TBR: Caffeine Extraction

[justadream]

TBR Ochem Book II page 274 #23



In the extraction procedure, TBR uses water and methylene chloride. I understand that caffeine mostly dissolves in the methylene chloride.



Here is the beginning of the procedure

“Initially, 2.0 g of tea leaves are added to 10 mL of boiling water for 20 mins. The aq solution is then extracted 3 times with exactly 5.0 mL of methylene chloride.”



So in the first step, TBR puts the tea leaves in hot water. From this, TBR suddenly concludes that caffeine can partially dissolve in water? How does TBR make that conclusion?



In addition, what’s the purpose of dissolving the caffeine in hot water anyways? I don’t understand why you would do that when you are using the solubility difference of caffeine in methylene chloride v. water (not the solubility difference of caffeine in hot water v. cold water)?

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[NextStepTutor_1]

Looking at the structure of caffeine in the passage, you can see that it has a couple dipole moments (mostly caused by oxygen) allowing it to undergo hydrogen bonding as a hydrogen bond receiver, but the bulk of caffeine only allows for minimal water solubility at best. This lack of water solubility is hinted at in the last few sentences that state that caffeine can be extracted from an organic solvent (hydrophobic layer) with the use of an acid (protonates caffeine creating a positive charge and thus increases it's solubility in water).

So why is water used?

Most likely because the experimenter wanted to break down the tea leaves so that the maximal amount of caffeine is available in the solution. This will allow for maximal extraction of product. Water will break down anything with enough time - and time is the reason the water is hot. Heat increases the reaction rate - think about marshmallows in a hot chocolate drink as opposed to a room-temp chocolate drink. The marshmallows dissolve much, much quicker in the hot chocolate drink, though while still eventually dissolving in the room-temp chocolate drink.