Chapter15

Chapter15 - Chapter 15 Conjugated Systems Orbital Symmetry...

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Unformatted text preview: Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Chaper 15 2 Definitions • Conjugated double bonds are separated by one single bond. Examples: 1,3-pentadiene • Isolated double bonds are separated by two or more single bonds. Example: 1,4- pentadiene. • Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. O C H H 3 C H CH 3 allenes Chaper 15 3 Conjugated Dienes from heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes ∆ H-237 (-56.5) 1,3-Butadiene-126 (-30.1)-127 (-30.3) kJ (kcal)/mol Name 1-Pentene 1-Butene trans- 1,3-Pentadiene 1,4-Pentadiene trans- 2-Butene-115 (-27.6) cis- 2-Butene-120 (-28.6)-226 (-54.1)-254 (-60.8) Structural Formula =2*1-Pentene Releases less energy than two double bonds The difference between the heat of hydrogenation of the conjugated diene and the sum of two double bonds is the resonance energy-237 kJ/mol – 2*(-127 kJ/mol) = 17 kJ/mol Chaper 15 4 Relative Stabilities twice 1-pentene more substituted => Chaper 15 5 Constructing Molecular Orbitals • Pi molecular orbitals are the sideways overlap of p orbitals. • p orbitals have 2 lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charge. • When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed. • When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. => Chaper 15 6 Ethylene Pi MO’s • The combination of two p orbitals must give two molecular orbitals. • Constructive overlap is a bonding MO. • Destructive overlap is an anti-bonding MO. => Chaper 15 7 π 1 MO for 1,3-Butadiene • Lowest energy. • All bonding interactions. • Electrons are delocalized over four nuclei. => Chaper 15 8 π 2 MO for 1,3-Butadiene • 2 bonding interactions. • 1 antibonding interaction. • A bonding MO. => Chaper 15 9 π 3 * MO for 1,3-Butadiene • Antibonding MO. • Empty at ground state. • Two nodes. => Chaper 15 10 π 4 * MO for 1,3-Butadiene • All antibonding interactions. • Highest energy. • Vacant at ground state. => Chaper 15 11 MO Energy Diagram The average energy of electrons is lower in the conjugated compound. => Chaper 15 12 Conformations of 1,3-Butadiene • 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 => Chaper 15 13 Allylic Cations • Carbon adjacent to C=C is allylic. • Allylic cation is stabilized by resonance. • Stability of 1 ° allylic ≈ 2 ° carbocation....
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This note was uploaded on 09/19/2011 for the course CHM 262 taught by Professor Staff during the Spring '08 term at Purdue.

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Chapter15 - Chapter 15 Conjugated Systems Orbital Symmetry...

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