The technique used to separate an organic compound from a mixture of compounds is called Extraction. Extraction process selectively dissolves one or more of the mixture compounds into an appropriate solvent. The solution of these dissolved compounds is referred as the Extract. Caffeine is a naturally occurring alkaloid (a class of naturally occurring compounds containing nitrogen and having the properties of an organic amine base) found in coffee, tea, Cocoa, Guarana (Paullinia cupana) and kola nuts.
Caffeine belongs to a family of naturally occurring compounds known as xanthines. Caffeine is chemically 1,3,7-trimethylxanthine (C8H10N4O2). Caffeine acts as a stimulant. It stimulates the heart, respiration, the central nervous system, and is a vasodilator (relaxes the blood vessels) as well as a diuretic (increases urination). It is a beverage produced by steeping in freshly boiled water the young leaves and leaf buds of the tea plant, Camellia sinensis. Two principal varieties of tea are used, the small-leaved China plant (C. sinensis) and the large-leaved Assam plant (C. assamica). The leaves may be fermented or left unfermented. Fermented teas are referred to as black tea, unfermented teas as green tea, and partially fermented teas as oolong tea. Tea leaves consist mostly of cellulose (a water-insoluble polymer of glucose), caffeine, tannins (phenolic compounds, compounds that have an -OH directly bonded to an aromatic ring) and a small amount of chlorophyll. The solubility of caffeine in water is 22 mg/ml at 25·C, 180 mg/ml at 80·C, and 670 mg/ml at 100·C. Here the organic solvent dichloromethane is used to extract caffeine from aqueous extract of tea leaves because caffeine is more soluble in dichloromethane (140 mg/ml) than it is in water (22 mg/ml). However, the tannins that are slightly soluble in the dichloromethane can be eliminated by converting it to their salts (phenolic anions by adding sodium carbonate) (tannins are phenolic compounds of high molecular weight and being acidic in nature can be converted to salts by deprotonation of the -OH group) which remain in the water.
Place 30 g of the tea leaves in a 500 ml beaker. Add 250 mL of distilled water and 5 g of sodium carbonate and stir the contents of the beaker with a glass rod. Boil the contents of the beaker on a hot plate/water bath for 10 minutes. Cool the tea solution to room temperature using an ice-water bath. Filter the cooled solution using glass wool or muslin cloth or cheese cloth. Transfer the filtrate into a separating funnel and extract 3-4 times (4 x 10-25 mL) using dichloromethane. Take care to avoid formation of emulsion for each extraction. Don’t shake the separating funnel vigorously, but gently swirl the two immiscible layers for 5 minutes. After each extraction, remove the lower organic layer into 250 mL beaker, leaving any emulsion layer behind. Add anhydrous sodium sulphate to the combined extracts. The sodium sulfate will remove any water and water soluble salts that are retained in the dichloromethane (organic layer) or accidentally transferred during decantation. Filter to remove the solid sodium sulphate and transfer the dry solution to a pre-weighed 250 mL beaker. Evaporate it to dryness by boiling it on a water bath. (It can also be evaporated under vacuum or by blowing dry air or nitrogen gas on the surface of the liquid). The residue will be crude caffeine (usually slight green in color) (determine its weight-the yield will be 0.5 g approx for 25 g of tea leaves). Purify the crude caffeine either by sublimation or by recrystallization using methanol. Caffeine is reported to sublime at 170°C. Pure caffeine appears as white glistering needles. It is bitter in taste. Determine the melting point using a sealed capillary. The melting point of caffeine is 238°C.
Add few crystals of caffeine with 3-4 drops of conc. nitric acid in a porcelain dish and evaporate to dryness. Addition of 2 drops of ammonium hydroxide to the residue gives purple color.
To assess the purity of caffeine extracted perform TLC on both for your caffeine sample and an authentic sample of caffeine. Add a few drops of ethanol to caffeine on your watch glass. This will not dissolve all of it, but enough will dissolve to allow you to do TLC on the solution. Spot the TLC plate with the isolated caffeine and with authentic caffeine from a solution of caffeine in ethanol. Label both the sample and authentic sample on your plate. Develop the chromatogram using a mixture of ethyl acetate (95%) and acetic acid (5%) as mobile phase. After the development air dry the plate and examine the plate under UV light to observe the spots. Outline the spots with a pencil and confirm the purity of caffeine from Rf value (It is the ratio of distance traveled by sample spot from the origin to the solvent front from the origin) of both the spots.
Dr. A. Rajasekaran, Additional Professor,
School of Pharmacy, Faculty of Medicine and Health Sciences,
Asian Institute of Medicine Science and Technology, Sungai Petani, Kedah, Malaysia
Phone: 006-04-4422884 Fax: 006-04-4422881