25.3 The Chemistry of Phenol & 25.4 Directing Groups

Question 1

What are phenols?

Answer 1

• A group characterised by having an OH group bonded to an aromatic ring

Question 2

What is phenol?

Answer 2

• C6H5OH (benzene with an OH group bonded to it)

Question 3

How can you tell if aromatic compounds with an OH group are alcohols or phenols?

Answer 3

• If the OH group is bonded to a side group, it’s an alcohol
• If it’s directly bonded to the aromatic ring, it’s a phenol

Question 4

How does the solubility of phenol compare to other alcohols, and why?

Answer 4

• It is less soluble than them because the benzene ring is non-polar

Question 5

What happens when phenol dissolves in water? What does this mean?

Answer 5

• It partially dissociates to form a phenoxide ion and a proton (which is a reversible reaction)
• This makes phenol a weak acid

Question 6

How does the acidity of phenols compare to other groups? In what 2 ways can this be proven?

Answer 6

• Phenols are more acidic than alcohols, but less acidic than carboxylic acids
• By comparing Ka values
• By looking at reactions with bases

Question 7

What do Ka values show, and how does this prove phenols’ relative acidity when compared to alcohols and carboxylic acids?

Answer 7

• Ka is the acid dissociation constant, which shows the extent of dissociation of a weak acid
• The higher the Ka value of a substance, the more acidic it is
• Phenols have a higher Ka value than alcohols, but not carboxylic acids

Question 8

How can the relative acidity of phenols be measured using bases?

Answer 8

• Stronger acids can react with both weak and strong bases
• Weaker acids can only react with strong bases
• Very weak acids cannot react with bases at all
• Using sodium carbonate as a weak base and sodium hydroxide as a strong base, alcohols react with neither, carboxylic acids are strong enough to react with both, but phenols only react with sodium hydroxide

Question 9

Why is the reaction of organic acids with sodium carbonate useful?

Answer 9

• It can be used to distinguish between carboxylic acids and phenols, as carboxylic acids would react with sodium carbonate and release carbon dioxide- you would be able to observe bubbles

Question 10

What happens when phenol reacts with sodium hydroxide?

Answer 10

• Neutralisation
• Sodium phenoxide (salt) and water are formed

Question 11

Which mechanism does phenol undergo? Give 2 examples.

Answer 11

• Electrophilic substitution
• Bromination and nitration

Question 12

What is the overall equation for the bromination of phenol? Name the main product.

Answer 12

• Phenol + bromine water -> 2,4,6-tribromophenol + hydrogen bromide
• C6H5OH + 3Br2 -> 2,4,6-tribromophenol + 3HBr

Question 13

What can be observed during bromination of phenol?

Answer 13

• Bromine water decolourises
• A white precipitate is formed (2,4,6-tribromophenol)

Question 14

What are the conditions for the bromination of phenol?

Answer 14

• Room temperature
• No catalyst

Question 15

What is the overall equation for the nitration of phenol?

Answer 15

• Phenol + dilute nitric acid -> 2-nitrophenol + water
• Phenol + dilute nitric acid -> 4-nitrophenol + water
• A mixture of 2-nitrophenol and 4-nitrophenol is formed

Question 16

What are the conditions for the nitration of phenol?

Answer 16

• Room temperature
• No catalyst

Question 17

Why are the reaction conditions for phenol and benzene different?

Answer 17

• Phenol is more reactive than benzene

Question 18

Why is phenol more reactive than benzene?

Answer 18

• The lone pair of electrons from the p-orbital of the oxygen atom is partially delocalised into the pi-system/ ring
• The electron density of the delocalised ring is therefore increased
• This makes the ring more susceptible to electrophilic attack, which attracts and polarises electrophiles more strongly than in benzene

Question 19

How is the position of substitution on aromatic rings determined?

Answer 19

• It depends on whether the functional groups ALREADY bonded to the benzene ring are electron donating or electron withdrawing

Question 20

What are electron donating functional groups? Using 2 examples, explain why they are electron donating.

Answer 20

• Functional groups that can donate electrons into the delocalised benzene ring
• NH2 and OH are both electron donating as they have a lone pair

Question 21

What are 2 effects electron donating groups have?

Answer 21

• They increase the electron density in the benzene ring, which allows it to react more readily with electrophiles (therefore react like phenol)
• They cause groups in further reactions to be substituted at the 2- and 4- positions (they are 2- and 4-directing groups)

Question 22

In reactions where substituents are substituted at the 2- and 4- positions, which will be the major product, and why?

Answer 22

• The product where the substitution has taken place on the second carbon, as there are 2 ‘second carbons’

Question 23

What are electron withdrawing functional groups? Using an example, explain why it is electron withdrawing.

Answer 23

• Functional groups that withdraw electrons from the delocalised ring
• NO2; the N has a positive dipole

Question 24

What are 2 effects that electron withdrawing groups have?

Answer 24

• They decrease the electron density in the benzene ring, which causes the molecule to react less readily with electrophiles (causing a need for conditions similar to those benzene needs)
• They cause groups in further reactions to be substituted at the 3- position

Question 25

What else can 2-, 3- and 4-directing groups be called?

Answer 25

• ortho-directing
• meta-directing
• para-directing