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

Options

A. `1-`Butene
B. trans `-2-`Butene
C. `cis-2-`Butene
D. `1,3-`Butadiene

Answer

d

Explanation

`1,3-`Butadiene shows the smallest heat of hydrogenation due to its higher stability which is due to resonance.
trans `-2-` Butene is more stable than cis-2- butene on the basis of heat of hydrogenation.
`1-` Butene is less stable than `-2-` butene due to hyperconjugation (shows six no bond resonating structures) `9` hyperconjugative structures, while `1-` butene due to hyperconjugationi shows no bond resonating structures. (Resonance is more powerful than hyper`-` conjugation.).
Order of stability `:`
`1,3-`Butadiene `gt` trans `-2-`Butene `gt` cis-2-Butene `gt 1-`Butene
Heat of hydrogenation
`1-`Butene `gt cis -2-` Butene `gt `trans `-2-`Butene `gt 1,3-`Butadiene