The concept of resonance explains various properties of compounds.The molecules with conjugated system of pi bonds, are stabilized by resonace and have low heat of hydrogenation.Hyperconjugative stabilization also decreases heat f hydrogenation.In aromic rings a functional group with a lone pair of electron exerts +m effect. Some functional groups like -NO, -NC, -CH=CH_2 can function both as electron releasing (+m,+R) or electron withdrawing (-m,-R) groups.More extended conjugation provides more stabilization. The correct heat of hydrogenation order is (p)1,3-Pentadiene , (q)1,3-Butadiene , (r)2,3-Dimethyl-1,3-butadiene , (s)Propadiene

The concept of resonance explains various properties of compounds.The

1.	The concept of resonance explains various properties of compounds.The molecules with conjugated system of pi bonds, are stabilized by resonace and have low heat of hydrogenation.Hyperconjugative stabilization also decreases heat f hydrogenation.In aromic rings a functional group with a lone pair of electron exerts +m effect. Some functional groups like -NO, -NC, -CH=CH_2 can function both as electron releasing (+m,+R) or electron withdrawing (-m,-R) groups.More extended conjugation provides more stabilization. The correct heat of hydrogenation order is (p)1,3-Pentadiene , (q)1,3-Butadiene , (r)2,3-Dimethyl-1,3-butadiene , (s)Propadiene
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