Aromatic chemistry

• What if benzene's formula and structure

  C6H6

  
• What is another name for arenes

  • AROMATIC compounds
  • It is called this because it was first found in sweet-smelling dyes
• What is the most common type of reaction of benzene

  SUBSTITUTION

  Of a H for a DIFFERENT functional group
• What is the shape of benzene

  • Flat and REGULAR HEXAGON
  • Bond angle = 120
• What is the bond length between adjacent C atoms

  Intermediate between C-C and C=C
• What happens to the 4th electron in the p orbital of each C atom in benzene

  • It delocalises to form rings of electron density above and below the hexagon
• What is the effect on benzene's stability of the rings of electron density

  • Makes benzene very stable
  • Even though it is UNSATURATED
• Draw the SKELETAL structure of cyclohex-1,3,5-triene

  ..

  
• What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene

  • Hydrogenation of cyclohexene = -120KJmol^-1 -> cyclohexa-1,3,5-triene = -360KJmol^-1
  • Benzene hydrogenation = -208KJmol^-1 so benzene is 152KJmol^-1 more STABLE
• Why else is cyclohexa-1,3,5-triene not a suitable model for benzene

  • Would NOT be symmetrical (C=C bond is SHORTER than C-C) but benzene is STABLE
  • Would easily undergo ADDITION reactions across the double bonds -> benzene DOES NOT
  • Would form two isomers on the addition of Br2 or similar -> benzene DOES NOT
• What is the appearance of benzene at 298K

  COLOURLESS LIQUID
• Why does benzene have a relatively HIGH melting point

  Close packing of FLAT HEXAGONAL molecules when solid
• Is benzene soluble in water

  • NO
  • because it is NON-POLAR
• What is the danger of benzene

  it is a carcinogen
• How do you name compounds containing a benzene ring

  • -benzene
  • Phenyl -
  • Can designate position on ring using numbers if there is more than one substituent
• Why can benzene be attacked by electrophiles

  • HIGH electron density above and below the ring due to DELOCALISED electrons
• What is delocalisation energy and what is the effect of this on benzenes reactions

  • The large amount of energy that is needed to BREAK the aromatic ring apart
  • Results in the aromatic ring almost always staying intact
• What is seen when benzene is COMBUSTED

  • SMOKY flames due to soot from unburnt carbon
  • This is because of the HIGH carbon:hydrogen ratio
• Draw a general electrophilic substitution mechanism of benzene using El+ to represent an electrophile

  • Arrow HAS TO COME FROM RING
  • The crescent HAS TO BE FACING WHERE THE ELECTRON IS BEING LOST FROM

  
• Which ion is used to nitrate benzene

  • NO2+
  • THE + IS ON THE NITROGEN
  • nitronium ion
• How is this NO2+ generated

  • CONCENTRATED H2SO4 AND CONCENTRATED HNO3
  • H2SO4 + HNO3 -> HSO4- + NO2+ + H2O
• How is the H2SO4 catalyst regenerated in the nitration of benzene

  HSO4- + H+ -> H2SO4

  H+ comes from the benzene ring
• Draw a mechanism and write an overall equation for the nitration of benzene

  • C6H6 + HNO3 -> C6H6NO2 + H2O

  
• What are the uses of nitrated arenes

  • Production of explosives e.g. TNT
  • TNT = 1-methyl-2,4,6-trinitrobenzene
  • TNT releases LOTS of heat and gas on explosion
• How do substituents with a positive inductive effect (e.g alkyl groups) affect further substitution

  • They release electrons into the DELOCALISED electron ring
  • INCREASING the electron density
  • Thus making further substitution reactions more likely/quick
  • Direct substituents to the 2,4,5 positions
• How do substituents with a negative inductive effect (e.g NH2) affect further substitution

  • REMOVE electrons from the DELOCALISED electron ring
  • DECREASING the electron density and making further substitution reactions less likely/quick
  • DIRECT substituents to 3,5 positions
• What type of catalyst is used for Friedel-crafts reactions

  A HALOGEN CARRIER

  E.G AlCL3
• Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl2 and RCOCL

  • AlCL3 + RCOCl -> AlCl4- + RCO+
  • in RCO+ the + is on the C
  • RCO+ can attack benzene
• What is happening when AlCl4- is formed in terms of electrons

  Chlorine atom's lone pair of electrons is forming a coordinate bond to Al
• How is the AlCl3 catalyst reformed

  AlCl4 - + H+ -> HCl + AlCl3

  H+ is from benzene
• How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring

  • Use a halogeoalkane and AlCl3 to create an electrophile that can attack benzene
• Draw the mechanism for the ACYLATION of benzene from RCO+ 


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• If you are considering cyclic compounds, what might happen if two double bonds are NEXT TO EACH OTHER

  • C=C bond are in close proximity
  • Electrons in the pi cloud can PARTIALLY DELOCALISE
  • This means they can move between the two C=C double bonds
• What effect would electrons in p orbitals moving between the two C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation

  • MAKES THE MOLECULE MORE STABLE
  • Makes the enthalpy of hydrogenation more POSITIVE