How does one purify a mixture of structurally similar neutral compounds that is about 80% one isomer and 20% the other? If you adopt fractional crystallization the most likely outcome is that you do purify the major compound but the recovery is about 60%. The lost material is in the mother liquors in an approximate 50:50 w/w ratio with the minor constituent.
You could try liquid-liquid partition, even trying several of these in series resembling a rough counter-current extraction. The problem is that there aren’t that many liquid phases that are mutually immiscible and more frequently at least one component of any pair that is immiscible will exhibit poor solubility for most of the multifunctional organic compound mixtures that you want to separate. Yes- water and hydrocarbons are immiscible but neither one dissolves most organics well. Yes- acetonitrile and hydrocarbons are immiscible, but most organic mixtures do not partition competitively between them. Yes, hydrocarbons and perfluorocarbons are immiscible but again distribution between them is usually overwhelmingly into one or the other. Then there are less well-known ones such as MIBK/sulfolane which might be promising, but these are few.
What is needed is a way to modify water so that it has an increased capacity to dissolve organic compounds of interest while still remaining substantially immiscible with those common organic solvents which also have a good ability to dissolve a target mixture. This is what hydrotropes can do.
Two important strengths of the methodology: (i) the solubilization capacity of the hydrotrope is a strong function, usually exponential, of the hydrotrope concentration and
(ii) mere dilution of the hydrotrope with water is enough to recover dissolved materials.
(ii) mere dilution of the hydrotrope with water is enough to recover dissolved materials.
No comments:
Post a Comment