Introduction to aromatic chemistry

                                        

Naming compounds based on benzene:

 

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:

Or

 

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

 

Reactivity

Practical

Practical

 

Electrophilic substitution

1)  Nitration of benzene

         HNO3     +       H2SO4                NO2+        +       HSO4-         +          H2O

                                                                  Nitryl ion is electrophile

 

 

 

 

 

+           H+
H+ + HSO4- H2SO4    
      H2SO4      
C6H6 + HNO3 C6H5NO2 + H2O
      50oC      

 

Mechanism:

 

 

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+

 

Mechanism:

 

 

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

Phenols

 

Reactions of phenol

 

Practical

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