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1 2008 Lecture 18 Dienes - Chem 2203/2207 Answers to Test 2...

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Chem 2203/2207 Answers to Test 2 are now posted on http://http-server.carleton.ca/~pbuist/
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Dienes Chapter 14
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Dienes 1) Conjugated double bonds 2) Evidence for conjugation 3) Reactions of dienes Isoprene Ergosterol
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Conjugated double bonds Conjugate means “to link” 1-butene 1,3-butadiene 1,4-pentadiene 1,3( E )-pentadiene “Non-conjugated” or “isolated “ double bonds “Conjugated” double bonds
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H H H H Non-conjugated diene H H H H sp 2 sp 3 sp 2 2 Electrons in the π -Bonding Molecular orbital of each double bond are insulated from each other
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H H H H H H H H Conjugated diene sp 2 π Molecular orbitals now linked to allow electrons to flow between carbons: resonance forms - + - +
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2. Evidence for conjugation A. Bond Lengths + - 1.48 Å H 3 C–CH 3 1.54 Å H 2 C=CH 2 1.33 Å Bond distance between that of a single and a double bond
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2. Evidence for conjugation B. Barrier to rotation “transoid” “cisoid” Single bonds between conjugated pi bonds are slightly stiffer than normal C-C bonds. E
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2. Evidence for conjugation C. Thermochemical H H 2 Pt / E Conjugation makes compound more stable by 12-16 kJ/mol - less energy released upon hydrogenation.
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