JEE AdvancedHydrocarbonsHard

Question

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

Options

A.1-butene
B.trans-2butene
C.cis-2-buteme
D.1, 3-butadiene

Solution

(i) Conjugated dienes (e.g., 1,3-butadiene) are more stable due to resonance stabilization.
(ii) Trans-alkene will be mor stable than cis-alkene due to lees steric repulsion.
(iii) is less stable than -2-butene because 2-butene may have six hyperconjugation structures while 1-butene may have only two.
On this basis of arguments (i) to (iii).
Order of stability of given compund as 1, 3-butadiene > trans-2 butene > cis-2butene > 1-butene
∴ Heat of hydrogenation
1-butene > cis-2-butene > trans-2 butene > 1, 3 -butadiene

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