[synthon]

I can't find a straight answer, even in the archives, so why does charcoal remove colored impurities?  I've seen that charcoal like to absorb large organic molecules, compared to ions and small organics, but nothing directly relating to color.   Is this just because large compounds and polymers are typically colored while the small organics which are typically products are colorless?

Just curious.

[Borek]

Is this just because large compounds and polymers are typically colored while the small organics which are typically products are colorless?

Something like that. Colored molecules are usually large enough to be easily adsorbed and absorbed from the solution.

[fledarmus]

Also because among organic molecules, most of the colored molecules are those which have either long strands of conjugated double bonds, multiple aromatic groups, or combinations of both. If you make models of these compounds, you will find that they mesh very nicely with graphite sheets making excellent pi stacking interactions.

[synthon]

Very helpful, thanks!  The classification of colored compounds is very general and somewhat misleading.

[fledarmus]

Yes it is - this is a purely empirical method. If you get a yellow or brown solution, you throw in a little activated carbon and filter it off to see if it helps. If it works, great, if it doesn't, you don't bother with it the next time you run the reaction.

[orgopete]

I don't want anyone thinking we don't know what we are doing. There is method to our madness. If you were going to recrystallize a compound, the first thing you should do is run a TLC on it to estimate the kind of problem you maybe facing. If there was a very slow moving impurity and you were to try to recrystallize the sample from dichloromethan-hexane mixture, that slow moving impurity is likely more polar and less soluble. It may crystallize out first or it may precipitate as an amorphous solid and interfere with your recrystallization. If that were to happen, I would add a small amount of silica gel to absorb that impurity, filter, and crystallize the filtrate.

Alternately, I could do the recrystallization from a methanol-water mixture. In this case the polarities are reversed. A slow moving impurity is more likely to remain in solution. If there is an impurity that is precipitating before my crystals form, then I would treat it with charcoal, the polar opposite of the non-polar/silica gel example above. The material that precipitates first from methanol is likely more strongly absorbed by charcoal. The effectiveness of the charcoal treatment does depend upon its absorption, but it also depends upon the polarity of the solvent. One doesn't treat a non-polar solvent with charcoal nor does one treat a polar solvent with silica gel. Charcoal and silica gel use the same absorbent properties that make them effective in chromatography to remove impurities from samples.

[Babcock_Hall]

This is a bit of a tangent, but if one hydrolyzes protein and treats with charcoal, it removes the aromatic amino acids (Phe, Tyr, Trp) selectively.  This is useful for phenylketonurics, because then phenylalanine can be added back in a controlled amount.  The aromatic amino acids are essential, meaning that they must be in our diet, but sufferers of PKU cannot catabolize an excess amount of it properly.
http://www.biochemj.org/bj/053/0431/0530431.pdf

[discodermolide]

Just to give my two cents worth. Charcoal doesn't just like coloured compounds, it will also adsorb normal non-coloured substances as well.

[synthon]

Points to orgopete for a thorough explanation, very well said.