Aromatic Chemistry
27.1 Introduction of Arenes:
- Arenes are hydrocarbons based on benzene 27.1 Introduction to arenes
C6H6. 27.2 arenes – physical properties, naming and reactivity
- It is a hexagonal ring
27.3 – reactions of arenes
structure.
- 𝜋 electrons are delocalised around the ring above and below the ring.
o
- All bond angles are 120 .
- Benzene does not decolourise bromine water or undergo electrophilic addition
reactions.
- Benzene’s carbon-carbon bonds are in between the length of a single carbon-carbon bond and a double
carbon-carbon bond.
- The enthalpy of hydrogenation of cyclohexane is -120kJmol-1. Kekule’s structure with 3 double bonds
would have an expected enthalpy of hydrogenation of 3 x -120 = -360kJmol-1. The actual
hydrogenation of benzene is -208kJmol-1.
- Benzene is more stable because of its delocalised electrons.
27.2 Arenes – Physical Properties, Naming and Reactivity:
- Benzene has a similar boiling point to hexane, but a much higher melting point. This is because benzene’s
flat hexagonal molecules are able to pack together very well as a solid. This makes them harder to
separate.
- Benzene is a colourless, sweet smelling, highly flammable liquid. It is obtained from crude oil and is a
component of petrol and cigarette smoke.
- Benzene is a carcinogen
- Reactivity of aromatic compounds:
o The ring has a high electron density so is attacked by electrophiles.
o Aromatic ring is very stable, a lot of energy must be put in to break the ring, so it almost
always stays intact in reactions.
27.3 Reactions of Arenes:
Combustion:
- Arenes burn with a smoky flame because they have a high carbon to hydrogen ratio, this means there
is more incomplete combustion which produces soot from the unburnt carbon.
- A smoky flame suggests an aromatic compound.
Electrophilic Substitutions
- The electrons from the delocalised ring of benzene are attracted to the electrophile, the electrophile
substitutes for a hydrogen atom on the benzene ring.
Nitration:
- Benzene reacts slowly with nitric acid to form nitrobenzene.
- A water bath is used to maintain a steady temperature.
- A temperature above 50oC will result in further substitution.
- 70oC will produce dinitrobenzene.
27.1 Introduction of Arenes:
- Arenes are hydrocarbons based on benzene 27.1 Introduction to arenes
C6H6. 27.2 arenes – physical properties, naming and reactivity
- It is a hexagonal ring
27.3 – reactions of arenes
structure.
- 𝜋 electrons are delocalised around the ring above and below the ring.
o
- All bond angles are 120 .
- Benzene does not decolourise bromine water or undergo electrophilic addition
reactions.
- Benzene’s carbon-carbon bonds are in between the length of a single carbon-carbon bond and a double
carbon-carbon bond.
- The enthalpy of hydrogenation of cyclohexane is -120kJmol-1. Kekule’s structure with 3 double bonds
would have an expected enthalpy of hydrogenation of 3 x -120 = -360kJmol-1. The actual
hydrogenation of benzene is -208kJmol-1.
- Benzene is more stable because of its delocalised electrons.
27.2 Arenes – Physical Properties, Naming and Reactivity:
- Benzene has a similar boiling point to hexane, but a much higher melting point. This is because benzene’s
flat hexagonal molecules are able to pack together very well as a solid. This makes them harder to
separate.
- Benzene is a colourless, sweet smelling, highly flammable liquid. It is obtained from crude oil and is a
component of petrol and cigarette smoke.
- Benzene is a carcinogen
- Reactivity of aromatic compounds:
o The ring has a high electron density so is attacked by electrophiles.
o Aromatic ring is very stable, a lot of energy must be put in to break the ring, so it almost
always stays intact in reactions.
27.3 Reactions of Arenes:
Combustion:
- Arenes burn with a smoky flame because they have a high carbon to hydrogen ratio, this means there
is more incomplete combustion which produces soot from the unburnt carbon.
- A smoky flame suggests an aromatic compound.
Electrophilic Substitutions
- The electrons from the delocalised ring of benzene are attracted to the electrophile, the electrophile
substitutes for a hydrogen atom on the benzene ring.
Nitration:
- Benzene reacts slowly with nitric acid to form nitrobenzene.
- A water bath is used to maintain a steady temperature.
- A temperature above 50oC will result in further substitution.
- 70oC will produce dinitrobenzene.
