Extraction of caffeine from tea

Extraction of caffeine from tea

The introduction to the extraction of caffeine is provided in Chapter 7 Extraction of Macroscale and Microscale Experiments 5th edition by Williamson).¹  It is required reading!  Read the theory pertaining to extractions on pages 135 to 145, as well as the theory pertaining to the extraction of caffeine from tea on pages 162 and 163.

Liquid-liquid extraction ^2,3

An organic chemist in the laboratory must separate and purify mixtures of compounds.  Mixtures of compounds may be obtained when synthesizing a particular substance by a chemical reaction.  When the chemical reaction is complete, it is necessary to do a workup, that is, to separate and purify the desired product from the mixture of by-products and residual starting material.  Mixtures may also result when a natural product is isolated from its source.  For example, once eugenol (a dental analgesic and antiseptic) is isolated from cloves, it is necessary to separate and purify eugenol so that it may be characterized.

Liquid-liquid extraction is a technique that is commonly used to separate mixtures of compounds.  This technique uses (relies on, is based on) solubility differences and acid-base principles to carry out the separation.  In a liquid-liquid extraction, two insoluble solvents are used:

• Water or an aqueous solution such as 10 % NaOH or 10 % NaHCO₃ AND
• An organic solvent such as dichloromethane (CH₂Cl₂)_, diethyl ether or hexane.
  

Compounds are separated by their solubility differences in a aqueous and organic solvent. 

An item of glassware called a separatory funnel is used for the extraction.  When two insoluble liquids are added to the separatory funnel, two layers form, with the less dense liquid on top and the more dense liquid on the bottom.

Scenario 1

Suppose a mixture of cyclohexanol (C₆H₁₁OH ) and NaCl is added to a separatory funnel containing an organic solvent, CH₂Cl₂, and H₂O.  Following the predictive rule “like dissolves like,” cyclohexanol would dissolve in the organic CH₂Cl₂ layer, and NaCl would dissolve in the water layer.  Separating the organic and aqueous layers, and placing them into different flasks separates the cyclohexanol and NaCl from each other. 

Remember:  Water and organic solvents are two different kinds of solvents.  Water is very polar (in fact the most polar), and is capable of forming hydrogen bonding intermolecular interactions with a solute.  Many organic solvents are either nonpolar, like CCl4 and hexane, or weakly polar like dichloromethane (CH2Cl2) and diethyl ether (CH3CH2OCH2CH3).

Ionic compounds are held together by strong electrostatic forces, so only very polar solvents are able to dissolve them.  Most ionic compounds are soluble in water, but are insoluble in organic solvents.  To dissolve an ionic compound, the strong ion-ion interactions must be replaced by many weaker ion-dipole intermolecular interactions. 

Most organic compounds are soluble in organic solvents (remember like dissolves like).  An organic compound is water soluble ONLY if it contains approximately one polar functional group for approximately every 5 C atoms it contains.  In other words, a water-soluble organic compound has an O- or N-containing functional group that solubilises its nonpolar 5-carbon backbone. 

 Scenario 2

How could we separate a mixture of benzoic acid (C₆H₅COOH) and cyclohexanol?  Both compounds are organic, and as a result, both are soluble in an organic solvent such as CH₂Cl₂ and insoluble in water.  If a mixture of benzoic acid and cyclohexanol were added to a separatory funnel containing CH₂Cl₂ and water, both would dissolve in the organic CH₂Cl₂ layer, and the two compounds would not be separated from each other. 

Is it possible to use liquid-liquid extraction to separate two compounds of this sort that have similar solubility properties?  If a carboxylic acid is one of the compounds, the answer is YES.  Why?  Because we can use acid-base chemistry to change the solubility properties of the carboxylic acid.  This type of liquid-liquid extraction is, more specifically, called an acid-base liquid-liquid extraction, and involves carrying out simple acid-base reactions to separate compounds from each other. 

When benzoic acid (a stronger organic acid) is treated with aqueous NaOH, benzoic acid is deprotonated, forming sodium benzoate (see reaction 1 below).  Because sodium benzoate is ionic, it is soluble in water, but insoluble in organic solvents. 

Remember:  Acid-base reactions always favor the formation of the weaker acid and weaker base.

Reaction 1.  The reaction of benzoic acid with sodium hydroxide

Thus, a carboxylic acid can be separated from other organic compounds by converting it into a water-soluble carboxylate anion by an acid-base reaction. 

A similar acid-base reaction does not occur when cyclohexanol is treated with NaOH, because organic alcohols are much weaker organic acids, and, therefore, they can only be deprotonated by a very strong base such as NaOH (see reaction 2 below).  NaOH is not strong enough to form significant amounts of the sodium alkoxide.  

Reaction 2.  The reaction of cyclohexanol with sodium hydroxide

Diagram 1.  Separation of benzoic acid (strong acid) and cyclohexanol (weak acid) by a liquid-liquid extraction procedure with 10% NaOH and dichloromethane (CH2Cl2)

References

1. Williamson KL, Minard R, Masters KM. Chapter 7 Extraction. In: Macroscale and microscale: organic experiments. 5th edition. Boston: Houghton Mifflin; 2007. p. 135.
2. Gorzynski Smith J. Chapter 19.12 Extraction. In: Organic chemistry. 2nd ed. Boston: McGraw-Hill; 2008. p. 707
3. Schoffstall AM, Gaddis BA, Druelinger ML. Chapter 1 Technique I. In: Microscale and miniscale organic chemistry laboratory experiments. 2nd ed. New York: McGraw-Hill Higher Education; 2004. p. 72-83.