Introduction to aromatic chemistry


Naming compounds based on benzene:



  • Benzene is the feedstock for many other compounds including phenol, styrene, dyestuff.

  • The major problem with benzene is that it is a carcinogen (cancer causing) molecule.

  • Its main source is from crude but occurs naturally from volcanoes.

The structure of benzene:



Questions 1-5  P5


The structure of benzene


Problems with Kekule structure:

Benzene's low reactivity:

Kekule's equilibrium model of benzene



The carbon - carbon bond length in benzene:



C C              0.153nm                    cyclohexane


C = C              0.134nm                    cyclohexene


C C              0.139nm                    benzene


This suggests that the C - C bond is somewhere between a single and double bond


Hydrogenation of benzene



Qu 1 - 3  P 7


The delocalised model of benzene


The delocalised model of benzene

  1) 6 carbon's and 6 hydrogen's.
  2) Arranged in a hexagonal ring.
  3) The shape around each carbon atom is trigonal planar with a bond angle of 120o.
  4) Carbon carbon bond lengths are all the same.


P orbital
  • Remember that sigma bonds, s are covalent bonds with a maximum overlap.
  • In alkenes, the second covalent bond is due to the overlap of 2 adjacent p orbitals to form a p bond.
  • The alkenes have 4 electrons between the carbon carbon double bond making it electron rich enough to polarise an electrophile.
  • These electrons are localised - only in the C=C.
s bond with p orbitals overlapping to form a p bond in alkenes
  • if 2 x p orbitals can overlap forming a p bond then 6 x p orbitals can overlap forming a system of p orbitals spread over all 6 carbons.
  • The 6 electrons can be anywhere in this system.
  • This means that the electrons are not localised around 1 carbon but delocalised over all 6 carbons.
6 x p orbitals p delocalised orbital  

     Represents 2es in a bond

=     Represents 4es in a double bond

The delocalised model of benzene and chemical reactivity:

Reaction with Alkenes Benzene
Decolorise bromine water
Strong acids, HCl
Halogens, Cl2

Qu 1 - 4   P 9


Benzene and its reactions






Electrophilic substitution

1)  Nitration of benzene

         HNO3     +       H2SO4                NO2+        +       HSO4-         +          H2O

                                                                  Nitryl ion is electrophile






+           H+
H+ + HSO4- H2SO4    
C6H6 + HNO3 C6H5NO2 + H2O





Nitration of methylbenzene


2)      Halogenation of benzene

FeCl3, FeBr3, AlCl3, AlBr3            (depending on which halogen you are adding)


Fe can be used on its own as it will react with any halogen forming FeHal3

The reaction with chlorine and bromine:

Function of the halogen carrier (catalyst):

                      FeBr3            +          Br2                                  FeBr4-       +         Br+





Regeneration of the halogen carrier (catalyst):

                      FeBr4-       +         H+                                  FeBr3            +          HBr


Other halogens

Mechanisms for benzene:

Full mechanism:


Qu  1 - 4  P11  /  1 - 3  P13


The reactivity of alkenes and benzene

Cyclohexene vs Benzene


Cyclohexene Benzene
  • We already know that an alkene such as cyclohexene will decolourise bromine water:

  • When benzene reacts with bromine, a halogen carrier is required:

The difference between an alkene and benzene is the electron density


  • An alkene has 2e from a s bond and 2e from the localised p bond = 4e
  • This has a high electron density = electron rich.
  • This will polarise an incoming electrophile like bromine sufficiently that it will react with the alkene readily.
  • Alkenes do not need Br+ so do not need a halogen carrier.
  • Benzene has 6 delocalised p electrons distributed over 6 bonds.  This averages 1e per C-C bond.
  • Benzene's C-C bonds have 2e from a s bond and 1e from the delocalised p system = 3e
  • This does not have the electron density of an alkene = less electron rich.
  • This will not polarise an electrophile sufficiently enough to react.
  • Benzene requires Br+ (more positive) so needs a halogen carrier:
The mechanism:

The mechanism:

 FeBr3   +    Br2           FeBr4-       +     Br+                FeBr4-       +         H+                                  FeBr3            +          HBr

  • Alkenes add electrophiles (chlorine) to themselves.
  • Benzene substitutes electrophiles (bromine) with hydrogen.
Electrophilic addition Electrophilic substitution

Qu 1-2  P15



Reactions of phenol



1.      Solubility in water

2.      Acidic properties

                                                                                 Phenol                                                 Phenoxide


a)  Reaction with sodium hydroxide:

                                Acid   +   metal hydroxide        salt    +    water


                              C6H5OH    +    NaOH         C6H5O-Na  +    H2O

b)  Reaction with sodium:

                                Acid   +   metal        salt    +    hydrogen


                              2C6H5OH   +   2Na        2C6H5O-Na+    +    H2

3.  Reaction with bromine

                                                                                            2,4,6 tribromophenol

Uses of phenols:

Qu 1 - 3   P17  /  1 - 3  P19


Qu  1- 4  P41


Qu  1 - 3  P43