Arenes - Benzene

Revision of Benzene

1. Arenes
Benzne

2. Contents
•Arenes and Benzene
•Kuklé
model + Problems with it
–Low reactivity
–Carbon-Carbon bond lengths
–Hydrogenation of Benzene
•Delocalised model
•Reactions
–Electrophilic substitution
–Halogenation of Benzene
•Definitions
•Summary
–Reactions
–Electrophilic substitution

3. Arenes and Benzene
•Arenes are aromatic
hydrocarbons containing
one or more benzene
rings
•Aromatic comes from the
Latin ‘aroma’ meaning
fragrance
•Arenes occur in crude oil
and coal
•Benzene has a molecular
formula of C₆H₆ and has a
molecular weight of 78
•The ring is classified as an
aromatic compound
•It is a carcinogen which can
cause cancer or anaemia
•It is a colourless liquid with a
sweet smell and highly
flammable
•Benzene is used in
detergents, explosives,
pharmaceuticals and dyes
Back to Contents

4. Kuklé model
•Kuklé was the first
person to come up with
the idea of a ring
•He suggested that it
had a planar, cyclic
structure
•He also said there were
alternating double and
single bonds
Back to Contents

5. Problems: Low Reactivity
•If there were C=C bonds then it would act
like an alkene
•It did not react with bromine water though
which is the test for double bonds
•Benzene also takes part is substitution
reactions not addition reactions at
expected with double bonds
•It was thought that the bonds create an
equilibrium which is represented in the
resonance hybrid
Back to Contents

6. Problems: Carbon-Carbon Bond
Lengths
•Kuklé supposed there were 3 C–C bonds
and 3 C=C bonds
•When X-ray studies revealed that C–C
bonds and C=C bonds had different lengths
•However, it was found that all the bond
lengths in benzene were equal and between
the two lengths (0.139nm)
•So benzene looked like this:
Back to Contents

7. Problems: Hydrogenation of
Benzene
•Benzene was compared with
cyclohexene
•When looking at the enthalpy
change of cyclohexene it was
-120 kJmol⁻¹
•Thus benzene should have
been 3 times this
•However it was found to be
-208 kJmol⁻¹ rather than -360
kJmol⁻¹
•This shows that it is more
stable than thought
•This explains why it is less
reactive than it was previously
thought
Back to Contents

8. Delocalised model
•The theory suggests that
there are 3 localised bonds
and 6 delocalised electrons
•This forms p-orbitals and
these overlap to create pi (π)
bonds
•This would give the planar
structure and no double
bonds
•The structure is stable and
resists attempts to break it
down by addition reactions
•But substitution of hydrogens
would not affect the
delocalised theory
Back to Contents

9. Reactions: Electrophilic substitution
•Reagents: conc. Nitric acid and conc. Sulphuric
acid (catalyst)
•Conditions: reflux at 55C
•Equation:
•Mechanism:
•Restoring the Catalyst:

Back to Contents

10. Reactions: Halogenation of
Benzene
•Reagents: Chlorine and a halogen carrier (catalyst)
•Conditions: reflux and halogen carrier
•Equation:
•Mechanism:
•Halogen Carriers: iron, iron (III) chloride, iron (III)
bromide, aluminium chloride
•Chlorine is non polar so anhydrous aluminium chloride
can act as a catalyst
Back to Contents

11. Definitions
•Delocalised electrons – shared between more than two atoms
•Addition reaction – a reactant is added to an unsaturated molecule
to make a saturated molecule
•Substitution reaction – an atom or group of atoms is replaced with a
different atom or group of atoms
•Electrophile – an atom (or group of atoms) that is attracted to an
electron-rich centre, where it accepts a pair of electrons to form a
new covalent bond
•Electrophilic substitution – a type of substitution reaction in which an
electrophile is attracted to an electron-rich centre or atom, where it
accepts a pair of electrons to form a new covalent bond
•Reaction mechanism – a series of steps that, together, make up the
overall reaction
•Curly arrow – a symbol used in reaction mechanisms to show the
movement of an electron pair in the breaking or formation of
covalent bonds
Back to Contents

12. Summary - Reactions
Nitrobenzne Chlorobenzene
Bromobenzene
Benzene
Br₂
FeBr₃
Conc. HNO₃
Conc. H₂SO₄
50°C
Cl₂
AlCl₃
Back to Contents

13. Summary – Electrophilic Substitution
Back to Contents