6.1.1 - aromatic compounds

• The empirical formula of benzene is CH

  the molecular formula of benzene is C6H6
• Benzene is drawn as a hexagon with a circle inside of it
• Benzene is a liquid at room temperature
• The bond angle of benzene is 120 degrees
• Three features that dont support Kekules model

  1. benzene is resistant to addition reactions
  2. enthalpy change of hydrogenation of benzene is more stable than predicted
  3. all the carbon bonds are the same length
• X ray diffraction was used to find the bond lengths of benzene
• Kekules model suggested benzene was a planar molecule with alternating single and double bonds
• In the delocalised model, the 4th electron in the p orbital of each carbon is delocalised forming rings of delocalised electrons above and below the hexagon
• The rings of electron density in benzene make it very stable, this is aromatic stability
• Benzene is attacked by electrophiles because of its high electron density due to delocalised electron rings
• The general electrophilic substitution mechanism of benzene is

  
• Nitration is a type of electrophilic substitution reaction
• An NO2+ ion is used To nitrate benzene
• The catalyst in nitration of benzene is concentrated sulfuric acid
• The H2SO4 catalyst in the nitration of benzene is regenerated by:

  HSO4- + H+ ——> H2SO4
• The overall equation for the nitration of benzene is:

  C6H6 + HNO3 ——> C6H5NO2 + H2O
• A friedel crafts reaction uses a halogen carrier as a catalyst eg, AlCl3
• Benzene does not react directly with halogens as the aromatic ring is too stable
• When AlCl4- is formed, the lone pair of electrons on the Cl atom is forming a dative covalent bond to Al
• The AlCl3 catalyst is reformed by:

  AlCl4- + H+ ——> HCl + AlCl3
• To add a methyl group to a benzene ring, Use a halogenoalkane and add AlCl3 to create an electrophiles that can attack benzene
• The NO2+ ion is generated by:

  H2SO4 + HNO3 ——> H2O + NO2+ + HSO4–

  both H2SO4 and HNO3 are concentrated
• The structure of phenol is an aromatic ring with a -OH functional group attached
• To test and show the weak acidity of phenol first a neutralisation reaction with NaOH occurs. But when you react phenol with carbonates no reaction happens
• The equation between phenol and bromine is:

  C6H5OH + 3Br2 ——> 2,4,6-tribromophenol + 3HBr
• The equation for the reaction between phenol with dilute nitric acid is:

  C6H5OH + HNO3 ——> 2-nitrophenol + H2O

  C6H5OH + HNO3 ——> 4-nitrophenol + H2O
• It is easier for electrophilic substitution to occur with phenol compared to benzene because the lone pair of electrons on the oxygen of the -OH are delocalised into the pi system. This increases the electron density of the ring so electrophiles are more attracted to phenol.
• The directing effect of electron donating groups OH and NH2 is 2,4
• The directing effect of electron withdrawing group NO2 is 3
• Benzene has a relatively high melting point because of close packing of flat hexagonal molecules when solid
• Benzene is insoluble in water because it is non-polar
• Benzene is not used in schools because it is a carcinogen.
• To name compounds containing a benzene ring it is either

  -benzene (if benzene is the functional group) orr

  phenyl- (if phenol is the functional group)