Chapter10

Chapter10 - Organic Chemistry CHE 275 Chapter 10...

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Unformatted text preview: Organic Chemistry CHE 275 Chapter 10 Conjugation in Dienes and Allylic Systems The Double Bond as a Substituent allylic carbocation C + C C allylic radical C C C conjugated diene C C C C C C C H H H H H The Allyl Group R Vinylic versus Allylic C C C vinylic carbons allylic carbon vinylic hydrogens are attached to vinylic carbons Vinylic versus Allylic C C C H H H allylic hydrogens are attached to allylic carbons Vinylic versus Allylic C C C H H H Vinylic versus Allylic C C C X X X vinylic substituents are attached to vinylic carbons Vinylic versus Allylic C C C X X X allylic substituents are attached to allylic carbons Allylic Carbocations C C C + the fact that a tertiary allylic halide undergoes solvolysis (S N 1) faster than a simple tertiary alkyl halide Cl CH 3 CH 3 CH 3 relative rates: (ethanolysis, 45C) 123 1 Allylic Carbocations C Cl CH 3 CH 3 C H 2 C CH provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH 3 CH 3 CH 3 formed faster Allylic Carbocations C C H 2 C CH + + CH 3 CH 3 provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH 3 H 2 C=CH stabilizes C+ better than does CH 3 Allylic Carbocations C C CH 3 CH 3 H 2 C CH + + CH 3 CH 3 Delocalization of electrons in the double bond stabilizes the carbocation resonance model orbital overlap model Stabilization of Allylic Carbocations Resonance Model CH 3 CH 3 H 2 C CH + C CH 3 CH 3 H 2 C CH + C C CH 3 CH 3 H 2 C CH + + Orbital Overlap Model + + Orbital Overlap Model Orbital Overlap Model Orbital Overlap Model H 2 O Na 2 CO 3 S N 1 Hydrolysis of an Allylic Halide Cl CH 3 CH 3 C H 2 C CH (85%) OH CH 3 CH 3 C H 2 C CH (15%) + CH 3 HOCH 2 CH C CH 3 H 2 O Na 2 CO 3 Corollary Experiment (85%) OH CH 3 CH 3 C H 2 C CH (15%) + CH 3 HOCH 2 CH C CH 3 CH 3 ClCH 2 CH C CH 3 CH 3 ClCH 2 CH C CH 3 Cl CH 3 CH 3 C H 2 C CH and give the same products because they form the same carbocation CH 3 CH 3 H 2 C CH + C CH 3 CH 3 H 2 C CH + C CH 3 HOCH 2 CH C CH 3 OH CH 3 CH 3 C H 2 C CH + (85%) (15%) more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH 3 CH 3 H 2 C CH + C CH 3 CH 3 H 2 C CH + C Allylic Free Radicals C C C Allylic free radicals are stabilized by electron delocalization C C C C C C Radicals are Stabilized by Electron Delocalization Spin density is a measure of the unpaired electron distribution in a molecule. The unpaired electron in allyl radical "divides it time" equally between C-1 and C-3. Spin density in allyl radical Radicals are Stabilized by Electron Delocalization Free-radical Stabilities are Related to Bond-Dissociation Energies CH 3 CH 2 CH 2 H 410 kJ/mol CH 3 CH 2 CH 2 + H 368 kJ/mol + H CHCH 2 H H 2 C CHCH 2 H 2 C CH bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane ClCH 2 CHCH 3 Cl addition 500 C substitution CHCH 3 H 2 C + Cl 2 CHCH 2 Cl H 2 C + HCl Allylic Halogenation: Chlorination of Propene selective for replacement of allylic hydrogen free radical mechanism allylic radical is intermediate Allylic Halogenation...
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Chapter10 - Organic Chemistry CHE 275 Chapter 10...

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