Week 5 Slides - Organic Chemistry II UC-Berkeley Extension...

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Organic Chemistry II UC-Berkeley Extension x36B Fall2011 October 17, 2011 Silence/Turn off your Cell Phones Week 5: Conjugated Systems: Conjugated, unconjugated and cumulated dienes, stability of conjugated systems, effect of conjugation on kinetic vs. thermodynamic pathways, UV-Visible spectroscopy, electronic transitions between π bonding and π * anti-bonding molecular orbitals, Beer’s law, degree of conjugation.
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Week 5 Structure of Alkenes 2 A double bond consists of One ± bond formed by the overlap of sp 2 hybrid orbitals and one π bond formed by the overlap of parallel 2 p orbitals. the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°.
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Week 5 Allene 3
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Week 5 Conjugated Dienes 4 From heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes. Δ 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
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Week 5 Conjugated Dienes Butadiene 5 Conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol . 2H 2 + catalyst Δ H 0 = 2(-127 kJ/mol) = -254 kJ/mol) 2 2 2H 2 + Δ H 0 = -237 kJ/mol catalyst
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Week 5 Conjugated Dienes Butadiene 6 Conjugation of double bonds in butadiene gives the molecule an additional stability of approximately 17 kJ/mol.
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Week 5 Conjugated Systems 7 Systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds. 3-Cyclohexenone (less stable) 2-Cyclohexenone (more stable) O O
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Week 5 Structure of Butadiene MOs 8 Combination of four parallel 2p atomic orbitals gives two π -bonding MOs (this screen) and two π -antibonding MOs (the next screen).
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Week 5 Structure of Butadiene MOs 9 the two π -antibonding MOs of butadiene.
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This note was uploaded on 01/22/2012 for the course CHEM II taught by Professor G during the Fall '11 term at University of California, Berkeley.

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Week 5 Slides - Organic Chemistry II UC-Berkeley Extension...

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