Question

Identify the alkene with the highest value of heat of hydrogenation:
(A)- propene
(B)- 2-methylpropene
(C)- cis-2-pentene
(D)- 2-methyl-2-butene

Answer 1

Hint: The heat of hydrogenation is the energy released on the catalytic hydrogenation of the alkenes. As more the stability of the alkene, less is the value of heat of hydrogenation.

Complete step by step answer:
The hydrogenation process of alkenes involves the breaking of the double bond in the alkene compounds, which is further dependent on the strength/ stability of the alkenes. The energy required to break these bonds is thus related to the stability.
The stability of the alkene is affected by the amount of substitution of the hydrogen atoms in the alkene. With the increase in the substitution, the stability increases due to the process of hyperconjugation in the substituted alkenes. Thus, more stable the alkene, stronger is its double bond and so the energy released on breaking the bond is less. That is, the stability of alkene is inversely proportional to the heat of hydrogenation.
Also, the number of double bonds in the compound affects the heat of hydrogenation. As with the increase in the number of double bonds in the alkene, the heat of hydrogenation increases. So, the order of stability of the given compounds is as follows:
$$2-methyl-2-butene2-methylpropene>cis-2-pentene>propene$$

Therefore, option (A)- propene with the least stability, has the highest heat of hydrogenation.

Note: In the hyperconjugation of the substituted alkenes, which depends on the number of hydrogens on the $$\alpha -\text{carbon}$$, adjacent to the double bond. As more the number of hydrogens, the resonance structures increases. So, more is the hyperconjugation and more is the stability of the alkene.
 Also, this hydrogenation process is an exothermic reaction, that is, release of energy on the catalytic breaking of the double bond and addition of hydrogens.