1. aromatic compounds

• Aromatic chemistry is the study of compounds containing benzene rings.
• what are aromatics important in?

  synthesis of drugs, dyes, explosives and plastics
• methyl orange 


  pH indicator frequently used in titrations because of its clear and distinct colour change
• T.N.T


  explosive material
• aspirin


  medication used to treat pain, fever, or inflammation
• evidence for structure of benzene

  first discovered in 1825 by Michael Faraday
• what did Michael Faraday find out?

  empirical formula = CH

  relative mass = 78

  molecular formula = C6H6
• Kekule structure - 1865


  structure with alternate single and double bonds between carbon atoms

  
• problems with Kekule's structure - 1

  • suggests that benzene should readily undergo electrophilic addition reactions - similar to alkenes
  • unlike alkenes, benzene doesn't decolourise bromine water
  • benzene has lower chemical reactivity than would be predicted by Kekule's structure
• problems with Kekule's structure - 2

  enthalpy of hydrogenation for benzene doesn't agree with the Kekule structure

  
• enthalpy of hydrogenation of benzene?

  -208 kJmol-1
• what does the enthalpy of hydrogenation of benzene show?

  the actual structure of benzene is more stable than Kekule's structure - releases less energy when it reacts
• problems with Kekule's structure - 3

  1929, X-ray diffraction studies at the University of Leeds by Kathleen Lonsdale finally probed that Kekule's structure for benzene was incorrect
• Electron density map of benzene shows that all carbon-carbon bond lengths are equal and the electron density around each carbon is equal. This would not be the case if Kekule’s structure was correct.

  
• X-ray studies revealed all bonds were of an equal length – 0.140 nm, somewhere in between that which would be expected for a single and a double bond. ​

  
• structure of benzene

  
• • each carbon has 3 bonds
  • each carbon has a spare electron in a p orbital
  • 6 of these p orbitals can overlap forming a pi orbital where 6e- spread over all 6 carbons
  • the 6 electrons can be anywhere in the pi orbital formed
  • these electrons are said to be delocalised
• delocalisation stability 


  The extra stability of benzene is from the electrons in the double bonds - not fixed in one place but are delocalised around the benzene ring.
• naming aromatics


  -benzene / phenyl-