aromaticity

aromaticity - Chapter 15 (pp. 498-515)� Benzene and...

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Unformatted text preview: Chapter 15 (pp. 498-515)� Benzene and Aromaticity� Suggested Problems:� 15.4, 15.10, 15.19, 15.27, 15.31, 15.32, 15.33, 15.35, 15.36, 15.38, 15.41� 15A Introduction and Nomenclature� 15B Stabilityof Aromatic Compounds/Huckel’s Rule� 15C Aromatic Ions� 15D Aromatic Heterocycles� 15E Polycyclic Aromatic Compounds� Effect of Aromatic Stabilization on Reactions with Benzene� ADDITION Bonds broken Br Br2 Br Bonds made p bond 2 C-Br Br-Br bond Br2 If reaction occurred p bond No reaction Br-Br bond 2 C-Br BUT would lose AROMATIC STABILIZATION� INSTEAD - SUBSTITUTION FeBr3 /Br2 Br Heat evolved upon catalytic hydrogenation (DHo) A MEASURE OF ALKENE STABILITY ALKANE ALKENE H2/Pd-C ENERGY (-) 28.3 kcal/mol (-) 2 7.4 kcal/mol (-) 30.3 kcal /mol Catalytic hydrogenation of benzene and various cyclohexenes Conjugated Isolated diene diene Benzene 3 x -28.6 kcal/mol Isolated alkene 2 x -28.6 kcal/mol Magnitude of aromatic stabilization = 36 kcal/mol Benzene (C6H6) is not “cyclohexatriene!� C=C 1.34Å C-C 1.54Å Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring 1879 Landenberg Criteria for Aromaticity� 1. 2. 3. 4. Cyclic Unhybridized p orbital in continuous cyclic system� Able to adopt a planar geometry Fulfills Huckel’s Rule and includes (4n + 2) electrons� n = 0, 1, 2, 3, 4…….. (the 2, 6, 10, 14….electrons)� •Initially, all cyclic conjugated hydrocarbons were proposed to be aromatic •However, cyclobutadiene is so reactive that it dimerizes before it can be isolated mix of single and double bonds non-planar geometry •Cyclooctatetraene adds Br2 readily. •Look at Molecular Orbitals (MOs) to explain aromaticity in benzenelike molecules Constructing Molecular Orbitals� • p molecular orbitals are the sideways overlap of p orbitals •p orbitals have 2 lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charge •When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed •When + and - lobes overlap, waves cancel out and a node forms; antibonding MO node DESTRUCTIVE OVERLAP + - - p* p* (ANTI-BONDING) H C H + Energy of isolated p orbitals on C1 and C2 ENERGY + - ENERGY + - p (BONDING) CONSTRUCTIVE OVERLAP H C H MO Rules for Benzene� •Six overlapping p orbitals must form six molecular orbitals •Three will be bonding, three antibonding •Lowest energy MO will have all bonding interactions, no nodes •As energy of MO increases, the number of nodes increases •System symmetric so 2 pairs of degenerate orbitals ENERGY ENERGY Energy Diagram for Benzene� 6 atomic orbitals - 6 molecular orbitals System symmetric so 2 pairs of degenerate orbitals The six electrons fill three bonding pi orbitals. All bonding orbitals are filled (“closed shell”), an extremely stable arrangement (AROMATIC STABILIZATION). Energy Diagram for Cyclobutadiene� Following Hund’s rule, two electrons are in separate orbitals because they are at same energy. Most stable if filled with an electron pair (as with benzene) If cyclobutadiene adopted a coplanar geometry - two of the molecular orbitals would each have a single unpaired electron very unstable. Applies to any (4 n) system Cyclobutadiene is ANTIAROMATIC Aromatic Anti-aromatic� (4n + 2) (4n) All bond lengths same Combination of single and double bonds The Acidity of the Pyridinium Ion� •� Heterocyclic aromatic compound. •� Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8. The Acidity of Protonated Pyrrole� Also aromatic, but lone pair of electrons is delocalized: much weaker base. Aromatic Cations and Anions� Many Benzene Derivatives are Useful Drugs CO2H CO2H Et O HN N O O HO O OH acetyl salicylic acid ASPIRIN ibuprofen ADVIL H2N procaine NOVOCAINE HO bronchodilator ALBUTEROL O O S N H N CO2H O HO acetaminophen TYLENOL MeO H CF3 naproxen ALEVE appetite suppresant FENFLURAMINE O O rocecoxib VIOXX Et ...
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