THE ISOLATION OF CAFFEINE FROM TEA

Introduction

Caffeine (C8H10N4O2 – chemical structure given in Figure 1) is an example of a type of organic weak base called an alkaloid – alkaloid means ‘alkali-like’. Caffeine is a stimulant and is found naturally in tea, coffee and kola nuts. Other examples of alkaloids include cocaine (a drug of abuse), strychnine (a poison), morphine (a pain killer), piperine (found in black pepper) and quinine (used to treat malaria and added to tonic!). Clearly some alkaloids are beneficial and others extremely harmful.

Figure 1Chemical structure of caffeine

In this experiment you will extract caffeine from tea. You will encounter the following techniques:

  1. Büchner filtration
  2. Solvent extraction
  3. Distillation using a rotary evaporator

Experimental Procedure

At room temperature caffeine is only sparingly soluble but at high temperature (100C) it is very soluble, so by heating the tea bag in water we dissolve the caffeine.

  • Place about 25g of dry tea bags, 10g of calcium carbonate powder, and 250ml of hot water in a 500ml beaker. Use a Hot plate to heat the mixture until it is gently boiling. Continue to gently boil for about 15min, occasionally stirring the mixture with a glass rod.
  • Next we need to separate any solid material from our mixture, so while the solution is still hot, vacuum filter it into a CLEAN 250ml Büchner flask. Place an unfolded filter paper into a Büchner funnel. The paper should be just the right size for the funnel. Place the funnel through a rubber seal into a CLAMPED Büchner flask. Clamping is essential with Büchner filtration because the apparatus is very 'top-heavy' even before you add liquid to the funnel.

Apply suction and dampen the filter paper with the solvent you will be using before pouring your suspension onto the filter paper (it is often beneficial to swirl your mixture before pouring to prevent the filter paper becoming clogged with very small particles). You can rinse your solid with a little fresh solvent to wash off residual solution

Figure 2Büchner filtration

  • Keep the filtrate (the filtered liquid) and dispose of the remaining tea bags into the container provided. Cool the filtrate to room temperature.

Solvent extraction is used extensively in synthetic chemistry as a way of purifying reaction mixtures. It is based on the different solubility of compounds in different types of solvent. At room temperature, caffeine is more soluble in dichloromethane than water.

  • Making sure that the tap is CLOSED, add the filtrate and 50ml of dichloromethane to a CLEAN 500ml separating funnel (see Figure 3).

Figure 3Figure 4Venting a separating funnel.

Separating funnel in use

  • Stopper the separating funnel and gently invert it. DO NOT SHAKE THE SEPARATING FUNNEL.
  • Vent the separating funnel (Figure 4) by opening the tap to release any built-up pressure.
  • Close the tap, shake the mixture once and vent the separating funnel.
  • Close the tap, shake the mixture twice and vent the separating funnel.
  • Repeat (increasing the amount of shaking) until no more vapour is expelled.
  • Close the tap, place the separating funnel in the ring and take out the stopper.

The organic (lower layer) and aqueous layers should separate after standing for several minutes although some emulsion will be present in the organic layer. The emulsion can be broken and the organic layer ‘dried’ (to remove any water) simultaneously by passing the lower organic layer slowly through anhydrous magnesium sulphate as follows:

  • Place a small piece of cotton wool in the neck of a filter funnel and add a 1cm layer of anhydrous magnesium sulphate on top of the cotton wool.
  • Carefully open the tap and run off the organic layer directly from the separating funnel into the drying agent and collect the filtrate in a CLEAN, DRY flask. Rinse the magnesium sulphate with 1-2ml of fresh dichloromethane.
  • Repeat the extraction of the filtrate with another 50ml portion of dichloromethane added to the aqueous layer remaining in the separating funnel and repeat the drying as described above with a FRESHportion of anhydrous magnesium sulphate. Collect the organic layer in the flask containing the first dichloromethane extract.

The extracts should now be clear, showing no visible signs of water contamination. If some water should pass through the filter, repeat the drying as described above with a fresh portion of drying agent and collect the extracts in a dry flask.

The organic layers contain dissolved caffeine. Pour the dry organic extracts into a round-bottomed flask and, AFTERCONSULTING A DEMONSTRATOR, distil off the dichloromethane on a rotary evaporator. This will leave you with a solid which is caffeine.

SAFETY DATA

Dichloromethane / HARMFUL / Wear protective gloves. RESIDUES TO CHLORINATED WASTE SOLVENTS.