6.1.1 Aromatic Compounds

Question 1

What are aromatic compounds?

Answer 1

Have a cyclic or ‘ring’ structure and a delocalised pi system of electrons
Many of which have odours.
Benzene is a planar molecule with hybrid bonds between each carbon atom.

Question 2

Evidence for the structure of benzene: reactions

Answer 2

Does not decolourise bromine water.
Unlike alkenes, it does not undergo addition reactions due to the stability of the pi electron system.
It undergoes electrophilic substitution reactions which do not disrupt the delocalised electron ring.
Able to distinguish between alkenes and benzene or other arenes.

Question 3

2: carbon-carbon bond length

Answer 3

The bond length between all carbon atoms in benzene is equal at 139pm, which is less than the length of a carbon-carbon single bond, but greater than that of a carbon-carbon double bond.

Question 4

3: resonance enthalpy

Answer 4

The enthalpy change of hydrogenation of benzene is less than would be expected for the kekulé structure.
This difference in energy is known as the resonance enthalpy of benzene.
It shows that the structure of benzene is stabilised by the delocalised pi electron system.

Question 5

Nitration of benzene

Answer 5

Substitution of a nitronium group to form nitrobenzene.
Reagents: conc nitric and sulphuric acids
Conditions: 50 degrees
Reaction type: electrophilic substitution

Question 6

Halogenating of benzene

Answer 6

Electrophilic substitution of a halogen into the benzene ring.
Chlorination: forms chlorobenzene
Reagents: chlorine
Conditions: dark - to avoid homolytic fission and radicalisation of chlorine.
- aluminium chloride or iron (III) chloride catalyst.
Bromination: forms bromobenzene
Reagents. Bromine
Conditions: room temp in the dark
- aluminium bromide or iron (III) bromide catalyst.

Question 7

Alkylation of benzene

Answer 7

Electrophilic substitution of an alkyl group into the benzene ring.
E.g form methylbenzene
Reagents: chloromethane
Conditions: reflux in anhydrous conditions
- aluminium chloride catalyst

Question 8

Why is the carbon-chlorine bond in chlorobenzene stronger than in chloroalkanes?

Answer 8

Due to the overlap of lone pair electrons in p orbitals of chlorine electrons with the pi electron system of the benzene ring.
This overlap gives the C-Cl bond some pi character and increases bond strength.
Makes chlorobenzene less reactive than chloroalkanes.

Question 9

Why is the carbon-chlorine bond in chlorobenzene stronger than in chloroalkanes?

Answer 9

Due to the overlap of lone pair electrons in p orbitals of chlorine electrons with the pi electron system of the benzene ring.
This overlap gives the C-Cl bond some pi character and increases bond strength.
Makes chlorobenzene less reactive than chloroalkanes.

Question 10

Naming monosubstituted benzene molecules

Answer 10

Alkane chain depends on length, if the same or shorter than benzene then it is substituent.
Use a prefix before ‘benzene’
For alkyl - ‘alkyl’benzene
Halogen atoms - ‘halogeno’benzene
Nitrile atoms - ‘nitrile’benzene

Question 11

Naming benzene bonded to an alkyl with a functional group

Answer 11

Consider the benzene molecule to be the substituent of the alkyl chain
Benzene ring is given the prefix ‘phenyl’ - phenol, phenylamine
Benzaldehyde and benzoic acid are exceptions to this rule.

Question 12

What is acylation?

Answer 12

Electrophilic substitution
Adding an acylchloride to benzene to form an aromatic ketone.

Question 13

Why is phenol more reactive than benzene

Answer 13

Phenol reacts more readily with electrophiles than benzene
In phenol, a lone pair of electrons in a p orbital on the oxygen atom is donated into the delocalised electron structure of the benzene ring
Therefore this increases the electron density in the benzene ring, so there is a greater attraction between the benzene ring and electrophiles

Question 14

Bromination of phenol

Answer 14

Phenol reacts readily with bromine water at room temp.
Forming a white precipitate of 2,4,6-tribromophenol
The bromine water decolourises (orange to colourless)
Due to increased electron density of the benzene ring in phenol, the phenol molecule can induce a dipole in a molecule of bromine.
Does not require a halogen carrier catalyst

Question 15

Nitration of phenol

Answer 15

Use dilute nitric acid at room temp
Make a mixture of 2-nitrophenol and 4-nitrophenol.
Requires milder conditions than with benzene due to increased electron density in the benzene ring of phenol.

Question 16

Acidity of phenol

Answer 16

Compared to other alcohols, phenol is less soluble on water, due to the benzene ring which is non-polar.
When dissolved in water, phenol is a weak acid.
A proportion of the phenol molecules reversible dissociate.
More acidic than alcohols but less acidic than carboxylic acids.
Can react with string base - sodium hydroxide but not sodium carbonate.