1 / 21

Terrence P. Sherlock Burlington County College 2004

CHE 242 Unit VI The Study of Conjugated Systems, Aromaticity and Reactions of Aromatic Compounds CHAPTER 15. Terrence P. Sherlock Burlington County College 2004. Definitions. Conjugated double bonds are separated by one single bond. Example: 1,3-pentadiene.

frayne
Download Presentation

Terrence P. Sherlock Burlington County College 2004

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CHE 242Unit VIThe Study of Conjugated Systems, Aromaticity and Reactions of Aromatic CompoundsCHAPTER 15 Terrence P. Sherlock Burlington County College 2004

  2. Definitions • Conjugated double bonds are separated by one single bond. Example: 1,3-pentadiene. • Isolated double bonds are separated by two or more single bonds. 1,4-pentadiene. • Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. => Chaper 15

  3. Resonance Energy • Heat of hydrogenation for trans-1,3-pentadiene is less than expected. • H for 1-pentene is 30.0 kcal/mol and for trans-2-pentene is 27.4 kcal/mol, so expect 57.4 kcal for trans-1,3-pentadiene. • Actual H is 53.7 kcal, so the conjugated diene is more stable. • Difference, (57.4 – 53.7) 3.7 kcal/mol, is the resonance energy. => Chaper 15

  4. Relative Stabilities twice 1-pentene more substituted => Chaper 15

  5. => Structure of 1,3-Butadiene • Most stable conformation is planar. • Single bond is shorter than 1.54 Å. • Electrons are delocalized over molecule. Chaper 15

  6. Allylic Cations • Carbon adjacent to C=C is allylic. • Allylic cation is stabilized by resonance. • Stability of 1 allylic  2 carbocation. • Stability of 2 allylic  3 carbocation. => Chaper 15

  7. 1,2- and 1,4-Additionto Conjugated Dienes • Electrophilic addition to the double bond produces the most stable intermediate. • For conjugated dienes, the intermediate is a resonance stabilized allylic cation. • Nucleophile adds to either carbon 2 or 4, both of which have the delocalized positive charge. => Chaper 15

  8. => Addition of HBr Chaper 15

  9. Major product at -80C => Kinetic vs. Thermodynamic Control Major product at 40C Chaper 15

  10. + HBr + Br  => Allylic Bromination Chaper 15

  11. Bromination Using NBS • N-Bromosuccinimide (NBS) provides a low, constant concentration of Br2. • NBS reacts with the HBr by-product to produce Br2 and prevent HBr addition. => Chaper 15

  12. SN2 Reactions of Allylic Halides and Tosylates => Chaper 15

  13. => Diels-Alder Reaction • Otto Diels, Kurt Alder; Nobel prize, 1950 • Produces cyclohexene ring • Diene + alkene or alkyne with electron-withdrawing group (dienophile) Chaper 15

  14. Endo Rule The p orbitals of the electron-withdrawing groups on the dienophile have a secondary overlap with the p orbitals of C2 and C3 in the diene. => Chaper 15

  15. => Regiospecificity The 6-membered ring product of the Diels-Alder reaction will have electron-donating and electron-withdrawing groups 1,2 or 1,4 but not 1,3. Chaper 15

  16.   * for ethylene and butadiene => Chaper 15

  17. Obtaining a UV Spectrum • The spectrometer measures the intensity of a reference beam through solvent only (Ir) and the intensity of a beam through a solution of the sample (Is). • Absorbance is the log of the ratio • Graph is absorbance vs. wavelength. => Chaper 15

  18. The UV Spectrum • Usually shows broad peaks. • Read max from the graph. • Absorbance, A, follows Beer’s Law: A = cl where  is the molar absorptivity, c is the sample concentration in moles per liter, and l is the length of the light path in centimeters. Chaper 15

  19. UV Spectrum of Isoprene => Chaper 15

  20. Sample UV Absorptions => Chaper 15

  21. POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION”L.G. WADEALL MATERIALS USED WITH PERMISSION OF AUTHORPRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGEORGANIC CHEMISTRY COURSEBY:ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN Chaper 15

More Related