Aromatic Compounds - Southern Methodist University

Aromatic Compounds - Southern Methodist University

Organic Chemistry, 5th Edition L. G. Wade, Jr. Chapter 16 Aromatic Compounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District 2003,Prentice Hall Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard

Mitscherlich who determined molecular formula to be C6H6. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. => 2 Chapter 16 Kekul Structure Proposed in 1866 by Friedrich Kekul, shortly after multiple bonds were suggested. Failed to explain existence of only one isomer of 1,2-dichlorobenzene. H H C

C H C C C H C H => H

Chapter 16 3 Resonance Structure Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. => Chapter 16 4 Unusual Reactions

Alkene + KMnO4 diol (addition) Benzene + KMnO4 no reaction. Alkene + Br2/CCl4 dibromide (addition) Benzene + Br2/CCl4 no reaction. With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. => Chapter 16 5 Unusual Stability Hydrogenation of just one double bond in benzene is endothermic!

=> Chapter 16 6 Annulenes All cyclic conjugated hydrocarbons were proposed to be aromatic. However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. And cyclooctatetraene

adds Br2 readily. Look at MOs to explain aromaticity. => Chapter 16 7 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. => Chapter 16 8 MOs for Benzene => Chapter 16 9 Energy Diagram for Benzene

The six electrons fill three bonding pi orbitals. All bonding orbitals are filled (closed shell), an extremely stable arrangement. => Chapter 16 10 MOs for Cyclobutadiene => Chapter 16 11

Energy Diagram for Cyclobutadiene Following Hunds rule, two electrons are in separate orbitals. This diradical would be very reactive. => Chapter 16 12 Polygon Rule

The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom. Chapter 16 13 => Aromatic Requirements Structure must be cyclic with conjugated pi bonds. Each atom in the ring must have an unhybridized p orbital.

The p orbitals must overlap continuously around the ring. (Usually planar structure) Compound is more stable than its openchain counterpart. => Chapter 16 14 Anti- and Nonaromatic Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart. Nonaromatic compounds do not have a

continuous ring of overlapping p orbitals and may be nonplanar. => Chapter 16 15 Hckels Rule If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic. If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic. =>

Chapter 16 16 [N]Annulenes [4]Annulene is antiaromatic (4N e-s) [8]Annulene would be antiaromatic, but its not planar, so its nonaromatic. [10]Annulene is aromatic except for the isomers that are not planar. Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. => Chapter 16

17 MO Derivation of Hckels Rule Lowest energy MO has 2 electrons. Each filled shell has 4 electrons. Chapter 16 18 => Cyclopentadienyl Ions

The cation has an empty p orbital, 4 electrons, so antiaromatic. The anion has a nonbonding pair of electrons in a p orbital, 6 e-s, aromatic. => Chapter 16 19 Acidity of Cyclopentadiene pKa of cyclopentadiene is 16, much more acidic than other hydrocarbons. H H

H + _ OC(CH3)3 + pKa = 16 HOC(CH3)3 pKa = 19 =>

Chapter 16 20 Tropylium Ion The cycloheptatrienyl cation has 6 p electrons and an empty p orbital. Aromatic: more stable than open chain ion H H OH +

H , H2O + => Chapter 16 21 Dianion of [8]Annulene Cyclooctatetraene easily forms a -2 ion. Ten electrons, continuous overlapping p orbitals, so it is aromatic. +

2K + + 2K => Chapter 16 22 Pyridine Heterocyclic aromatic compound.

Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8. Chapter 16 23 => Pyrrole Also aromatic, but lone pair of electrons is delocalized, so much weaker base. => Chapter 16

24 Basic or Nonbasic? N N Pyrimidine has two basic nitrogens. N N H

Imidazole has one basic nitrogen and one nonbasic. N N H N Purine? => N Chapter 16

25 Other Heterocyclics Chapter 16 26 => Fused Ring Hydrocarbons Naphthalene Anthracene

Phenanthrene => Chapter 16 27 Reactivity of Polynuclear Hydrocarbons As the number of aromatic rings increases, the resonance energy per ring decreases, so larger PAHs will add Br2. H Br

Br H H Br H Br (mixture of cis and trans isomers) Chapter 16 =>

28 Fused Heterocyclic Compounds Common in nature, synthesized for drugs. => Chapter 16 29 Allotropes of Carbon Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. Diamond: a lattice of tetrahedral Cs.

