The hydroxyl group (OH) in phenol ‘activates’ the benzene ring, making it more reactive with electrophiles.
Phenol is able to react directly with bromine (unlike benzene) in multiple substitution reacts to produce 2,4,6-tribromophenol.
Nitration of phenol is easier than nitration of benzene. Only nitric acid (HNO ) at room temperature is required (for nitration of benzene: nitric acid, sulfuric acid and temperature of 55ºC).
The OH group in phenol has a directing effect, meaning substitutions are most likely to occur on the second, fourth and sixth carbon (carbon one is occupied by the OH group).
Phenol can react directly with bromine, whereas benzene is unable to.
The hydroxide group is described as ‘activating’ the ring – because of the extra electron density it gives the ring, making the ring more reactive with electrophiles.
The increased electron density in the ring within phenol enables the ring to polarise the bromine molecule enough to form an electrophile.
Benzene does not have enough electron density in the ring for this to happen. This is why benzene needs a halogen carrier to react with bromine but phenol does not.
A lone pair of electrons on the oxygen atom in phenol is donated or ‘merged’ into the pi-bonding system in the benzene ring. The electron density inside the benzene ring increases a result, and this increased electron density enables phenol to react with electrophiles more easily than benzene does.
Phenol also undergoes multiple substitutions, forming 2,4,6-tribromophenol. Benzene only undergoes a single substitution, forming bromo-benzene.
Nitration of Phenol
Just like with benzene, phenol can react with a nitronium ion (NO⁺ ) and a nitrate group is added onto the carbon ring.
Just like with the bromination reactions, phenol is more reactive with electrophiles than benzene. So, the nitration of phenol happens more easily than the nitration of benzene. Room temperature and dilute nitric acid are the only reactants and conditions required (with the nitration of benzene, higher temperatures (55ºC) and concentrated sulfuric acid are also necessary).
Directing Effect of Phenol
The positions that the bromine substitutes to in phenol are as a result of electrons from the oxygen atom being moved around the ring. This detail is not needed for A-level chemistry, but it does explain why this happens.
If we draw out the Kekule structure of the ring in phenol it’s easy to see how the electrons from the negatively charged oxygen cause the positions of double bonds to move around the ring. We know this is not a good representation of the benzene ring, as there are technically no double bonds in benzene, but it is the easiest model to use to describe how the electrons move around the ring in this situation.
The phenol dissociates in solution and releases a H⁺ ion from the hydroxide group.
The electrons added to the ring from the oxygen atom repel the electrons in the double bond nearest the carbon attached to the oxygen. The repulsion moves the electrons in the bond to the next carbon, making it negatively charged.
This negatively charged carbon is more likely to bond with an electrophile than any other carbon atoms in the ring, this is why the electrophile bonds to the 2nd carbon. Making it 2-bromo-phenol in the case of bromination.
The negative charge does not just remain on the 2nd carbon though – it causes repulsion with the next closest double bond, forcing those electrons onto the second furthest carbon (4th carbon), making the 4th carbon negatively charged. This 4th carbon is now the most likely to react with an electrophile, so the next electrophilic substitution takes place on the 4th carbon.
This same process also occurs with the 6th carbon.
The movement of charge around a molecule may look very strange (and to A-level students I will repeat – you do not need to know this), but it is just a way to show how the charge can be spread around the molecule. The structures that result from the charge moving around the molecule are called resonance structures.