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Chemistry 242-002

Chemistry 242-002. Organic Chemistry II with Professor Virgil Percec Tue. And Thu. 9:00 AM-10:30 AM. Calendar. Professor Virgil Percec E-mail: percec@sas.upenn.edu Office: Vagelos Labs Room 4003 Office hours: T-Th 10:30 am - 12:30 pm or by appointment. Brad Rosen

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Chemistry 242-002

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  1. Chemistry 242-002 Organic Chemistry II with Professor Virgil Percec Tue. And Thu. 9:00 AM-10:30 AM

  2. Calendar

  3. Professor Virgil Percec E-mail: percec@sas.upenn.edu Office: Vagelos Labs Room 4003 Office hours: T-Th 10:30 am - 12:30 pm or by appointment Brad Rosen E-mail: bradr@sas.upenn.edu Office: Vagelos Labs Room 4080 Office Hours Thu Jan 17th and Tue Jan 24th 10:30 am – 12:30 pm Contact and Other Information Class info on Blackboard: https://courseweb.upenn.edu/ Sign-up for Workshops: http://www.penntutoring.info/orgochem/

  4. Course Policy • Text & Other Requirements: (packaged at considerable price savings): Solomons, Organic Chemistry, 9th Edition (John Wiley & Sons); Solomons, Study Guide and Solutions Manual for Solomons 9th Edition (John Wiley & Sons); Hehre, Shusterman & Nelson; Stull, Science on the Internet; John Wiley & Sons Molecular Model Set for Organic Chemistry. Students must read the assigned chapters before and after lectures for complete understanding of the material. Problem solving is an essential part of the course, and you should always try to do the problems before looking up the answers. Always read questions carefully when solving problems, both in the homework and in the tests. • Recitations & Workshops: teaching assistants and specific rooms are assigned for recitations and workshops and you are encouraged to take advantage of as many of these sessions as you can.

  5. Some more Course Policy • Exams, Grading & Regrading: there will be four exams and one cumulative final. There are no re-exams and no exams are dropped. However, in case of illness, etc. with an appropriate excuse, given before rather than after the exam, a student may be allowed to miss and reschedule one exam. I expect that A- to A+ will be given for final scores of 80 or 85 to 100% and B- to B+ for final scores of 60 or 70 to 80 or 85%. A very good class is expected to obtain up to 65 or 70% A and B. Regrading must be done within two days from the time the exam is returned. Questions must be directed to the grader in writing. You must not write on your exam in any fashion until after it has been regraded. Mid-Term Exams are scheduled for 5:00 – 7:00 PM in the locations posted above. • Drops, Withdrawals, or Incompletes: the deadlines for dropping or withdrawing must be rigorously observed. • Final grades: Final exams are scheduled by the Registrar’s office. Students missing a final examination must obtain permission to take the make-up exam the following semester (also scheduled by the registrar’s office) from an advisor in the SAS Dean’s office. The organic faculty has adopted a policy of not posting grades. You must obtain your grade by requesting in writing via email. Please Direct all Questions Regarding Course Policy to Professor Percec

  6. Lecture 1: Conjugated Unsaturated Systems Chapter 13 in Solomons 9/e

  7. What Are Conjugated Unsaturated Systems • Any system where there is a p-orbital adjacent to a double (or triple) bond

  8. Motivation: Synthetic Target • One often desires to make or study Medicinal Natural Products Containing Conjugated Unsaturated Systems

  9. Motivation: Unique Reactivity

  10. Motivation: Biology • Vision

  11. Allylic Substitution and the Allylic Radical

  12. Allylic Substitution and the Allylic Radical Why Allyl versus Vinyl or Alkyl Substitution ?

  13. Allylic versus Vinylic Substitution Allylic Proton is easier to homolyze by 96 kJ/mol

  14. Allylic Bromination Note: N-chloro-succinimide and N-iodo-succinimide exist and react in a similar way

  15. Rules of Resonance 1) The most important rule of resonance is that resonance structures are not real. They are merely a tool for rationalizing chemical behavior. We will revisit this in terms of the allylic and other conjugated systems.

  16. Rules of Resonance 2) In resonance we move only electrons, not atoms. And when we do it is usually π electrons. Processes which involve “resonance” of atoms such as keto-enol tautomerization (Chapter 17) are true chemical equilibria with where each Isomer truely exists in solution

  17. Rules of Resonance 3) All resonance structures must be true Lewis Structures (Chapter 1.5) 4) Resonance structures must have the same number of unpaired electrons.

  18. Rules of Resonance 5) Another very important rule is that systems in resonance need to be coplanar. However 2,3-di-tert-butyl-1,3- Butadiene is twisted out of plane and is not in resonance transoid 1,3 Butadiene coplanar and in resonance

  19. Rules of resonance 6) For reason which will be explained shortly, the energies of structures in resonance are always lower than those of their prototypical resonance forms. 7) Equivalent resonance structures make equivalent contributions to energies of the resonating compound 8) The more stable the resonance structure the larger its contribution

  20. Assessing Resonance Structure Stability • The more covalent bonds the better the structure. • The more complete valance shells the better the structure. • The less charge separation the better.

  21. Source of Allylic Radical Stability: Resonance One explanation for the peculiar stability of the Allyl Radical is through implications of resonance.

  22. Allyl Cation also Stabilized via Resonance ? Indeed according to suggested stability via resonance , the allyl cation is unusually stable

  23. Resonance Structures are Just A Tool • Keep in mind that resonance structures do not really exist. • Resonance Structures allow a chemist to quickly ascertain stabilities and relativities of compounds from their line drawings. • More accurate energies and electron distributions require computational chemistry.

  24. Molecular Orbital Description of Ethene E=α+β E=α-β

  25. The Molecular Orbital (MO) Approach to the Allyl System E=α+1.41β E=α E=α-1.41β

  26. Polyunsaturated Systems:Nomenclature and Classification AKA isolated dienes

  27. Molecular Orbital Description of 1,3 Butadiene E=α+1.62β E=α+0.62β E=α+β E=α-β E=α-0.62β E=α-1.62β

  28. Conformations of 1,3 Butadiene

  29. Stability of Conjugated Dienes Conjugative Stability

  30. UV-Vis Spectroscopy Beer’s Law A=ε x c x l = log (Io/I) ε=extinction coefficient/molar absorptivity c=concentration l= path length

  31. UV-VIS of Extended trans π-systems All π π* transitions

  32. Electrophillic Attack on Conjugated Dienes Expected Markovnikov Product

  33. Mechanism of Electrophillic Addition to Conjugated Dienes

  34. Comparison of 1,4 Addition

  35. Kinetic versus Thermodynamic Control Kinetic Control Thermodynamic Control

  36. Diels-Alder Reaction:The Basics S-cis diene required, s-trans does not work

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