Chapter 15 Benzene and Aromaticity Aromatic Compounds • Aromatic – Originally used to describe fragrant substances – Refers to a class of compounds that meets Hückel criteria for aromaticity 2 Aromatic Compounds • Aromatic – Originally used to describe fragrant substances – Refers to a class of compounds that meets Hückel criteria for aromaticity 3 Aromatic Compounds • The Hückel 4n + 2 Rule – Developed by Erich Hückel in 1931 – States that a molecule can be aromatic only if: • It has a planar, monocyclic system of conjugation • It contains a total of 4n + 2 molecules – n = 0,1,2,3… • 4n electrons are considered antiaromatic 4 Aromatic Compounds: Source • Coal and petroleum are the major sources of simple aromatic compounds • Coal primarily comprises Coal of large arrays of conjoined benzene-like rings • When heated to 1000°C, coal thermally breaks down to yield coal tar A representative structure of bituminous coal Proc. Natl. Acad. Sci. USA 79, 3365 (1982) https://grist.files.wordpress.com/2013/09/lump-o-coal.jpg 5 Aromatic Compounds: Source Fractional distillation of coal tar yields many aromatic compounds © 2016 Cengage Learning. 6 Aromatic Compounds: Source • Petroleum primarily comprises alkenes and few aromatic compounds • Formation of more aromatic molecules occur when alkanes are passed over a catalyst Petroleum at high pressure and temperature http://www.investigroup.com/wp-content/uploads/2014/07/5.jpg 7 Aromatic Compounds: Nomenclature • Aromatic compounds naming system uses: – Nonsystematic names © 2016 Cengage Learning. 8 Aromatic Compounds: Nomenclature • Aromatic compounds naming system uses: – Nonsystematic names – International Union of Pure and Applied Chemistry (IUPAC) Rules • Allows use of widely used names 9 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Monosubstituted benzenes have systematic names with –benzene being the parent name © 2016 Cengage Learning. 10 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Arenes are alkyl-substituted benzenes • Alkyl-substituent benzenes are smaller than the ring (<6 carbons) – Phenyl-substituted benzenes • Phenyl-substituted benzenes are larger than the ring (>7 carbons) – The term phenyl (Ph or Φ) is used in substituent benzene ring – C6H5 – The term benzyl is used for the C6H5CH2– group 11 © 2016 Cengage Learning. Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Disubstituted Benzenes • Names based on the placement of substituents – Ortho- is 1,2 disubstituted – Meta- is 1,3 disubstituted – Para- is 1,4 disubstituted – Provides clarity in the discussion of reactions © 2016 Cengage Learning. 12 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Disubstituted Benzenes • Names based on the placement of substituents – Ortho (o), meta (m) , and para (p) » Provide clarity in the discussion of reactions © 2016 Cengage Learning. 13 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Benzenes +2 or more substituents • Numbers with the lowest possible values are chosen • List substituents alphabetically with hyphenated numbers • Common names, such as toluene can serve as root name(as in TNT) © 2016 Cengage Learning. 14 Worked Example • Provide the IUPAC name for the following compound • Solution: – The compound is 1-Ethyl-2,4-dinitrobenzene • Substituents on trisubstituted rings receive the lowest possible numbers © 2016 Cengage Learning. 15 STRUCTURE AND STABILITY OF BENZENE 16 Aromatic Compounds: Stability of Benzene • The reactivity of benzene is much lesser than that of alkenes despite having six fewer hydrogens – Benzene - C6H6 – Cycloalkane - C6H12 © 2016 Cengage Learning. 17 Aromatic Compounds: Stability of Benzene Comparison of the heats of hydrogenation proves the stability of benzene Remember • Heat of Hydrogenation is the heat produced when alkene is reduced to an alkane – Alkene with lower (less negative) value is more stable – Reduction is exothermic (converting weaker pi bond to stronger sigma bond) – Depends on degree of substitution of double bond (greater substitution, lower heat of hydrogenation) – Trans alkene is lower than cis alkene © 2016 Cengage Learning. 18 Aromatic Compounds: Stability of Benzene Comparison of the heats of hydrogenation proves the stability of benzene 19 © 2016 Cengage Learning. Aromatic Compounds: Structure of Benzene • All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds • Electron density in all six C-C bonds is identical • Structure is planar, hexagonal 1.39 Å © 2016 Cengage Learning. 