Question

Hydrogenation of unsaturated hydrocarbons to saturated hydrocarbons is an exothermic reaction. One mole of each of the following unsaturated hydrocarbon is subjected to complete hydrogenation.

$CH_3-CH_2-CH = CH_2$ (1)

$CH_3-CH = CH - CH_3$ cis (11)

$CH_3-CH = CH – CH_3$ trans (III)

$CH_2=CH- CH = CH_2$ (IV)

Arrange them in the increasing order of magnitude of heat of hydrogenation.

• A. III < IV < II < I

Answer 1

Answer: (A)

III < IV < II < I

Answer 2

1. Concept: The heat of hydrogenation is inversely related to the stability of the alkene. More stable alkenes release less heat on hydrogenation.

2. Comparison of alkenes:

   • (III) trans-2-butene: Most stable due to trans configuration.
   • (IV) 1,3-butadiene: Conjugation gives extra stability, though less than trans-2-butene.
   • (II) cis-2-butene: Less stable than trans isomer.
   • (I) 1-butene: Terminal alkene; least substituted and least stable.

3. Order: Thus, in increasing order of heat of hydrogenation (i.e. lowest to highest heat release):

   $$\text{III} < \text{IV} < \text{II} < \text{I}$$