Stabilities of Dienes Heat of hydrogenation for 1,4-pentadiene is -252 kJ/mol,about twice that of 1-pentene. ·△Hfor1-pentene is-126 kJ/mol and for trans-2-pentene is -116 kJ/mol,so expect -242 kJ/mol for trans-1,3-pentadiene. ·Actual△His-225kJ/mol,so the conjugated diene is more stable. Difference,-242 kJ(-225 kJ)=17 kJ/mol,is the resonance energy. => Chaper 15 3 Chaper 15 3 Stabilities of Dienes • Heat of hydrogenation for 1,4-pentadiene is –252 kJ/mol, about twice that of 1-pentene. • H for 1-pentene is -126 kJ/mol and for trans-2-pentene is -116 kJ/mol, so expect -242 kJ/mol for trans-1,3-pentadiene. • Actual H is -225 kJ/mol, so the conjugated diene is more stable. • Difference, -242 kJ (-225 kJ) = 17 kJ/mol, is the resonance energy. =>