Oxidise phenylmethanol to benzoic acid using KMnO4 in alkaline conditions

Draw a diagram of this experiment

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Step 1

Add phenylmethanol into a conical flask

Step 2

Add saturated potassium permanganate and sodium carbonate

Step 3

Clamp conical flask in a beaker of boiling water for 20 minutes

Step 4

Cool solution by running cold water from the tap over the outside of the conical flask

Step 5

Add hydrochloric acid to help precipitate the benzoic acid crystals in the fume cupboard

Step 6

Dip glass rod into conical flask and touch it against blue litmus paper (Non-acidic)

Step 7

Add sodium sulfite solution until brown precipitate dissapears

Step 8

Stir flask until brown specks of unreacted manganese dioxide disappear

Step 9

Place conical flask in ice

Step 10

White crystals form

Step 11

Filter off crystals using a Buchner flask + filter paper

Step 12

Transfer filtered liquid into conical flask and rinse benzoic crystals

Step 13

Filter solution and rewash crystals

Step 14

Place damp filter paper containing crystals on a clock glass + allow crystals to dry overnight

Step 15

Use electronic balance to record mass of benzoic acid crystals formed

Step 16

Calcculate % yield of benzoic acid

Why should you stir the flask to remove the unreacted specks of manganese dioxide

Contaminate the benzoic acid crystals

What is appearance of phenylmethanol?

Colourless liquid

Why is sodium carbonate added to the reaction mixture?

Ensures an alkaline mixture as this oxidation works best at alkaline pH

Why is a concentrated/saturated solution of potassium permanganate used?

Ensures there is enough potassium permanganate to oxidise all the phenylmethanol to benzoic acid

Why is concentrated hydrochloric acid added to the reaction? (3)

Convert sodium benzoate to benzoic acid. Neutralise excess sodium carbonate and produce potassium hydroxide. Acidic conditions help reduce Mn4+ to Mn2+

Why was it placed in a fume cupboard when concentrated HCL was added?

Concentrated HCL is volatile and toxic if inhaled

Why was sodium sulfite solution added to the reaction?

To reduce insoluble Mn4+ ions to soluble Mn2+ which are filtered out of the benzoic acid crystals

What is observed when sodium sulfite is added to the conical flask? (2)

Brown manganese dioxide precipitate disappears. White crystals of benzoic acid become visible

Suggest a reason as to why a student may have difficutly getting crystals and smelling almonds. How to fix this?

Phenylmethanol hasn't been fully oxidised to benzoic acid as some remained as benzaldehyde. Use excess KMnO4

Why are the crystals washed?

Removes any soluble impurities on the crystals or filter paper

Why does the water used to rinse the crystals have to be ice cold?

Benzoic acid is soluble in warm water