Chapter 25: Phenol

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

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

• 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

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

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

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

• 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

• 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

• 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

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

• Electrophilic substitution • Bromination and nitration

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

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

• Room temperature • No catalyst

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

• Room temperature • No catalyst

• Phenol is 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, and it therefore attracts and polarises electrophiles more strongly than in benzene

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

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

• 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)

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

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

• 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