MO for 1,3-Butadiene Strongly all antibonding antibonding. Very high. 4 Vacant at ground state. node node node Chapter 15 11
Chapter 15 11 4 * MO for 1,3-Butadiene ▪ Strongly antibonding. ▪ Very high. ▪ Vacant at ground state
MO for 1,3-Butadiene and Ethylene butadiene ethylene The bonding MOs of both 1,3-butadiene and ethylene are filled and the antibonding MOs are empty. p orbital ·Butadiene has lower energy than ethylene. This lower energy is the resonance stabilization of the conjugated diene. Copyright 2010 Pearson Prentice Hall,Inc Chapter 15 12
Chapter 15 12 MO for 1,3-Butadiene and Ethylene ▪ The bonding MOs of both 1,3-butadiene and ethylene are filled and the antibonding MOs are empty. ▪ Butadiene has lower energy than ethylene. ▪ This lower energy is the resonance stabilization of the conjugated diene
Conformations of 1.3-Butadiene The s-trans conformer is more stable than the s-cis by 12 kJ/mol (2.8 kcal/mol). Easily interconvert at room temperature. s-trans s-cis H Chapter 15 13
Chapter 15 13 Conformations of 1,3-Butadiene ▪ The s-trans conformer is more stable than the s-cis by 12 kJ/mol (2.8 kcal/mol). ▪ Easily interconvert at room temperature. H H H H H H s -trans s -cis H H H H H H
Allylic Cations H H H2C=C-CH2 H2C-C=CH2 The positive charge is delocalized over two carbons by resonance giving the allyl cation more stability than nonconjugated cations. Chapter 15 14
Chapter 15 14 Allylic Cations ▪ The positive charge is delocalized over two carbons by resonance giving the allyl cation more stability than nonconjugated cations. H2C C H CH2 + H2C C H CH2 +
Stability of Carbocations H3C+<1°<2°，allyl<3°，substituted allylic X H2C---CH--=CH2 is about as stable as CH3-CH-CH3 Copyright 2010 Pearson Prentice Hall,Inc. Stability of1 allylic≈2°carbocation. "Stability of2°allylic≈3 carbocation. Chapter 15 15
Chapter 15 15 Stability of Carbocations ▪ Stability of 1 allylic 2 carbocation. ▪ Stability of 2 allylic 3 carbocation