Acidity of Phenol
The structure of phenol
- In phenol the OH group is directly bonded to the benzene ring
- Phenol can behave as a very weak acid
- Depending on the concentration, the typical pH of a solution of phenol will be 5-6
- A hydrogen ion can break away from the -OH group and transfer to a base
C6H5OH + H2O ⇌ C6H5O- + H3O+
- Phenol is able to lose a hydrogen ion because the phenoxide ion (C6H5O-) formed is fairly stable
- The more stable the ion is, the more likely it is to form
- One of the lone pairs of electrons on the oxygen is delocalised with the electron cloud of the whole phenoxide ion
- Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen
Delocalisation in the phenoxide ion
- Oxygen is the most electronegative element in the ion and the delocalised electrons will be drawn towards it
- That means that there will still be a lot of charge around the oxygen which will tend to attract the hydrogen ion back again
- This is why phenol behaves as just a weak acid
- This behaviour is demonstrated by the reactions of phenol with sodium hydroxide and sodium metal
- Phenols dissolve in alkaline solutions and undergo acid-base reactions with bases to form a soluble salt and water
Reaction of phenol with sodium NaOH
- Phenols react vigorously with reactive metals such as sodium (Na)
- A soluble salt is formed and hydrogen gas is given off
Reaction of phenol with Na