Learners can distil oil of wintergreen, and prepare salicylic acid, through the preparation of gaultheriae procunbers leaves

Many organic compounds are found in plants. 2-Hydroxybenzoic acid (salicylic acid) can be made from methyl 2-hydroxybenzoate which is obtained as oil of wintergreen by distillation from the leaves of Gaultheriae procunbers.

Oil of wintergreen is 98% methyl 2-hydroxybenzoate. This oil can be hydrolysed by boiling with aqueous sodium hydroxide for about 30 minutes. The reaction produces sodium 2-hydroxybenzoate which can be converted into 2-hydroxybenzoic acid by adding hydrochloric acid.

The preparation of 2-hydroxybenzoic acid - diagram - student sheet

Apparatus

  • Measuring cylinder, 10 cm3
  • Pear shaped flask fitted with a reflux condenser, 50 cm3
  • Anti-bumping granules
  • Water bath

NB the reaction can be heated directly over a low Bunsen burner flame, but care must be taken not to heat too strongly, causing bumping.

  • Beaker surrounded with ice and water in a larger beaker, 100 cm3
  • Stirring rod
  • Buchner flask and suction apparatus
  • Watch glass

Health, safety and technical notes

  • Read our standard health and safety guidance here.
  • Wear eye protection.
  • Sodium hydroxide can cause severe burns to the skin and is dangerous to the eyes, see CLEAPSS Hazcard HC091a.

Procedure

Stage 1

Set up apparatus suitable for heating about 30 cm3 of reaction mixture using a water bath. Use a condenser to prevent any volatile liquids escaping. Get your apparatus checked by your teacher before you start the reaction. Put on your eye protection. 

Put 2 g of oil of wintergreen into your flask and add 25 cm3 of 2 mol dm–3 sodium hydroxide (CARE!). Aqueous sodium hydroxide is particularly prone to bumping, so you will need some anti-bumping granules. Then heat over a boiling water bath for 30 minutes. 

While the mixture cools, make a list of the possible compounds present in the mixture.

Stage 2

Pour the mixture into a small beaker surrounded by a mixture of ice and water. Add concentrated hydrochloric acid (CARE!) to the mixture dropwise until it is just acidic, stirring all the time. Why do you need to keep the mixture cool during this process? 

Stage 3

Filter the product using a Büchner funnel and suction apparatus. Wash the product with a little ice-cold water and transfer it to a weighed watch glass. Allow to dry overnight. 

Questions

Students should include the answers to the following questions in their write-up.

  1. How can you tell from observing the process that a new substance has been formed in the reaction?
  2. What has happened to the methanol formed in the reaction?
  3. What is oil of wintergreen used for nowadays?
  4. What mass of product was formed from 2 g of oil of wintergreen?
  5. What percentage yield is this?

Answers

  1. There are two layers in the flask at the start of the preparation, but when the reaction is complete, the mixture is homogeneous.
  2. The methanol produced remains in the filtrate because it is completely miscible with water.
  3. Oil of wintergreen is 98% methyl salicylate, and it shows the medicinal properties of salicylates in general. It is not usually given by mouth but is readily absorbed by the skin. It is applied as a liniment for the relief of pain in lumbago, sciatica and rheumatic conditions, as well as for minor sports injuries.
  4. A good yield can be obtained in this experiment. Two grams produces 1.7 g of dry 2-hydroxybenzoic acid

Downloads

Download all

Additional information

These resources were compiled by David Lewis and edited by Colin Osborne and Maria Pack.

Aspirin book