hydrogenation order would be $$5>3>2>4>1$$

Hence, the correct answer is Option D.

Note:

An alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as a synonym of olefin, that is, any hydrocarbon containing one or more double bonds. Also, it should be noted down that internal alkenes are more stable compared to terminal ones.