CHE 242 Unit VI The Study of Conjugated Systems, Aromaticity and Reactions of Aromatic Compounds CHAPTER 15 Terrence P. Sherlock Burlington County College 2004

Chaper 152 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 153 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 154 Relative Stabilities twice 1-pentene more substituted =>

Chaper 155 Structure of 1,3-Butadiene Most stable conformation is planar. Single bond is shorter than 1.54 Å. Electrons are delocalized over molecule. =>

Chaper 156 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 157 1,2- and 1,4-Addition to 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 158 Addition of HBr =>

Chaper 159 Kinetic vs. Thermodynamic Control Major product at 40  C Major product at -80  C =>

Chaper 1510 Allylic Bromination + HBr + Br  =>

Chaper 1511 Bromination Using NBS N-Bromosuccinimide (NBS) provides a low, constant concentration of Br 2. NBS reacts with the HBr by-product to produce Br 2 and prevent HBr addition. =>

Chaper 1512 S N 2 Reactions of Allylic Halides and Tosylates =>

Chaper 1513 Diels-Alder Reaction Otto Diels, Kurt Alder; Nobel prize, 1950 Produces cyclohexene ring Diene + alkene or alkyne with electron- withdrawing group (dienophile) =>

Chaper 1514 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 1515 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 1516 =>    * for ethylene and butadiene

Chaper 1517 Obtaining a UV Spectrum The spectrometer measures the intensity of a reference beam through solvent only ( I r ) and the intensity of a beam through a solution of the sample ( I s ). Absorbance is the log of the ratio Graph is absorbance vs. wavelength. =>

Chaper 1518 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 1519 UV Spectrum of Isoprene =>

Chaper 1520 Sample UV Absorptions =>

Chaper 1521 POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5 TH EDITION” L.G. WADE ALL MATERIALS USED WITH PERMISSION OF AUTHOR PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE ORGANIC CHEMISTRY COURSE BY: ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN