Abstract

In this lab, the primary objective was to determine the yield and percent of caffeine extracted from tea leaves. The experiment employed the solvent extraction method in which two bags of tea leaves were heated in hot boiling water for a few minutes to obtain aqueous tea solution. Sodium carbonate was then added to the mixture to increase the solubility of acidic substances, such as tannin, present in tea leaves. Next, caffeine was isolated from the solution through a subsequent extraction of the mixture with dichloromethane. After the isolation, the organic solvent was removed through evaporation at a boiling point of 40°C to get crude caffeine. The amount of caffeine obtainedfrom the experiment was 0.0145g while the percent yield was 0.322%. However, the caffeine yield was lower compared to the expected range of 0.05-0.07g. This significant difference might have been due to the occurrence of errors such as inappropriate calibration of the measurement scales.

Introduction

Mechanism

Caffeine is one of the essential alkaloids that naturally occurs in plants. This nitrogenous organic compound is often found in a variety of products such as coffee, tea, and kola nuts. The alkaloid has the formula, C₈H₁₀N₄O₂. In this experiment, the solvent extraction approach was used to extract caffeine from tea leaves.

Background of the Experiment

The Liquid-Liquid Extraction (LLE), or Solvent Extraction, is the technique of separating compounds based on their relative solubility in two liquids that are immiscible.  For example, although caffeine dissolves in hot water, it is more soluble in organic solvents such as dichloromethane (Lab Manual).Therefore, it is possible to extract caffeine from a sample of tea solution using dichloromethane.

Significance

Tea leaves contain many substances. These include acidic tannins, caffeine, cellulose, undecomposed chlorophyll, and pigments. However, in this laboratory exercise, caffeine was of more interests than the other substances because it is physiologically active in humans. Additionally, caffeine is a stimulant of the central nervous system, and therefore, affects the health of thousands of individuals who consume tea (Williamson & Masters, 2010). Thus, it is necessary to analyze and understand the caffeine amount of caffeine present in tea leaves.

Procedure

In order to extract caffeine from tea, two bags of tea-leaves each 2.25g were placed in a 100-mLbeaker containing 50-mL of hot boiling water. The tea was allowed to steep for approximately 10 minutes after which the pockets were squeezed to press the liquid out. The resulting aqueous solution was transferred to a separating funnel, and 0.5g of solid sodium carbonate added. Once the solid was dissolved, the mixture was cooled to room temperature before adding 20-mL of dichloromethane. The final solution was allowed to separate, forming two distinct layers with water (density=1 gm/mL) floating on dichloromethane (1.3g/mL).

In order to isolate the mixture, the organic layer was drained from the funnel into a 125-mL Erlenmeyer. The second 20-mL portion of dichloromethane was used to extract the aqueous layer. Next, the organic layer formed was transferred to the 125-mL Erlenmeyer, and a small amount of anhydrous sodium sulfate added to obtain a less cloudy solution as shown in Figure 1 (b).

Figure 1: (a) Separation of the layers (b) Drying of the organic layer(Lab Manual).

The combined dichloromethane was transferred to a pre-weighted round-bottomed flask. Finally, a rotary evaporator was used to evaporate the excess dichloromethane. The evaporation resulted in the formation of a coat of crude caffeine on the bottom of the flask.

Results and Discussion

Results

1. Yield
2. %Caffeine

• Experimental Error in the Yield:

The Infrared of the spectrum of caffeine shows two absorption signals at 1600-1700. The signals designate the two carbonyl groups in caffeine molecule. The NMR of the compound, on the other hand, shows three large signals in the spectrum, which indicate the hydrogen ions on the methyl groups.

Discussion

Caffeine is essentially a purine with three functional groups including an alkene, amide, and an amine.  However, the fundamental properties of caffeine originate from the lone pair of electrons, which exists around the nitrogen (Williamson& Masters, 2010). Accordingly, the structure of caffeine influences its functions.

Figure 2: The structure of Caffeine (Lab Manual).

Besides caffeine, a solution of tea leaves has acidic tannin and cellulose within it. Although the presence of the latter does not affect the extraction process as it is relatively insoluble in water, both tannin and caffeine readily dissolve in hot water. Thus, to extract caffeine from the mixture, the solubility of the acidic substances is increased by treating the water extract with a base (sodium carbonate).

In the subsequent extraction, an organic compound is used to extract caffeine from the aqueous solution. Dichloromethane is preferred for the purpose because it readily dissolves caffeine at room temperature(Williamson&Masters, 2010). Additionally, the compound is ideal for the isolation of caffeine from tea solution because it does not react with the solute. Furthermore, dichloromethane is immiscible with water due to their differences in densities (Williamson& Masters, 2010). Thus, these properties allow caffeine to be isolated from the aqueous mixture. On the other hand, the anhydrous sodium sulfate is used as a drying agent to remove excess water. A near-pure sample of caffeine can, therefore, be obtained by evaporating dichloromethane at room temperature.

In the experiment, 0.0145g of caffeine was obtained from 4.5g of tea leaves. Thus, the calculated percent yield from the sample was 0.322 %.  The caffeine yield was lower compared to the expected range of 0.05-0.07g. The difference might have been due to the loss of a substantial amount of the product throughout the procedure because of the failure to wash the organic solvent. Moreover, a 100% caffeine yield was impossible due to the incomplete reactions of chemicals during the experiment. Additionally, the occurrence as a result of improper calibration of the measurement scales might have contributed to inaccurate results. The overall percent error was approximated to be 75.83%.

Conclusion

In this experiment, the primary objective was to determine the yield and percent of caffeine extracted from tea leaves by the solvent extraction method. 0.0145g of caffeine was recovered from 4.5g of tea leaves. The percent yield was 0.322% while the experimental error was found to be 75.83%. The error can be reduced by washing dichloromethane and conducting several trials to obtain the average yield.

References

Lab Manual: The extraction of caffeine from tea leaves and coffee.

Williamson, K. L., & Masters, K. M. (2010). Macroscale and microscale organic experiments. Cengage Learning.

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