CHEM 2211 1st Edition Lecture 24Outline of Last LectureI. Know your mechanismsII. Intro to RetrosynthesisIII. Example #1IV. Example #2V. Practice ProblemOutline of Current LectureI. Delocalized ElectronsII. Bonding in benzeneIII. Drawing Resonance structuresIV. Factors that decrease stabilityV. Delocalization adds stabilityVI. Criteria for AromaticityCurrent LectureI. Delocalized ElectronsA. Electrons are shared by 3 or more atomsB. All hydrogens are equivalent to each other (benzene)II. Bonding in benzeneA. Has bond angle of 120° aka PLANAR moleculeB. P-orbitals overlap neighboring p-orbitalsIII. Drawing Resonance structuresA. Resonance hybrids actual structure with delocalized electrons drawn as a dashed lineResonance HybridB. RULESi. Only electrons moveii. Only pi electrons and lone pairs can moveiii. Sigma bonds are not broken and substituents are not removedExample of ResonanceIV. Factors that decrease stabilityA. Atoms with incomplete octetsB. Negative charge on an atom that is not the most electronegative atom in the molecule or a positive charge on the most electronegative moleculeC. Charge separationREMEMBER that resonance increases stability and makes the molecule less reactive (and moreacidic )V. Criteria for AromaticityA. Must have an uninterrupted, cyclic cloud of pi electrons above and below molecule’s planeB. Must have an odd number of electron pairs or a Huckel numberi. Huckel’s rule: 4n + 2 where n is any integerC. If the molecule does not fit both requirements, it is NOT aromaticAromaticNOT
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