Objective


1. 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.





Fig. 1: Chemical structure of caffeine



2. Why Do We Extract Caffeine?

What is the difference between Panadol and Panadaol extra?



3. OBJECTIVE(S)

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

techniques:



1) Büchner filtration

2) Solvent extraction

3) D istillation using a rotary evaporator



4. CONCEPTS BEHIND THE TECHNIQUES

4.1 Solvent Extraction



A useful rule of thumb for judging solubility is “like dissolves like”. O rganic compounds tend

to be soluble in organic solvents. Polar materials, such as salts tend to be more soluble in

polar solvents, such as water.

Some organic compounds are exceptions and are more

soluble in water than in organic solvents. T his is because

they have a large number of polar functional groups, such

as hydroxyl groups, relative to the amount of hydrocarbon.

Glucose is an example of this kind of molecule.




In general, an organic compound and an inorganic salt can be easily separated using these

solubility differences. If a mixture of 1,4-dimethoxybenzene and lithium chloride is dissolved

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in a mixture of diethyl ether and water, both will dissolve. T he mixture will separate into two

clear layers. One will be the “organic layer” and it will contain the 1,4-dimethoxybenzene.

T he other will be the aqueous layer, and it will contain the lithium chloride. If the organic

layer is separated from the aqueous, dried to remove traces of water and then evaporated,

pure 1,4-dimethoxybenzene will be obtained.


How can we tell which phase is aqueous and which is organic?

H int: density. Think about your solutions and find out what your two layers are.


There are some other tricks that you need to know about. Usually, we do not extract once,

but twice or even more times. This is to ensure that all traces of the phase are removed.

Look out for this trick in the experimental procedure. Also, organic extracts must be dried

after being separated from the aqueous layer. T his is because organic solvents will contain

small amounts of dissolved water. T his is usually done by using a “drying agent” such as

anhydrous magnesium sulfate or anhydrous sodium sulfate. T hese absorb the water from

the organic solution

Students often put a lot of effort into extractions by shaking the separatory funnel vigorously

to mix the two layers. T his can be a bad idea. Such vigorous mixing, can result in the

formation of an emulsion – an intimate and inseparable mixture of the two layers. Emulsions

can be broken – but not easily! Milk is an everyday example of an emulsion. Swirling the

separatory funnel is a better technique



4.2 Rotary Evaporator

T his equipment is used to evaporate the solvent that your chemical compound of interest is

solubilized in. It is composed of hot water bath (where you can adjust the temperature) and

also a rotating glass tube (where you will connect your round bottom flask) connected in turn

to a condenser and to vacuum. A teaching assistant will show you and explain the

installation. Why do we rotate the flask while it is heating in water? Why do we need a

condenser? And last but not least what is the benefit to work under vacuum (think about the

relationship between pressure and temperature)?



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5. PROCEDURES

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.

1) Add about 25 g of dry tea bags, 10 g of calcium carbonate powder in 250 ml of hot water

in a 500 ml beaker. Use a H ot plate to heat the mixture until it is gently boiling. Continue

to gently boil for about 15 min, occasionally stirring the mixture with a glass rod.

2) N ext we need to separate any solid material from our mixture, so while the solution is

still hot, vacuum filter it into a CLEAN 250 ml Büchner flask. Place an unfolded filter

paper into a Büchner funnel. T he 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





Fig. 2: Büchner 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.



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3) Making sure that the tap is CLOSED , add the filtrate and 50 ml of dichloromethane to a

CLEAN 500 ml separating funnel (see Figure 3).





Fig. 4: Venting a separating funnel



Fig. 3: Separating funnel in use



4) Stopper the separating funnel and gently invert it. D O N O T SH AKE T H E SEPAR AT IN G

FUN N EL.

5) Vent the separating funnel (Figure 4) by opening the tap to release any built-up

pressure.

6) Close the tap, shake the mixture once and vent the separating funnel.

7) Close the tap, shake the mixture twice and vent the separating funnel.

8) R epeat (increasing the amount of shaking) until no more vapour is expelled.

9) Close the tap, place the separating funnel in the ring and take out the stopper.



T he organic (lower layer) and aqueous layers should separate after standing for several

minutes although some emulsion will be present in the organic layer. T he 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:

10) Place a small piece of cotton wool in the neck of a filter funnel and add a 1 cm layer of

anhydrous magnesium sulphate on top of the cotton wool.

11) 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 1-2 ml of fresh dichloromethane.

12) Repeat the extraction of the filtrate with another 50 ml portion of dichloromethane added

to the aqueous layer remaining in the separating funnel and repeat the drying as

described above with a FRESH portion 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, AFT ER CONSULTING A DEMONTRATOR, distil off the

dichloromethane on a rotary evaporator. T his will leave you with a solid which is caffeine.

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(SAFETY DATA

Dichloromethane HARMFUL Wear protective gloves.

RESIDUES T O CH LO R IN AT ED WASTE SOLVENTS.)



6. QUESTIONS

N ow you have extracted caffeine what techniques can you use to characterize your

compound and make sure you have caffeine?

H ow would you prepare decaffeinated coffee that is fit to drink?



7. APPENDIX

Safety And Regulations

Students and ALL accompanying staff will be expected to follow the division of CBC's rules

for visiting school scientists on health and safety in the laboratories as well as the specific

health and safety requirements of the experiments.

The lead teacher is asked to notify all students in advance of the visit.

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