Chapter 11 Arenes and Aromaticity 1 Arenes and Aromaticity Arenes are hydrocarbons based on the benzene structural unit 2 Benzene 1866 August Kekul proposed the six carbon atoms were joined together in a ring Reasoning 1 Benzene is C6H6 2 All hydrogens of benzene are equivalent 3 The structural theory requires that there be four bonds to each carbon 3 Benzene Structure It was expected that disubstitution would yield two isomers Kekul suggested these isomers rapidly interconvert 4 Benzene Structure Benzene is planar and shaped like a regular hexagon All C C bonds and all angles correspond to perfect sp2 hybridization Two Lewis structures are resonance forms Circle represents 6 delocalized p electrons 5 Benzene Stability Relative stability of related hydrocarbons can be compared via heats of hydrogenation These three compounds all yield cyclohexane upon hydrogenation cyclohexene 1 3 cyclohexadiene benzene 6 Benzene Stability Heats of Hydrogenation 7 Benzene Stability Alternatively 1 3 5 hexatriene can be used as a comparison Gives a resonance energy of 129 kJ mol Resonance energy is large Large resonance energy is what puts compounds in the aromatic category 8 Benzene Bonding Valence bond approach has all carbons connected by sigma bonds and a half filled p orbital each carbon that overlaps 9 Benzene Bonding MO theory concludes that 6 p orbitals are formed 3 bonding orbitals and 3 antibonding 10 Benzene Bonding Electrostatic potential map suggests the p electrons are not that tightly held 11 Benzene Derivatives Compounds that contain a benzene ring are aromatic Common names may contain benzene Other traditional common names have been retained 12 Benzene Derivatives Common Names 13 Benzene Derivatives There are three isomeric disubstituted benzenes Ortho 1 2 meta 1 3 and para 1 4 14 Benzene Derivatives Numbers must be used for trisubstituted benzenes The parent common name determines which carbon is C 1 Number so the next substituent has the lowest number List substituents in alphabetical order 15 Benzene Derivatives Naming trisubstituted derivatives 16 Benzene Derivatives As a substituent the C6H5 group is called phenyl C6H5CH2 is the benzyl group 17 Naming Biphenyls Biphenyl is the accepted name for the compound with two benzene rings connected One ring is numbered with unprimed numbers and the other with primed numbers 18 Polycyclic Aromatic Hydrocarbons This group has benzene rings fused together Most stable resonance contributor has greatest number of Kekule benzene rings 19 Polycyclic Aromatic Hydrocarbons Benzo a pyrene is a carcinogen Converted in the liver to an epoxy diol that is mutagenic 20 Fullerenes Nanotubes and Graphene Graphite and carbon were the two allotropes of carbon known before 1985 Buckminster fullerene and other forms were synthesized in 1985 Current research involves these and compounds like carbon nanotubes Graphite Buckminsterfullerene C60 Carbon nanotube 21 Benzene Physical Properties Boiling point of benzene is similar to the closely related compounds 22 Benzene Dimer There are four arrangements of benzenes in the noncovalently bonded dimer that are similar in energy Most stable 23 Benzene Crystal In the solid state benzene packing is known as a herringbone arrangement 24 Aromatics Melting Points Larger polycyclic aromatics tend to be more ordered parallel packing which results in higher melting points than related non aromatics 25 Benzylic Cations Anions and Radicals 26 Valence Bond Description of Benzyl Cation Anion and Radical The valence bond description has the cation unpaired electron or anion in a p orbital Greater stability due to electron charge delocalization 27 MO Description of Benzyl Cation Anion and Radical Important orbtials LUMO for benzyl cation SOMO for benzyl radical HOMO for benzyl anion LUMO of benzyl cation 28 SN1 Reactions of Benzylic Halides Benzylic halides react much faster than tertiary halides 29 Relative Stability of Benzyl Cations Electrostatic potential maps show the dispersion of the positive charge in benzylic cation less blue purple relative to t butyl cation 30 SN1 Reactions of Benzylic Halides Nucleophile only attacks the benzyl carbon 31 Relative Rates of SN2 Reactions Benzylic halides react faster than primary and allylic halides The nucleophile interacts with the LUMO of the benzylic halide 32 SN2 Reactions of Benzylic Halides Primary benzylic halides only undergo substitution Secondary and tertiary benzylic halides also undergo elimination 33 Stability of Benzyl Radicals Bond dissociation enthalpies help quantify the stability of the benzyl radical 34 Benzylic Bromination The benzylic halide is selectively formed 35 Stability of Benzylic Anions For anions the stability can be evaluated by considering the anion as the conjugate base of an acid The stronger the acid the more stable the base 36 Oxidation of Cumene The industrial radical oxidation of cumene followed by acid catalyzed decomposition of the peroxide yields phenol and acetone Key radical reactions 37 Oxidation of Alkylbenzenes Strong inorganic oxidants oxidize alkylbenzenes that have at least one benzylic hydrogen to form the corresponding carboxylic acids 38 Biochemical Oxidations Enzymes catalyze benzylic oxidation as well as oxidation of benzene Benzene oxide and its derivatives are carcinogenic 39 Synthesis of Alkenylbenzenes Dehydrogenation dehydration and dehydrohalogenation all yield alkenyl benzenes 40 Reactions of Alkenylbenzenes Alkenyl benzenes react like alkenes 41 Reactions of Alkenylbenzenes Reactions that proceed through carbocation intermediates will selectively form benzyl cation Mechanism 42 Reactions of Alkenylbenzenes Reactions that proceed through radical intermediates will favor formation of the benzyl radical 43 Mechanism of Radical Polymerization Step 1 Bond Homolysis Step 2 Radical Addition 44 Mechanism of Radical Polymerization Step 3 Radical Addition Step 4 Radical Addition 45 Reduction of Benzene A Group 1 metal in liquid ammonia and ethanol 46 Mechanism of the Birch Reduction Step 1 Electron Transfer Step 2 Protonation 47 Mechanism of the Birch Reduction Step 3 Electron Transfer Step 4 Protonation 48 Retrosynthetic Analysis and Synthesis Challenge is to synthesize Z 1 phenyl 2 butene 49 Retrosynthetic Analysis and Synthesis The reaction sequence in the forward direction 50 Cylcobutadiene and Cyclooctatetraene Are these compounds as stable as benzene Cyclooctatetraene 1st synthesized in 1911 Willstatter from a more complex