Chem 336 - Spring 2000
Organic Chemistry III
Dr. Carl C. Wamser

Chapter 19 - Benzene and Aromaticity

Aromaticity

• pleasant odor
• unusually low reactivity
  - substitution, not addition
• unusually stable
• characteristic ring structurewith delocalized pi bonding
• NMR ring current effect

Benzene - stability

• C6H6 (1,3,5-cyclohexatriene ?)
• no typical C=C reactions
  unreactive with HX, X2, KMnO4
• reaction requires extreme conditions
  when reaction does occur, it is substitution not addition
• Heats of hydrogenation:
  cyclohexene + H2 -28.6 kcal/mol (typical pi bond)
  1,3-cyclohexadiene + 2 H2 -55.8 kcal/mol
  (more stable than 2 pi bonds by 1.4 kcal/mol - conjugation)
  benzene + 3 H2 -49.8 kcal/mol
  (more stable than 3 pi bonds by 36 kcal/mol - aromaticity)

Benzene - structure

• all C are sp2 (trigonal, 120° angles)
  ideal for a planar hexagon
  
• all C-C bonds are the same (139 pm)
• compare C-C (154 pm), C=C (134 pm)
• cyclic conjugated pi bonds are unusually stable (resonance)

Nomenclature of Aromatics

• monosubstituted benzenes:
  there are many common names
• disubstituted benzenes:
  ortho (1,2-), meta (1,3-), para (1,4-)

p-nitrobenzoic acid / / / 2-chloro-6-ethylaniline

Nomenclature of Aromatics

• group names:
  phenyl C6H5
  benzyl C6H5CH2
  
  (E)-1-phenyl-1-butene

Spectroscopy

• NMR - ring current effect: aryl Hs are deshielded
  annulenes - outside Hs are deshielded, inside Hs are strongly shielded ( < 0 ppm)
• IR - typical C=C stretching and C-H bends depending on substitution patterns

Benzenoid Hydrocarbons

• fused six-membered rings
• know naphathlene, anthracene, phenanthrene

Graphite

• extended sheets of benzene rings
  electrically conductive, good lubricant

Fullerenes

• curved closed form of 60 carbon atoms in benzene rings with intervening 5-membered rings (soccer ball pattern)
• subject of the 1996 Nobel Prize in chemistry

Nonbenzenoid Hydrocarbons

• ring sizes other than six
  e.g., azulene

Aromatic Heterocycles

• six-membered rings - N replaces C
  e.g., pyridine, pyrimidine
• five-membered rings - lone pair on N, O, or S replaces a pi bond
  e.g., pyrrole, furan, thiophene, imidazole

Molecular Orbitals

• combination of n atomic orbitals (AOs) must give n molecular orbitals (MOs)
• six pi MOs for benzene (see text pictures)

Huckel's Rule - Aromaticity

• a compound will display aromaticity if
  it is a continuous cyclic array of p orbitals
  it is planar
  it has a total number of pi electrons of 2, 6, 10, 14, ... (4n+2)
• pi systems that are planar and cyclic but have 4n electrons are antiaromatic
• cyclobutadiene (rectangular - like two separate double bonds)
• cyclooctatetraene (tub-shaped - like four separate double bonds)

Aromatic Ions

• cyclopentadienyl anion (5 p orbitals but 6 pi electrons - aromatic)
  makes a good ligand for metal complexation C5H5 -1
  e.g., sandwich compounds like ferrocene Fe(Cp)2
• cyclooctatetraenyl dianion (8 p orbitals and 10 pi electrons - aromatic)
  makes a good ligand for large metals C8H8 -2
  e.g., sandwich compounds like uranocene U(COT)2
• cyclopropenyl cation C3H3+ , cycloheptatrienyl cation (tropylium ion) C7H7+
• MO patterns are visualized using the Frost Circle (see text)

Phenols

• alcohols on aromatic rings are called phenols
• C-O bond is strong, O-H bond is acidic (pKa ~ 10)
• phenolate anion has resonance stabilization
  note that anion can be delocalized to ortho & para positons (but not meta)
• Reactions:
  good nucleophile for Williamson ether synthesis
  Kolbe reaction - adds CO2 at ortho position (salicylic acid)
  easily oxidized to quinones

Reactions at Benzylic Positions

• anion, cation, or radical at a benzylic position can be stabilized by resonance delocalization
• alkylaromatics are easily oxidized to benzoic acid
• halogenation of alkylaromatics is preferred at the alpha (benzylic) position
• benzyl ethers are easily reduced to toluene plus alcohol (hydrogenolysis)
  a useful protecting group