eliminations - TOPIC 7. ELIMINATION REACTIONS (chapter 7...

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CHEM 2311 B Topic 7 slide 1 TOPIC 7. ELIMINATION REACTIONS (chapter 7 and parts of chapters 4,6,20) S N 1S N 2 E1 E2 H LG elimination H LG substitution H Nuc Mechanisms CHEM 2311 B Topic 7 slide 2 OBJECTIVES 1. Describe mechanisms for elimination of a leaving group and adjacent proton to form a pi-bond. 2. Discuss the effect of starting material (“substrate”), leaving group, and reaction conditions on the course and outcome of a reaction. 3. Describe syntheses of alkenes and alkynes. 4. Use combinations of elimination and substitution reactions in developing two-step syntheses of value-added compounds.
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CHEM 2311 B Topic 7 slide 3 REVIEW OF ALKENES: STRUCTURE, NOMENCLATURE, AND STABILITY Prob: 7.18-21 Structure Preview of Reactivity Additions (Topic 8) CC Cl H Cl H H Cl Cl H C H C Br C H C OH C H C H H 2 Pt or Pd HBr H 2 O H 2 SO 4 S:7.1-7.4 CHEM 2311 B Topic 7 slide 4 Systematic Nomenclature e.g., Assign Cahn-Ingold-Prelog priority to substituents on each sp 2 carbon Highest priority on same side: Z (zusammen=same) opposite sides: E (entgegen=opposite) Problem: Name the following compound CH 2 CH(CH 3 ) 2 3 H 3 C H F Br H 3 C H
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CHEM 2311 B Topic 7 slide 5 OH Problem: Provide IUPAC names for the following alkenes Problem: Determine the E / Z configurations of the double bonds in each of the following alkenes geraniol CHEM 2311 B Topic 7 slide 6 Relative Stability Heats of Hydrogenation C 4 H 8 + H 2 C 4 H 10 Pt or Pd H H > 0 H o < 0 H o = -127 kJ/mol H o = -120 kJ/mol H o = -115 kJ/mol + H 2 + H 2 + H 2 +H 2 +H 2 +H 2
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CHEM 2311 B Topic 7 slide 7 Overall Stability Number of substituents Ç Remember hyperconjugation? R R R R H H R H R H R H H R R H H H R R H R R R number of substituents CC C H H H CHEM 2311 B Topic 7 slide 8 C H H H H H H H H H Hyperconjugation accounts for the enhanced stability of cations, radicals and alkenes with higher degrees of substitution. relative stability stability explains Carbocation Radical Alkene
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CHEM 2311 B Topic 7 slide 9 Cycloalkenes CC HH cyclopropene cyclobutene cyclopentene cyclohexene cycloheptene cyclooctene cis cyclooctene trans cyclooctene strain energies: 15 kcal/mol 27 kcal/mol CHEM 2311 B Topic 7 slide 10 PREVIEW OF FORMATION OF ALKENES & ALKYNES BY ELIMINATION REACTIONS C H C Br C H C OH C Br C Br base acid C Br H C Br H base NaNH 2 Zn S:7.5
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eliminations - TOPIC 7. ELIMINATION REACTIONS (chapter 7...

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