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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.
kilomentor | 10 July, 2007 17:26
Wolf & Lamb reactions are reactions or reaction sequences wherein at least two mutually reactive agents are kept in the same reactor isolated from each other by being attached to separate solid phases, which cannot interpenetrate each other. For example one polymer may have an oxidant attached to it while another solid in the same reactor has a reducing agent attached to it but they cannot react with each other because each is held on a separate resin or porous solid. Alternately a strong base containing for example triphenyl methylide anions may be on one resin and the second resin may have acidic groups bound to it.
What is the characteristic of a transformation or set of consecutive reactions that can be performed more efficiently in a medium providing this site isolation possibility?
We can imagine a mental cartoon in which a Substrate (S) moves to an immobilized reaction site and a reaction happens there because
i) there is a reagent tethered there
ii) the environment there is different (pH, solvent composition, ionic strength)
iii) there is a catalyst immobilized there
iv) there is a trapping agent for a functional group there (this last possibility applies more to the product of a reaction on another immobilizing solid).
Intuitively some transformations seem more addaptable to site isolation reaction:
Why would this situation be advantageous?
An excellent paper to give you a better idea about some of the possibilities is Wolf and Lamb Reactions: Equilibrium and Kinetic Effectts in Multipolymer Systems, B.J. Cohen, M.A. Kraus and A. Patchornik, J. Am. Chem.. Soc. 103(25), 7620, (1981).
Insoluble reagents which are not polymers can also be classified as site isolation reagent:. an example would be activated manganese dioxide.
One can imagine the use of manganese dioxide with a strong base bound to a resin also combined with an epoxidation peracid bound to a second resin combined with semicarbazide adsorbed on silica gel. This combination might be expected to convert an olefin to an epoxide using the peracid; the epoxide could be isomerised to an alllylic alcohol by the tethered strong hindered base; the allylic alcohol could be oxidized to an apha- beta unsaturated ketone by manganese dioxide and the ketone could be trapped and immobilized on the silica by the semicarbazide carbonyl derrivatizer.
I’m not saying this would work! It illustrates the concept.
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