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CHE 242 Unit VI The Study of Conjugated Systems, Aromaticity and Reactions of Aromatic Compounds CHAPTER 15 Terrence P. Sherlock Burlington County College 2004
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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. =>
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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. =>
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Chaper 154 Relative Stabilities twice 1-pentene more substituted =>
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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. =>
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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. =>
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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. =>
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Chaper 158 Addition of HBr =>
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Chaper 159 Kinetic vs. Thermodynamic Control Major product at 40 C Major product at -80 C =>
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Chaper 1510 Allylic Bromination + HBr + Br =>
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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. =>
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Chaper 1512 S N 2 Reactions of Allylic Halides and Tosylates =>
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Chaper 1513 Diels-Alder Reaction Otto Diels, Kurt Alder; Nobel prize, 1950 Produces cyclohexene ring Diene + alkene or alkyne with electron- withdrawing group (dienophile) =>
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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. =>
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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. =>
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Chaper 1516 => * for ethylene and butadiene
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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. =>
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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.
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Chaper 1519 UV Spectrum of Isoprene =>
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Chaper 1520 Sample UV Absorptions =>
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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
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