What is the correct order of exothermicity for hydrogenation of the following hexadienes upon treatment with H2/Pd (more exothermic `gt` less exothermic)?

Options

A. `3 gt 1 gt 2`
B. `1 gt 2 gt 3`
C. `3 gt 2 gt 1`
D. `2 gt 1 gt 3 `

Answer

A

Explanation

More stable dienes release less heat upon hydrogenation than less stable dienes, i.e., the reaction of a more stable a diene is least exothermic (this is only valid when the alkenes being compared undergo hydrogenation to give the same alkane). The stability of these dienes can be assessed by considering the presence of conjugation and the number of alkyl substituents on sp2 carbon atoms. Compound 1 is a conjugated diene with a monosubstituted double bond and a disubstituted double bond. There is a less stable diene listed which would undergo a more exothermic hydrogenation