Graphite: layers of fused aromatic rings. => Chapter 16 30 Some New Allotropes Fullerenes: 5- and 6-membered rings arranged to form a soccer ball structure. Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings. => Chapter 16

31 Common Names of Benzene Derivatives OH CH3 phenol toluene H C

styrene CH2 OCH3 NH2 aniline anisole O

O O C C C acetophenone CH3 benzaldehyde

Chapter 16 H OH benzoic acid => 32 Disubstituted Benzenes The prefixes ortho-, meta-, and para- are commonly used for the 1,2-, 1,3-, and 1,4positions, respectively. Br

Br o-dibromobenzene or 1,2-dibromobenzene NO2 HO p-nitrophenol or 4-nitrophenol => Chapter 16 33

3 or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1. OH O2N O2N NO2 NO 2 NO2 2,4,6-trinitrophenol

NO2 1,3,5-trinitrobenzene => Chapter 16 34 Common Names for Disubstituted Benzenes CH3 O CH3

OH C OH CH3 CH3 m-xylene H3C CH3 mesitylene o-toluic acid

H3C p-cresol => Chapter 16 35 Phenyl and Benzyl Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon. CH2Br

Br phenyl bromide benzyl bromide => Chapter 16 36 Physical Properties Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points.

Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. Density: More dense than nonaromatics, less dense than water. Solubility: Generally insoluble in water. => Chapter 16 37 IR and NMR Spectroscopy C=C stretch absorption at 1600 cm-1. sp2 C-H stretch just above 3000 cm-1. 1H NMR at 7-8 for Hs on aromatic ring.

13C NMR at 120-150, similar to alkene carbons. => Chapter 16 38 Mass Spectrometry => Chapter 16 39 =>

UV Spectroscopy => Chapter 16 40 End of Chapter 16 Chapter 16 41

Recently Viewed Presentations

  • Important Terms Two Terms to know: 1. Positionally:

    Important Terms Two Terms to know: 1. Positionally:

    James uses the locative case of " en . pistei " to emphasize the location in which the request is to be made (in the sphere of complete dependency). While James uses the locative case predominately, the instrumental case is...
  • History of Canada From European Contact to Canadian

    History of Canada From European Contact to Canadian

    Province of Canada. In 1837, Canadians began to rebel against British control. Britain sent a government reformer to examine the Canadian problem. As a result of the inquiry, Britain discovered an economic need to unify the 2 Canadian provinces. In...
  • Conducting Public Forums Copyright  2014 by The University

    Conducting Public Forums Copyright 2014 by The University

    Are a first step toward understanding the community's needs and resources. Why Conduct Public Forums? They offer valuable insights into the community. They provide a database for guiding and explaining actions. They help link your group with people who can...
  • UbD Lesson Plan: Personal Odyssey Scrapbook - An Interactive ...

    UbD Lesson Plan: Personal Odyssey Scrapbook - An Interactive ...

    Learners will create a timeline or graphic map of their personal hero journey to be used as a guide to later create a PowerPoint scrapbook that will synthesize their understanding of the hero journey and demonstrate use of multimedia aids...
  • The Hajii - WordPress.com

    The Hajii - WordPress.com

    The story begins when Hajji, receives a call from his brother's girlfriend, who informs him of his brother's illness and wish to die as a Muslim. Development Hajji goes with Catherine to see his brother, to decide whether he will...
  • APUSH DBQ Writing: Formulating your thesis statement

    APUSH DBQ Writing: Formulating your thesis statement

    State a relevant thesis that directly addresses all parts of the question.. Support the thesis or a relevant argument with . evidence from all, or all but one, of the documents. Incorporate . analysis. of all, or all but one,...
  • Childhood blindness in Qatar A high incidence of homocysteinuria

    Childhood blindness in Qatar A high incidence of homocysteinuria

    Richard Scawn, Dawn Sim, Charles Claoue ... The aim of our study was to determine the causes of childhood blindness in Qatar One previous survey of blindness in adults and children of Qatar was carried out in 1977 1 The...
  • Sources of Hydrocarbons 5.1

    Sources of Hydrocarbons 5.1

    Fehlings test is used to distinguish the diffference in functional groups. Reflux (20 mins) 5g of sodium dichromate. 5cm3 of sulphuric acid. Cool flask with tap water. Mix 2 cm3 of ethanol and . 10 cm3 of deionised water in...