Re: What are the properties of the caffeine molecule?

Dear User,

Extraction is one of the oldest chemical procedures known to 
mankind. Extraction and concentration of pharmaceutically active substances 
, dyes, etc. date back to ancient Egypt and Mesopotamia and probably 
earlier than that. Your question touches a very broad area of 
basic chemistry, and I am afraid that I would only be able to provide a 
very basic set of concepts and hints - for full explaination you should 
refer to any decent textbook on basic chemistry.

Solubility of substances in solvents depends on how efficiently the solvent 
molecules break interactions between molecules of the solute (i.e. 
substance to be dissolved) and how efficiently the solvent molecules form 
new interactions with the solute molecules. Roughly, substances can be 
divided into two very general classes - polar and non-polar, depending on 
their dipole moments. Also, very roughly, substances may be divided into 
hydrophilic and hydrophobic i.e. "loving water" and "hating water". These 
sets overlap but are not identical. Solvents, of course, also fall into 
these categories. 
Polar substances readily form interactions with polar substances. These 
interactions are based chiefly on Coulomb - type forces, i.e. on 
interaction of partial charges which result from polarization of the 
molecules. 
Non-polar molecules, likewise, interact with non-polar ones, mostly via 
surface-surface type of interactions (van der Waals etc.). This is of 
course a very broad generalization, but it will suffice. 
As an example, it is easy to observe water droplets sliding around on 
teflon-covered pan, without even wetting the pan - due to the fact that 
water is highly polar whereas teflon is not. Oil, on the contrary, spreads 
nicely over teflonized surfaces due to the fact that both oil and teflon 
are hydrophobic, relatively non-polar substances.

Of course, molecules generally can be assigned to both classes, partially. 
And thus, caffeine is a polar molecule, which has a significant hydrophobic 
portion (look at the structure). In water, caffeine dissolves chiefly via 
formation of hydrogen bonds involving atoms with free electron pairs (N and 
O). In Ch2Cl2, these interactions give way to generalized hydrophobic 
interactions of the nonpolar dichloromethane with hydrophobic parts of the 
molecule, such as -CH3 etc. Likewise, caffeine readily dissolves in 
chloroform and several other organic solvents.

 http://www.scimedia.com/chem-ed/sep/extract/extract.htm
 http://www.uwec.edu/Academic/Curric/cheniepj/CAFFEIN2.htm

Solubility of caffeine in water is about 9 times less than in 
dichlorometane - thus when a solution of caffeine in water is brought in 
contact with dichloromethane, most of the caffeine will migrate into the 
organic layer (see partition theory in your textbook). In order to complete 
the extraction, fresh solvent is used twice (i leave it as an excercise for 
you to calculate the percentage of caffeine extracted from a given volume 
of solution using one, two, three etc. portions of organic solvent but it 
can be easily seen that two changes will extract most and three - 
practically all of the alkaloid).

Once again, the above is a very general and incomplete explaination. The 
fact that you feel "muddled" about this procedure may indicate incomplete 
understanding of the basic chemistry principles (polarity, solubility, 
intermolecular interactions) which may be vital for you to pass the 
necessary exams. Often, this condition may be remedied by reading 
appropriate literature - such as university-grade textbooks on general 
chemistry, which may be found in any decent library. Feel free to contact 
me if you need assistence.

Hope it helps.

A.G.E.