CHEM 3332 1st Edition Lecture 12Outline of Last Lecture Chapter 16: Aromatic CompoundsWe can separate organic compounds into two large categories:Structure of Benzene – initially a puzzling compound1825 – Michael Faraday isolates “benzene” from lampback (pyrolyzed whale oil) - Determines empirical formula CH1834 – EilhardMitscherlich synthesizes “benzene” and determines molecular formula C6H6Examine the results of typical reactions of multiple bonds:Add Lewis acid catalyst during bromination:*Something gives benzene a special resistance to addition, a special stability: aromaticity1866-Friedrich Kekule proposes cyclic structure of alternating double and single bondsProblem: structure wiuld predict the existence of two 1,2 – dibromobenzene isomersSolution: To explain the observation, Kekule adjusts the proposalProblem solved? If the cyclic structure of alternating double and single bonds observed in benzenes is the key to aromaticity, should find other evidenceConsider cyclooctatetraene:Kekule’s mistakeStability of the BenzeneAromaticity is not the result of conjugation alone OR benzene is not 1,3,5 – cyclohexatrieneBond lengths:Heat of Hydrogenation:Molecular Oribat Description Huckel RuleThe Huckel Rule is applied to molecules that meet the following1. Cyclic2. Plana3. Each atom in the “cycle” has a p orbitalIf each of the above is true and the cycle has 4n+2 pi electrons, where n = 0,1,2,3…, the molecule isAromatic –Example:If one or more of the above is not true, the molecule is nonaromatic, regardless of the number x electronsExamples:Polygon Rule: inscribe the molecule in a circle with one vertex at the bottom of the circle; each point in which the molecule touches the circle represents the energy level of a molecular orbitalOutline of Current Lecture Annulenes – monocyclic compounds with alternating double and single bondsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Aromatic/ antiaromatic/nonaromaticExamples:Aromatic ionsHeterocyclic Aromatic CompoundsExample: aromaticantiaromatic/nonaromatic? (Do we count the lone pair?)Polynuclear Aromatic Hydrocarbons (PNA’s or PAH’s) – fused benzene ringsNomenclature of Benzene Derivatives:I. MonosubstitutedA. IUPAC – named as abenzeme derivativeB. Common Names – You MUST know the common namea on p. 790 of WadeII. DisubstitutedA. Number substituentsB. B. Ortho, para, meta systemIII. Trisubstituted – must use numbersIV. Benzene ring as a substituent: General Guide – larger portion is parentSpectroscopy Summary (Benzene and Derivative)IR:1. Ring “skeletal” vibrations 2. CH stretching 3100-300 cm-13. CH out of plane bending 900-675 cm -1Characteristic of ring substitutionCurrent
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