Arrange the following hexenes in decreasing order of heat of hydrogenation.
(I)
(II)
(III)
(IV)

Question

Arrange the following hexenes in decreasing order of heat of hydrogenation. (I) (II) (III) (IV)

TestHub · Accepted Answer

A less stable alkene has more potential energy, so it gives out more heat on hydrogenation : heat of hydrogenation is proportional to 1/stability.

The 2-hexenes have 5 α-hydrogens and the 3-hexenes have 4, so the 2-hexenes are more stabilised by hyperconjugation. Within each pair the trans isomer is more stable than the cis isomer.

Stability : III $>$ IV $>$ II $>$ I, so the heat of hydrogenation order is I $>$ II $>$ IV $>$ III.

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