CHM030 5.1: Aromatic Compounds Introduction

• phenyl group


  the characteristic structural unit of “aromatic” organic compounds
• In the early days of organic chemistry, the word aromatic was used to describe fragrant substances
• benzene


  from coal distillate
• benzaldehyde


  from cherries, peaches, and almonds
• toluene


  from tolu balsam
• aromatic


  this term is used to refer to the class of compounds structurally related to benzene
• toluene

  
• benzaldehyde

  
• benzene

  
• aromatic


  refers to the class of compounds that contain six-membered benzene-like rings with three double bonds
• benzene

  • causes bone marrow depression
  • causes depressed white blood cell count (leukopenia) on prolonged exposure
  • should not be used as a laboratory solvent
  • if used, it should be handled cautiously
• estrone


  steroidal hormone
• morphine


  pain medication
• diazepam (valium)


  treatment for anxiety disorders, alcohol withdrawal symptoms, or muscle spasms
• atorvastatin (lipitor)


  cholesterol lowering drug
• Structure of Benzene

  • has eight fewer hydrogens (C6H6) than the corresponding six-carbon alkane (C6H14)
  • unsaturated
  • represented as a six-membered ring with alternating double and single bond
• Structure of Benzene


  all C-C bonds of benzene have the same length:

  • 139 pm — intermediate between single bonds (154 pm) and double bonds (134 pm)
  • all C–C–C bond angles are 120 ⁰
• Structure of Benzene

  • each C is sp2-hybridized
  • has a p orbital perpendicular to the plane of the six-membered ring
  • each p orbital has one electron in it
  • the electron density in all six C - C bonds is identical
  • planar molecule with the shape of a regular hexagon
• TRUE OR FALSE:

  TRUE: It is IMPOSSIBLE to define three localized π bonds in which a given p orbital overlaps only one neighbouring p orbital
• Structure of Benzene

  • all six carbon atoms and all six p orbitals in benzene are equivalent
  • each p orbital overlaps equally well with both neighboring p orbitals leading to a picture of benzene in which all six π electrons are free to move about the entire ring
• Molecular Orbital Description of the Resonance in Benzene

  benzene is a hybrid of two equivalent forms (in resonance terms): neither form is correct by itself; the true structure of benzene is somewhere in between the two resonance forms but is impossible to draw with our usual conventions.
• Molecular Orbital Description of the Resonance in Benzene


  benzene is more stable and less reactive than a typical alkane because of resonance
• Molecular Orbital Description of the Resonance in Benzene

  benzene fails to undergo typical alkene addition reactions (addition product is not formed)

  
  

  Addition Reaction: Alkene + Br2/CCl4 -> dibromide (addition product)
• aniline

  
• phenol

  
• acetophenone

  
• benzoic acid

  
• ortho-Xylene

  
• styrene

  
• Naming Aromatic Compounds: monosubstituted benzenes


  systematically named in the same manner as other hydrocarbons, with -benzene as the parent name

  Examples:

  C6H5Br - bromobenzene

  C6H5NO2 - nitrobenzene

  C6H5CH2CH3 - ethylbenzene
• Naming Aromatic Compounds: Phenyl


  used for the -C6H5 unit when the benzene ring is considered as a substituent
• Naming Aromatic Compounds: aryl group

  • a generalized aromatic substituent is called an _
  • abbreviated as Ar
• benzyl group


  used for C6H5CH2- group

  
• Naming Aromatic Compounds: disubstituted benzenes


  named using one of the prefixes:

  • ortho- (has its two substituents in a 1,2 relationship)
  • meta- (has its two substituents in a 1,3 relationship)
  • para- (has its two substituents in a 1,4 relationship)
• Naming Aromatic Compounds

  • Benzenes with more than two substituents are named by (1) choosing a point of attachment as carbon 1 and (2) numbering the substituents on the ring so that the second substituent has as low a number as possible.
  • The substituents are listed alphabetically when writing the name.

  Note: -phenol and -toluene can be used as the parent names rather than -benzene.

  
• indene

  
• xylene

  
• naphthalene

  
• biphenyl

  
• anthracene