Chapter 25 - Aromatic Compunds

Question 1

What are problems with the Kekulé model?

Answer 1

• Lack of reactivity with bromine
• Equal lengths of C-C and C=C bonds
• Less exothermic hydrogenation enthalpy (more stable)

Question 2

What is the delocalised model of benzene?

Answer 2

• P-orbitals above and below the plane of benzene ring to form a planar ring
• Sideways overlap of p-orbitals to create delocalised ring of electron density above and below plane of benzene ring

Question 3

When is the phenyl- prefix used?

Answer 3

Alkyl chain attached to benzene has:
- Functional group
- 7+ carbon atoms

Question 4

What is the name of a benzene with a COOH functional group?

Answer 4

Benzoic acid

Question 5

What is the name of a benzene with a NH₂ functional group?

Answer 5

Phenylamine

Question 6

What is the name of a benzene with a CHO functional group?

Answer 6

Benzaldehyde

Question 7

What type of reaction does benzene undergo?

Answer 7

Electrophilic substitution

Question 8

What is the temperature involved in the nitration of benzene?

Answer 8

50º

Question 9

Why must a halogen carrier be present in reactions with benzene?

Answer 9

Delocalised π electrons creating a lower electron density in the C=C bonds of benzene so insufficient to polarise non polar molecule (e.g. bromine)

Question 10

Why is phenol less soluble in water than alcohols?

Answer 10

Due to the non polar benzene ring

Question 11

Why does phenol act as a weak acid?

Answer 11

When dissolved in water it partially dissociates forming a phenoxide ion and and proton (H⁺)

Question 12

Phenol is _____ acidic than alcohols
Phenol is _____ acidic than carboxylic acids

Answer 12

More
Less

Question 13

How to distinguish between a phenol and carboxylic acid?

Answer 13

• React with sodium carbonate (weak base)
• Phenol will NOT react but carboxylic acid will

Question 14

Compare the reactivities of ethanol, phenol and carboxylic acid relative to the strength of bases?

Answer 14

Ethanol - no reaction with strong or weak base
Phenol - reaction with strong base
Carboxylic acid - reaction with strong and weak base

Question 15

What is formed in the reaction of phenol with sodium hydroxide (strong base)?

Answer 15

Salt and water (neutralisation reaction)

Question 16

Compare the reactivities of benzene and phenol?

Answer 16

• Reactions of phenol occurs more readily
• Lone pair of electrons from oxygen of -OH group donated into π system of phenol
• Higher electron density of benzene ring
• Stronger attraction of electrophile

Question 17

What is the difference between the bromination of phenol as compared to benzene?

Answer 17

Phenol doesn’t require a halogen carrier

Question 18

What is the difference between the nitration of phenol as compared to benzene?

Answer 18

Phenol doesn’t require concentration nitric acid and sulfuric acid. Only needs dilute nitric acid

Question 19

What forms when phenol reacts with bromine

Answer 19

White precipitate

Question 20

What are activating groups?

Answer 20

• Electron donating groups
• 2- and 4-directing groups

Question 21

What are the 2- and 4-directing groups? (ortho and para)

Answer 21

• NH₂
• OH
• Halogens (deactivating groups!)

Question 22

What are deactivating groups?

Answer 22

• Electron withdrawing groups
• 3-directing

Question 23

What are the 3-directing groups? (meta)

Answer 23

• CHO
• COOH
• NO₂