Question

Which of the following has the smallest heat of hydrogenation per mole?

• A. $1-\text{Butene}$
• B. $Trans-2-\text{butene}$
• C. $Cis-2-\text{butene}$
• D. $1,3-\text{Butadiene}$

Answer 1

The correct option is B $Trans-2-\text{butene}$

Among alkenes, heat of hydrogenation depends on:
(a) The number of double bonds:-
More the number of double bonds, more is the amount of heat of hydrogenation.
Hence, $1,3-\text{butadiene}$ has the highest heat of hydrogenation among these.
(b) Relative stability of alkenes:-
Greater the stability of alkene, smaller is the heat of hydrogenation.
$1-\text{Butene}$ is less stable than $2-\text{butene}$ as $1-\text{butene}$ forms less number of hyperconjugative structures.

In between cis and trans, $trans-2-\text{butene}$ is the more stable.
So, among the given alkenes, $trans-2-\text{butene}$ has least heat of hydrogenation.