Aromatic Compounds

Question 1

What is an aliphatic?

Answer 1

Straight or branched chain organic substances

Question 2

What is an aromatic or arene?

Answer 2

Includes one or more rings of six carbon atoms with delocalised bonding

Question 3

How are the electrons bonded in benzene?

Answer 3

Carbon is bonded to two other carbons and one hydrogen via single sigma bonds. This leaves one electron in a p orbital to form a delocalised ring.

Question 4

What evidence is there for the delocalised structure of benzene?

Answer 4

• Bond length intermediate between short double bonded carbon atoms and long single bonded carbon atoms.
• The enthalpy of hydrogenation is less exothermic than expected when compared to the hydrogenation for the Kekule structure.
• Only reacts with Br2 at high temperatures or in the presence of a halogen carrier.

Question 5

Why does benzene not generally undergo addition reactions?

Answer 5

This would result in breaking up the stable delocalised system.

Question 6

Benzene usually undergoes…?

Answer 6

Electrophilic substitution

Question 7

Why do reactions with bromine occur more easily with alkenes rather than benzene?

Answer 7

In benzene, electrons in the pi bond are delocalised whereas in alkenes, the electrons in the pi bonds are localised between two carbon atoms.
Benzene therefore has a lower electron density and therefore polarises bromine less and induces a weaker dipole in bromine than an alkene would.

Question 8

Why does bromine react more readily with phenol than benzene?

Answer 8

OH is an electron donating group, making the ring more negative. This repels electrons in Br2 which makes it easier to induce a dipole. Phenol is more attractive to electrophiles.

Question 9

What is an electrophile?

Answer 9

Lone pair acceptor

Question 10

Nitration of Benzene:

• Change in functional group?
• Reagents?
• Mechanism?
• Electrophile?

Answer 10

• Change in functional group: Benzene -> Nitrobenzene
• Reagents: Concentrated HN)3, H2SO4 (catalyst)
• Mechanism: Electrophilic Substitution
• Electrophile: NO2+

Question 11

What is the equation for the formation of the electrophile in the nitration of benzene?

Answer 11

HNO3 + 2H2SO4 -> NO2+ + 2H2SO4- + H3O+

Question 12

Halogenation of Benzene:

• Change in functional group?
• Reagents?
• Mechanism?
• Electrophile?

Answer 12

• Change in functional group: Benzene -> Bromobenzene
• Reagents: Bromine, Iron bromide catalyst (Halogen Carrier)
• Mechanism: Electrophilic Substitution
• Electrophile: Br+

Question 13

What is the equation for the formation of electrophiles in the halogenation of benzene?

Answer 13

AlCl3 + Cl2 -> AlCl4- + Cl+
Febr3 + Br2 -> FeBr4- + Br+

Question 14

Friedel Crafts Alkylation:

• Change in functional group?
• Reagents?
• Mechanism?

Answer 14

• Change in functional group: Benzene -> Akylbenzene
• Reagents: Chloroalkane in the presence of anhydrous AlCl3 catalyst. Heat under reflux.
• Mechanism: Electrophilic Substitution

Question 15

What is the equation for the formation of the electrophile in the Friedel Crafts Alkylation?

Answer 15

AlCl3 + CH3CH2Cl -> CH3CH2+ AlCl4-

Question 16

Friedel Crafts Acylation:

• Change in functional group?
• Reagents?
• Mechanism?
• Electrophile?

Answer 16

• Change in functional group?
• Reagents?
• Mechanism?
• Electrophile?

Question 17

Phenols are weakly…?

Answer 17

Acidic. They are weaker than carboxylic acids.

Both phenols and carboxylic acids react with metals and metal hydroxide.

However, only carboxylic acids will react with metal carbonates.

Question 18

Are metal phenoxides more soluble than the original phenols?

Answer 18

Yes, so it will dissolve.