Benzyl Alcohol to Benzoic Acid and the Recrystallisation of Benzoic Acid

Appearance of phenylmethanol (benzyl alcohol) at room temperature

Colourless liquid

Why is potassium permanganate added?

Ensure the complete oxidation of phenylmethanol

Phenylmethanol + sodium carbonate + heat + potassium permanganate changes

Purple to brown
Forms sodium benzoate

Why is sodium carbonate added?

Speed up the reaction

Why was the reaction acidified with hydrochloric acid?

Neutralise sodium carbonate
Convert sodium benzoate to benzoic acid

Test the solution to make sure it's acidic enough

Blue litmus paper - turns red

What happens when hydrochloric acid and sodium sulphite is added and the solution is cooled?

Brown colour disappears and white crystals form
MnO2 is being reduced (Mn4+ --> Mn2+), crystallisation of benzoic acid occurs

Why was the flask placed in ice before filtration?

Ensure all benzoic acid crystallises out because it's less soluble in cold water, maximises yield

What causes the smell of almonds?

Benzaldehyde

Method to dry crystals

Place on radiator

Technique to purify benzoic acid crystals

Recrystallisation

Recrystallisation apparatus

Measuring the melting point of crystals

1. Place crystal sample on aluminium melting block
2. Heat slowly while observing crystals
3. Record temperature range at which melting occurs

Examples of impurities

MnO2, sodium sulphite, HCl

Melting point difference between pure and impure crystals

Melting point of pure crystals higher, closer to 122 degrees Celcius

Measure the melting point of benzoic acid