Chapter 25-Aromatic Compounds

Question 1

Evidence to disprove Kekulé’s model

Answer 1

1. Benzene is unreactive. If it had 3 double bonds it should decolorise bromine, but it doesn’t undergo electrophilic addition reactions
2. The bond length of every bond in benzene is the same, and in between the length of a C-C bond and a C=C bond
3. The enthalpy change of hydrogenation is less exothermic than expected for a substance with 3 double bonds

Question 2

Describe the delocalised model of benzene

Answer 2

Planar cyclic hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms
Each carbon uses 3 outer electrons to bond to other atoms
Each carbon has 1 electron in a p-orbital at right angles to the plane of carbon atoms in the hexagon
Adjacent p-orbitals overlap sideways above and below the plane of carbon atoms creating a ring of equal electron density
This creates a system of pi bonds which spread over all 6 carbon atoms
The pi system electrons are delocalised

Question 3

Nitration of benzene: conditions, reactant, product

Answer 3

Conditions: heat to 50°C. If you heat more, further substitution could occur
Reactants: Nitric Acid
Catalyst: concentrated sulfuric acid
Products: nitrobenzene, water

Question 4

Halogenation of benzene: reactants, conditions, product

Answer 4

Conditions: RTP
Reactants: halogen
Catalyst: halogen carrier, FeCl3, FeBr3, AlCl3, AlBr3
Product: halobenzene, HX

Question 5

Alkylation of benzene:reactants, conditions, product

Answer 5

Reactants: haloalkane
Catalyst: halogen carrier
Conditions: RTP
Product: alkylbenzene, HX

Question 6

Acylation reactions: conditions, reactants, products

Answer 6

Reactant: acyl chloride
Conditions: RTP
Catalyst: halogen carrier
Product:aromatic ketone, HX

Question 7

Why is benzene unreactive?

Answer 7

Benzene has delocalised pi electrons spread above and below the plane of carbon atoms of the ring
Electron density is the same across whole molecule and less than a C=C bond
Insufficient electron density to induce a dipole in halogens so no reaction takes place without a halogen carrier

Question 8

What is a phenol?

Answer 8

Organic chemical containing an -OH group bonded directly to an aromatic ring

Question 9

Compounds with an -OH group directly bonded to a side chain bonded to a benzene ring are

Answer 9

Classified as alcohols rather than phenols

Question 10

How acidic is phenol?

Answer 10

It is a weak acid
More acidic than alcohols but less than carboxylic acids
Partially dissociates to form the phenoxide ion

Question 11

Reactivity relative to phenol

Answer 11

Ethanol does not react with sodium hydroxide (strong base) or sodium carbonate (weak base)
Phenol reacts with strong bases
Carboxylic acids react with both strong and weak bases

Question 12

Reaction of phenol with sodium hydroxide:

Answer 12

Phenol + NaOH —————————> Sodium phenoxide + water

Question 13

Bromination of phenol

Answer 13

Phenol + 3Br2 —————————> 2,4,6-tribromophenol +3HBr
2,4,6-tribromophenol is a white precipitate
Conditions: RTP, no catalyst required

Question 14

Nitration of phenol

Answer 14

Phenol+ HNO3 —————————> 2-nitrophenol or 4-nitrophenol + water
2 pathways
The reaction creates a mixture of isomers
No catalyst required, RTP

Question 15

Reactivity of phenol vs benzene

Answer 15

Phenol is more reactive than benzene
A lone pair from the oxygen p-orbital of the -OH group is donated to the pi system of phenol
The electron density of the benzene ring is increased
This attracts electrophiles more and phenol is more susceptible to electrophilic attack
For halogens, the electron density in phenol is sufficient to polarise the molecules so no halogen carrier is required

Question 16

Activating group

Answer 16

An activating group allows the aromatic ring to react more readily with electrophiles
Directs the second substituent to positions 2 or 4

Question 17

Deactivating group

Answer 17

Makes the aromatic ring react less readily with electrophiles
Directs the second substituent to position 3