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 Annulene is antiaromatic (4N e-s) Annulene would be antiaromatic, but its not planar, so its nonaromatic. 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 Annulene Cyclooctatetraene easily forms a -2 ion. Ten electrons, continuous overlapping p orbitals, so it is aromatic. +
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
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
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