Chapter15 - Chapter 15 Conjugated Systems Orbital Symmetry...

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Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
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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
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Chaper 15 3 Conjugated Dienes from heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes H 0 -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
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Chaper 15 4 Relative Stabilities twice 1-pentene more substituted =>
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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. =>
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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. =>
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Chaper 15 7 π 1 MO for 1,3-Butadiene Lowest energy. All bonding interactions. Electrons are delocalized over four nuclei. =>
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Chaper 15 8 π 2 MO for 1,3-Butadiene 2 bonding interactions. 1 antibonding interaction. A bonding MO. =>
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Chaper 15 9 π 3 * MO for 1,3-Butadiene Antibonding MO. Empty at ground state. Two nodes. =>
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Chaper 15 10 π 4 * MO for 1,3-Butadiene All antibonding interactions. Highest energy. Vacant at ground state. =>
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Chaper 15 11 MO Energy Diagram The average energy of electrons is lower in the conjugated compound. =>
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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 =>
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Chaper 15 13 Allylic Cations Carbon adjacent to C=C is allylic. Allylic cation is stabilized by resonance. Stability of 1 ° allylic 2 ° carbocation. Stability of 2 ° allylic 3 ° carbocation. H 2 C C H CH 2 + H 2 C C H CH 2 + =>
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Chaper 15 14 Addition of HBr H 2 C C H C H CH 2 H + H 3 C C H C H CH 2 + H 3 C C H C H CH 2 + Br _ Br _ H 3 C C H C H CH 2 Br H 3 C C H C H CH 2 Br 1,2-addition product 1,4-addition product =>
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Chaper 15 15 1,2- and 1,4-Addition to Conjugated Dienes Electrophilic addition to the double bond produces the most stable intermediate.
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