Aromatics

Question 1

Give three pieces of evidence that disproved the Kekulé model of benzene.

Answer 1

• Lack of reactivity: does not decolourise bromine water when in normal conditions (without catalyst) as does not undergo any electrophilic addition reactions, so cannot contain any C=C bonds.
• Bond length: using X-ray diffraction, the bond length in benzene was found to be 0.139nm in length. This is between the length for a single and double C bond, 0.153nm and 0.134nm respectively.
• Hydrogenation enthalpies: If benzene had 3 C=C bonds, it would be excepted to have triple the hydrogenation enthalpy of cyclohexene (120 kJ mol-1). The real value was 208 kJ mol-1. It is therefore more stable.

Question 2

What is the delocalised model of benzene?

Answer 2

• Benzene is a planar, cyclic, hexagonal hydrocarbon with 6 Hs and 6 Cs.
• Each carbon atom is bonded to 2 adjacent carbons and 1 hydrogen.
• The remaining 1 electron per C is in a p-orbital.
• Adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the bonded carbon nuclei.
• This forms a ring of electron density and a system of pi bonds which spread over all 6 Cs, the electrons within it are said to be delocalised .

Question 3

How does the nitration of benzene occur?

Answer 3

1. (Conc.) HNO3 + (conc.) H2SO4 —> NO2+ + HSO4- + H2O
2. The electrophile (NO2+) accepts a pair of electrons from the benzene ring to form a dative covalent bond. The intermediate formed is unstable, and so the H atom donates its electron into the benzene ring, reforming it. This forms nitrobenzene and a H+ ion.
3. H+ + HSO4- —> H2SO4.

Question 4

How does bromination of benzene occur?

Answer 4

1. Br2 + FeBr3 —> FeBr4- + Br+
2. The bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The intermediate is unstable, and so the H donates its electrons into the ring, reforming a stable pi ring. This produces bromobenzene and a H+ ion.
3. H+ + FeBr4- —> FeBr3 + HBr.

Question 5

What is a halogen carrier?

Answer 5

• Halogen carriers are catalysts necessary to produce halogen electrophiles, as they do not readily react with benzene by themselves.

Question 6

What halogen carrier is used for the chlorination of benzene?

Answer 6

• AlCl3/ FeCl.

Question 7

What is alkylation of benzene?

Answer 7

• The substitution of a H atom in the benzene ring by an alkyl group.
• The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile.
• Alkylation increases the number of carbon atoms in a compounds by forming C-C bonds.

Question 8

What is acylation of benzene?

Answer 8

• Benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst to form an aromatic ketone by electrophilic substitution.
• Forms C-C bonds.

Question 9

Why is phenol classed as a weak acid?

Answer 9

• Phenol partially dissociates in water, forming the phenoxide ion and a proton.

Question 10

Is phenol more or less soluble than alcohols?

Answer 10

• Less. Due to the presence of the non-polar benzene ring.

Question 11

Does phenol react with solutions of strong bases?

Answer 11

• Yes, such as aqueous sodium hydroxide.

Question 12

How can you chemically test between a phenol and a carboxylic acid?

Answer 12

• React with sodium carbonate. The carboxylic acid reacts with sodium carbonate to produce CO2, which is evolved as a gas.

Question 13

What does the reaction between sodium hydroxide and phenol produce?

Answer 13

• Produces the salt (sodium phenoxide) and water.
• This is a neutralisation reaction.

Question 14

Why does phenol react more readily (e.g nitric acid and bromine) than benzene?

Answer 14

• The lone pair of electrons from the oxygen p-orbital of the OH- group is donated into the pi-system of phenol, increasing the electron density of its pi-ring.
• This means it more strongly attracts electrophiles.
• The aromatic ring in phenol is therefore more susceptible to electrophilic attack than benzene.
• For bromine, the electron density in the phenol ring is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required.

Question 15

What is produced by the reaction of phenol and bromine? What conditions?

Answer 15

• Forms a white precipitate of 2,4,6-tribromophenol.
• This decolourises the bromine water (orange to colourless).
• No catalyst is required and the reaction occurs at room temperature.

Question 16

What does the nitration of phenol produce? What conditions are needed?

Answer 16

• Phenol reacts readily with dilute nitric acid at room temperature.
• A mixture of 2-nitrophenol and 4-nitrophenol is formed.

Question 17

Are electron withdrawing groups activating or deactivating?

Answer 17

• Deactivating, they remove electron density from the ring.

Question 18

Are electron donating groups activating or deactivating?

Answer 18

• Activating, they increase the ring’s electron density and therefore make it more reactive.

Question 19

How does -NH2 direct groups? Activating or deactivating?

Answer 19

• positions 2 and 4 (ortho/para).
• Activating.

Question 20

What is the directing effect of -NO2? Activating or deactivating?

Answer 20

• position 3.
• Deactivating.

Question 21

Why are the halogens an exception to the ortho/para directing groups?

Answer 21

• They are the only 2,4 directing group that are deactivating.

Question 22

Does phenol react with NaOH?

Answer 22

Yes.