Guide to Separation Schemes

Step 1: Ask yourself which compounds are soluble in water at neutral pH. To determine this follow the general rules below.

a) For a given compound, if the ratio of carbon atoms to N-H, NH2, or OH groups is <> good bet that the compound is water soluble. Some examples:

b) If the compound is charged, then it is a good bet that the compound is water soluble. Some examples: c) Exceptions to Rule a, include compounds with very large dipole moments and <>

Step 2: Second, consider acid base chemistry that the compound can undergo. If you adjust the pH, can you form a salt, or reform a neutral compound from a salt. Knowing which compounds will and will not react when you are considering organic molecules is actually not terribly complicated. You just need to know approximate pKas for functional groups, and whether they will react with a strong or weak base, or an acid.

Carboxylic acids have a pKa around 5. This means that carboxylic acids can react with either a strong base like NaOH, or a weak base like NaHCO₃. This would form a carboxylic salt that would be soluble in water.

Alcohols generally have pKa’s around 17. Something with a pKa of 17 is not going to react with a strong base like NaOH. There is only one type of alcohol that can be deprotonated by NaOH-that is a phenolic alcohol. The pKa of a phenolic alcohol is significantly lower because of the aromatic ring, around 10. This is low enough to be deprotonated by NaOH, a strong base, but not by a weak base like NaHCO₃.

Amines have pKa’s around 35. They are bases, so they will not react with base. They can, however, become protonated by an acid to form the water soluble salt.

Neutral compounds remain soluble in organic solvents. Likewise if you do acid base chemistry that gets you back to the neutral compound it will no longer be soluble in water.

Step 3: Extraction is generally a preferred technique. If this cannot be used consider other methods.

a) Can a simple filtration be used? Is one a solid, and the other a liquid?

b) Is one of the compounds volatile? If the compound is low boiling it can be easily taken of by rotary evaporation. Some examples of volitle compounds are acetone, hexanes, ethyl acetate, methylene chloride, ethyl ether.

c) For remaining compounds, column chromatography would probably be a good choice. Column chromatography separates compounds based on polarity. An expert can separate almost anything on a column, even the most minor polarity differences.

Solubility applied to Vitamins

Solubility is the extent which a compound dissolves in a solvent. Compounds tend to dissolve in compounds that have similar intermolecular forces. In my undergraduate class we simplify this into the rule of 5. Basically if there are five or less carbons to each polar (oxygen and nitrogen containing) functional roup then it is probably water soluble. If there are 6 or more carbons per polar functional group then the molecule is likely insoluble in water. It will most likely be soluble in hexane or another less polar solvent. There are exceptions for small molecules like acetone, if there are a small number (1 or 2) carbons with a polar functional group then it is probably at least slighltly soluble in water.

One reason why it is okay to load up on vitamin C when you are sick is because it is water soluble. If you intake too much vitamin C you will just end up excreting it, and it will not harm you. The reason for this is because vitamin C is water soluble. Looking at the structure you see a cyclic ester with 4 hydroxy groups. That is 5 carbons to 5 polar functional groups. It is soluble in water.

Another one that is soluble in water is Niacin. The carboxylic acid and the nitrogen are polar functional groups, there are only 6 carbons, so this too is water soluble.

Some examples of water insolube vitamins are vitamin K and vitamin A. These are not so easlily excreted. Ingesting a moderate excess of probably wont hurt you, but in large excess they can build up and cause problems. Story has it that some artic explorers died from eating polar bear liver which contain an unusual amount of vitamin A, which is just one more reason not to eat polar bear livers.

The structures of the non water soluble vitamins have a few polar functional groups, but they have long carbon chains, their carbon to polar functional group ratio greatly exceeds 5:1, therefore they are water insoluble.
We all know the latin root for vita, but apparantly the -amin part comes from amine. Polish chemist Cashimir Funk named them vitamines because he thought that all of these compounds would be amines. The name has since lost the e and Funk's hypthesis has been refuted, but the rest of the letters have remained.