6.1.1 Aromatic Compounds

Question 1

What is the structure of benzene?

Answer 1

6 carbons each joined to two other carbons, one hydrogen and the spare electron goes into the delocalised rings of electrons (above and below the pi bonds)

Question 2

Why is Kekule’s model of Benzene wrong? Simple answer

Answer 2

1. Enthalpy change of hydrogenation than cyclohexane x3
2. Carbon-carbon bond length
3. Reactivity

Question 3

Explain why Kekule’s model of Benzene was wrong: enthalpy change of hydrogenation

Answer 3

• Enthalpy change of hydrogenation for Benzene should be 3 times the enthalpy change of cyclohexane
• But benzene has lower enthalpy change
• Therefore Benzene is more stable than cyclohexane and cannot be cyclohexa-1,3,5- triene

Question 4

Explain why Kekule’s model of Benzene was wrong: carbon- carbon bond length

Answer 4

• Benzene Carbon-carbon bonds in benzene are longer than C=C bonds but shorter than C-C bonds
• Therefore cannot be alternating double and single carbon bonds

Question 5

Explain why Kekule’s model of Benzene was wrong: reactivity

Answer 5

• Benzene doesn’t decolourise bromine water
• Therefore no electrophilic addition
• Therefore no double carbon bonds

Question 6

What was Kekule’s model of Benzene?

Answer 6

Alternating single and double carbon bonds in cyclic ring of 6 carbons

Question 7

What is an Arene?

Answer 7

Aromatic compound that contains a benzene ring

Question 8

How are Arenes named?

Answer 8

1. Ending in benzene, e.g. nitrobenzene

2. Phenyl for those with functional group, e.g. phenol (-OH) or phenylamine (-NH2)

Question 9

Arenes undergo electrophilic ______ and not electrophilic ______ because of the ____ of the delocalised rings of electrons

Answer 9

Substitution
Addition
Stability

Question 10

What is Friedal- Crafts acylation? Simple answer

Answer 10

An acyl group (RCO-) added to benzene ring

Question 11

What conditions are needed for Friedal- Crafts acylation?

Answer 11

Catalyst: halogen carrier, e.g. AlCl3
Reactant: acyl chloride

Question 12

What conditions are needed for the Nitration of benzene?

Answer 12

Catalyst: sulfuric acid
Reactant: nitric acid
Temperature: below 55C for single substitutions, above 55 = multiple

Question 13

How is the catalyst reformed in the Nitration of benzene?

Answer 13

H+ produced joins to reform sulfuric acid

Question 14

How is the catalyst reformed in the Friedal- Crafts acylation?

Answer 14

The extra Cl on halogen carrier bonds with H from benzene to form HCl, meaning halogen carrier back to having only 3 chlorines

Question 15

Why are phenols more reactive than benzene?

Answer 15

• Lone pair from the oxygen in -OH donated to delocalised ring of electrons
• Increasing the electron density of the aromatic ring
• Attracts electrophiles more strongly

Question 16

How does the presence of a nitronium ion on a benzene ring (nitrobenzene) affect disubstitution?

Answer 16

Directs to carbon 3

Question 17

How does the presence of a hydroxyl ion on a benzene ring (phenol) affect disubstitution?

Answer 17

Directs to carbon 2 and carbon 4

Question 18

How does the presence of a amide ion (-NH2) on a benzene ring affect disubstitution?

Answer 18

Directs to carbon 2 and carbon 4