Curved Arrow Press Forum

An organic chemistry forum of Curved Arrow Press. (Oh, would you believe we accept donations.) Registration closed.

You are not logged in.

  • Index
  •  » Orgo Lab
  •  » How does charcoal remove high molecular weight impurities?

#1 2010-09-27 06:42:23

orgopete
Administrator

How does charcoal remove high molecular weight impurities?

Yahoo!Answers in chemistry wrote:

Organic chem, just need simple explanation as to why activated charcoal was added to a solution I was trying to purify while recrystallizing.

This is a 'like dissolves like' phenomena for liquid-solid extraction rather than a liquid-liquid extraction. Because weak forces are involved, we don't specify the interactions. We use the like dissolves like to explain why a compound is soluble in water or the organic layer.

Chromatography is a similar phenomena in which a liquid-solid interface is used. If silica gel is the stationary phase (TLC), the more polar the compound, the slower it will move. The interactions of a polar compound will be stronger on the polar silica gel. If we wish to elute a polar solid faster on a plate, we need to use a more polar solvent.

Reverse phase high performance liquid chromatography (HPLC) is the polar opposite of silica gel. A C18 (actually C18H37) backbone is chemically bonded to silica gel as the solid support. The elution characteristics are now the  opposite of TLC. A polar solvent (mostly water) is used so the analyte will remain attached to the non-polar C18 backbone. To elute a compound, the solvent polarity may be decreased (to mostly organic). Reversed phase HPLC is virtually the universal chromatography method as it used polar solvents so very polar materials do not become bonded to the stationary phase. Then the solvent polarity can be changed to elute all of the organic materials.

If we have a small amount of an impurity in our sample, we may remove it by either charcoal (non-polar) treatment or silica gel (polar) treatment. Charcoal is a non-polar stationary phase. It works best by absorbing organic compounds (and usually the least polar) from a polar solvent like methanol or ethanol. Similarly, if you have a polar impurity, it may be removed by a silica get treatment in a non-polar (organic) solvent like hexane or dichloromethane.

I liked to test my sample with TLC. If my sample had an impurity that moved faster than my product, then I knew silica gel would not remove that impurity, but charcoal in a polar solvent might. Similarly, if my sample had an impurity that remained at the baseline, then I knew that it would remain attached to silica gel. Adding a small amount of silica gel would remove the impurity from an organic solvent.

These two processes are usually combined into one during crystallization. If you are using a polar solvent like methanol, the least polar material will be less soluble and may crystallize first. If that is an impurity, you may see your solution become cloudy. That cloudiness is the impurity interfering with the crystallization. If it is cloudy, that means the insoluble material is amorphous or without a crystal structure. Because the solvent is polar, charcoal can absorb the least polar impurity. (It can actually also absorb polar impurities as well if they have polar functional groups attached to large organic backbones.) Because the solvent is polar, if you add too much charcoal, it can also absorb your organic compound as well.

Similarly, if you are recrystallizing your sample from non-polar organic solvents, such as hexane-ether or hexane-dichloromethane mixtures, you may see your solution become cloudy. That cloudiness is the impurity interfering with the crystallization. If it is cloudy, that means the insoluble material is amorphous.  Because the solvent is non-polar, silica gel can absorb a polar impurity. Adding a small amount of silica gel and filtering can remove the polar impurity. Because the solvent is non-polar, if you add too much silica gel, it can also absorb your organic compound as well.

From experience with different techniques, you can learn to judge whether you can purify your compounds with small amounts of charcoal or silica gel. It will depend on your initial sample purity, mp (intermolecular bonds), similarity of impurity to product, and sometimes patience. Even with ideal conditions, some samples refuse to crystallize quickly. Also, you may find the cloudiness return as your sample cools or you add a co-solvent. You may need to repeat a silica gel or charcoal treatment and filter again. I have had sample in which I have repeated these treatments four or five times. (Yes, I liked really nice and sharp melting crystals.)

PW

Offline

 
  • Index
  •  » Orgo Lab
  •  » How does charcoal remove high molecular weight impurities?

Board footer

Powered by PunBB 1.2.16
© Copyright 2002–2005 Rickard Andersson

//google analytics added