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kilomentor | 22 July, 2007 13:37
Kilomentor in an effort to mentor organic synthesis and process development chemists is trying to bring underutilized methods of separation and purification to greater attention.
Hydrotropes are aqueous solutions over 1 mole/litre which have the property that the mixture can increase the solubility of solutes which have very poor solubility in water alone.
The typical hydrotrope forming material is reasonable priced for example:
Perhaps one can have a partitioning of compounds between the hydrotrope and an apolar solvent such as heptane, cyclohexane and methylcyclohexane. Use of these very non-polar solvents increases the proportion of the substrate extracted into the aqueous phase because the substrate has limited solubility in the hydrocarbon.
An example from recent literature is provided:
Hydrotropic separation of mixtures of o-/p-hydroxyacetophenones
KOPARKAR Y. P. ; GAIKAR V. G. ;
Separation science and technology (Sep. sci. technol.) 2004, vol. 39, no16, pp. 3879-3895
A new extractive separation technique has been developed for the separation of o-/p-hydroxyacetophenones (HAPs) using hydrotropy. Hydrotropes are freely water-soluble organic salts, which enhance solubility of otherwise water-insoluble or sparingly soluble organic compounds in aqueous solutions. The ability of hydrotropes to differentiate even isomeric organic compounds is explored in this extractive separation. o-/ p-HAPs were extracted from their solutions in organic solvents of different polarities using aqueous solutions of hydrotropes. The solvent nature has a significant effect on the selective extraction of both phenols. The combination of heptane and aq. Na-p-toluene sulfonate solution gave almost pure p-HAP in the aqueous phase, whereas with chloroform as the solvent, it was possible to extract with complete selectivity o-HAP into the aqueous hydrotrope solutions.
Another possibility is that the solid crude substance could be partially dissolved in the hydrotrope solution.
Any organic solvent can be used as the 2nd phase in a separation using an aqueous hydrotrope so long as that solvent is not strongly soluble in the hydrotrope. Such a combination can be used particularly when one only wants to remove a small portion of the substrate ie a small amount of impurity into the hydrotrope phase. This could be particularly used to purify a mateial with less than a percent of a particular impurity. When the impurity seems to be partially soluble in water but not sufficiently soluble to remove the impurity in a practical amount of washing, resorting to a hydrotrope solution could solve the problem.
The use of hydrotropes for extraction is an example of ‘salting in’ using a concentrated solution of an inexpensive hydrophobic salt.
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