Oxidation of Phenylmethanol to Benzoic acid with KMnO4 sol. in alka

Key steps + observations in the procedure for the oxidation of Phenylmethanol to Benzoic acid with KMnO4 sol. in alkaline conditions

• place phenylmethanol, KMnO₄ and sodium carbonate into a conical flask

• heat in a water bath for some time

  • a brown precipitate forms

• cool the flask and add concentrated hydrochloric acid + use blue litmus paper to ensure acidic conditions

• add sodium sulftie to reduce the Mn⁴⁺ ioms to Mn²⁺

  • white crystals form

• place conical flask in ice to crash out the crystals

• filter off crystals using a Buchner flask and funnel

• use the filtrate to wash out the conical flask

• crystals are allowed to air dry overnight

What colour changes are noticed in this experiment?

Purple to brown precipitate to white crystals

Why is KMnO₄ used?

it is an oxidising agent

3 Reasons why conc. HCl is used?

• converts sodium benzoate intermediate to benzoic acid

• neutralises any excess sodium carbonate

• helps Mn⁴⁺ ions to reduce to Mn²⁺

Why is sodium sulfite added?

• is a reducing agent that allows Mn⁺⁴ ions to be reduced to Mn⁺² ions

  • these ions are soluble in water which ensures they do not contaminate the benzoic acid crystals

Why was the flask placed in ice?

benzoic acid has poor solubility in cold water allowing them to crash out

How were soluble impurities removed from the crystals?

they are washed with ice cold water

What else could be used to dry the crystals?

place them in a desicator