25.3 The Chemistry of Phenol & 25.4 Directing Groups Flashcards
What are phenols?
- A group characterised by having an OH group bonded to an aromatic ring
What is phenol?
- C6H5OH (benzene with an OH group bonded to it)
How can you tell if aromatic compounds with an OH group are alcohols or phenols?
- 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
How does the solubility of phenol compare to other alcohols, and why?
- It is less soluble than them because the benzene ring is non-polar
What happens when phenol dissolves in water? What does this mean?
- It partially dissociates to form a phenoxide ion and a proton (which is a reversible reaction)
- This makes phenol a weak acid
How does the acidity of phenols compare to other groups? In what 2 ways can this be proven?
- Phenols are more acidic than alcohols, but less acidic than carboxylic acids
- By comparing Ka values
- By looking at reactions with bases
What do Ka values show, and how does this prove phenols’ relative acidity when compared to alcohols and carboxylic acids?
- 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
How can the relative acidity of phenols be measured using bases?
- 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
Why is the reaction of organic acids with sodium carbonate useful?
- 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
What happens when phenol reacts with sodium hydroxide?
- Neutralisation
- Sodium phenoxide (salt) and water are formed
Which mechanism does phenol undergo? Give 2 examples.
- Electrophilic substitution
- Bromination and nitration
What is the overall equation for the bromination of phenol? Name the main product.
- Phenol + bromine water -> 2,4,6-tribromophenol + hydrogen bromide
- C6H5OH + 3Br2 -> 2,4,6-tribromophenol + 3HBr
What can be observed during bromination of phenol?
- Bromine water decolourises
- A white precipitate is formed (2,4,6-tribromophenol)
What are the conditions for the bromination of phenol?
- Room temperature
- No catalyst
What is the overall equation for the nitration of phenol?
- Phenol + dilute nitric acid -> 2-nitrophenol + water
- Phenol + dilute nitric acid -> 4-nitrophenol + water
- A mixture of 2-nitrophenol and 4-nitrophenol is formed
What are the conditions for the nitration of phenol?
- Room temperature
- No catalyst
Why are the reaction conditions for phenol and benzene different?
- Phenol is more reactive than benzene
Why is phenol more reactive than benzene?
- 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
How is the position of substitution on aromatic rings determined?
- It depends on whether the functional groups ALREADY bonded to the benzene ring are electron donating or electron withdrawing
What are electron donating functional groups? Using 2 examples, explain why they are electron donating.
- Functional groups that can donate electrons into the delocalised benzene ring
- NH2 and OH are both electron donating as they have a lone pair
What are 2 effects electron donating groups have?
- 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)
In reactions where substituents are substituted at the 2- and 4- positions, which will be the major product, and why?
- The product where the substitution has taken place on the second carbon, as there are 2 ‘second carbons’
What are electron withdrawing functional groups? Using an example, explain why it is electron withdrawing.
- Functional groups that withdraw electrons from the delocalised ring
- NO2; the N has a positive dipole
What are 2 effects that electron withdrawing groups have?
- 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
What else can 2-, 3- and 4-directing groups be called?
- ortho-directing
- meta-directing
- para-directing