20 Aromatic Compounds: Structure of Benzene • Carbon atoms and p orbitals in benzene are equivalent – Impossible to define three localized bonds in which a given p orbital overlaps only one neighboring p orbital • All electrons move freely in the entire ring due to equal overlap of all p orbitals 1.39 Å © 2016 Cengage Learning. 21 Aromatic Compounds: Structure of Benzene • Structure is in resonance – Resonance influences its rate of reactivity 1.39 Å © 2016 Cengage Learning. 22 Aromatic Compounds: Structure of Benzene • Benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds – The ring does not indicate the number of electrons in the ring but is a reminder of the delocalized structure © 2016 Cengage Learning. 23 Aromatic Compounds: Structure of Benzene • Molecular orbital description of benzene – The 6 p-orbitals combine to give: • Three bonding orbitals with 6 electrons • Three antibonding with no electrons • Orbitals with the same energy are degenerate © 2016 Cengage Learning. 24 Aromatic Compounds • Observations about benzene and benzene like aromatic compounds – Unusually stable - Heat of hydrogenation 150 kJ/mol less negative than a hypothetical cyclic triene – Planar hexagon - Bond angles are 120°, carbon- carbon bond length is 139 pm – Undergoes substitution rather than electrophilic addition – Resonance hybrid with structure between two line-bond structures 25 AROMATICITY AND THE HÜCKEL 4N+2 RULE 26 Aromatic Compounds: Hückel Rule • States that a molecule can be aromatic only if: – It has a planar, monocyclic system of conjugation – It contains a total of 4n + 2 electrons • n = 0,1,2,3… • Antiaromatic if 4n electrons are considered 27 Aromatic Compounds: Hückel Rule • Does molecule contain (4n+2) or 4n pi electrons – Cyclobutadiene • Four pi electrons • Antiaromatic – It reacts readily and exhibits none of the properties corresponding to aromaticity – It dimerizes by a Diels-Alder reaction at –78 °C © 2016 Cengage Learning. 28 Aromatic Compounds: Hückel Rule • Does molecule contain (4n+2) or 4n pi electrons – Benzene possesses six electrons (4n + 2 = 6 when n = 1) – Aromatic © 2016 Cengage Learning. 29 Aromatic Compounds: Hückel Rule • Does molecule contain (4n+2) or 4n pi electrons – Cyclooctatetraene possesses eight electrons – Not aromatic – Comprises four double bonds © 2016 Cengage Learning. 30 Aromatic Compounds: Stability and Molecular Orbital Theory • Molecular orbitals for cyclic conjugated molecules – Always contain a single lowest-lying MO – Above lowest MO, MOs come in Energy Levels of the Six Benzene degenerate pairs Molecular Orbitals © 2016 Cengage Learning. 31 Worked Example • To be aromatic, a molecule must have 4n + 2 electrons and must have a planar, monocyclic system of conjugation – Explain why cyclodecapentaene has resisted all attempts at synthesis though it has fulfilled only one of the above criteria 32 Worked Example • Solution: – Cyclodecapentaene possesses 4n + 2 (n = 2) but is not flat – If cyclodecapentaene were flat, the starred hydrogen atoms would crowd each other across the ring • To avoid this interaction, the ring system is distorted from planarity © 2016 Cengage Learning. 33 Aromatic Ions • The 4n + 2 rule applies to ions as well as neutral substances – Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic © 2016 Cengage Learning. 34 Aromatic Ions • How are ions aromatic? – Starting with a neutral saturated hydrocarbon – Remove one hydrogen from the saturated CH2 – Rehybridize the carbon from sp3 to sp2 – Result is a fully conjugated product with a p orbital on every product © 2016 Cengage Learning. 35 Aromatic Ions • Methods to remove hydrogen from saturated CH2 – Removing the hydrogen with both electrons (H:–) from the C–H bond results in a carbocation – Removing the hydrogen with one electron (H·) from the C–H bond results in a carbon radical – Removing the hydrogen without any electrons (H+) from the C–H bond results in a carbanion © 2016 Cengage Learning. 36 Aromatic Ions: Cyclopentadienyl Anion • Disadvantages of the four--electron cyclopentadienyl cation and the five-- cyclopentadienyl radical – Highly reactive – Difficult to prepare – Not stable enough for aromatic systems • Advantages of using the six--electron cyclopentadienyl cation – Easily prepared – Extremely stable – pKa =16 • Acidicty of a hydrogen atom © 2016 Cengage Learning. 37 Worked Example • Cyclooctatetraene readily reacts with potassium metal to form the stable 2– cyclooctatetraene dianion, C8H8 – Explain why this reaction occurs so easily – Determine the geometry for the cyclooctatetraene dianion © 2016 Cengage Learning. 38 Worked Example • Solution: – When cyclooctatetrene accepts two electrons,