The following is a research idea. As far as KiloMentor is aware It has not been demonstrated. Before any trials, an up-to-date literature search is recommended.
A very large number of organic compounds are essentially insoluble in both pure hydrocarbon solvents and in water. As such they would be expected to also be insoluble in a two-phase mixture of water and hydrocarbon. Solutes that belong to this large group would be candidates for a crystallization/recrystallization procedure that, as far as I know, has not been tried to date.
Methyl and ethyl salicylates are very low melting solids and high boiling liquids: methyl salicylate mp -8.6 C; Bp 220-224 C; ethyl salicylate mp 1 C; Bp 232.5 C.
They share another common property. When stirred with aqueous alkali they are hydrolyzed to 2-hydroxybenzoate salts. What is less commonly recognized is that the presence of a separate hydrocarbon phase would not effectively inhibit this hydrolysis. The reason: the free phenolic substituent essentially drags the ester into the aqueous phase where the base attacks the ester functionality irreversibly after which it no longer has any affinity for the hydrocarbon layer.
Unlike hydrocarbons or water, these compounds will be good solvents for a wide variety of other organic materials. They can interact using Van der Waal dispersion forces, dipole-dipole interactions, and hydrogen bonding using both the phenolic hydrogen bond donor and the ester carbonyl hydrogen bond acceptor. Furthermore, these compounds are not particularly expensive and are readily available at an industrial scale. They have been demonstrated to be safe. Methyl and ethyl salicylates are flavoring and perfume chemicals.
Suppose we choose to dissolve a solute of interest in a combination of a highly apolar poor solvent, like the hydrocarbon heptane for example, and as solubilizing agent methyl or ethyl salicylate. Such a combination will have the property of having a boiling point at least as high as the hydrocarbon used but will have the enhanced dissolving power provided by the additive. When the mixture is all a single solution it is cooled to ambient temperature and an immiscible aqueous solution of base is added. Even with only very weak stirring, hydrolysis in the two-phase medium will result in the salicylate being taken into the aqueous phase. Now with its solubilizer degraded neither hydrocarbon nor aqueous phases will appreciably solubilize the substrate so it is likely to slowly crystallize out.
The mechanism by which the methyl or ethyl salicylate gets hydrolyzed and retained in the aqueous phase as carboxylate salt is the same extractive hydrolysis that was featured in another KiloMentor blog.
Wide Range of Acceptable Solutes
Even solutes containing functional groups sensitive to aqueous alkali can be expected to safely undergo this treatment. Molecules without active hydrogens (such as phenols, carboxylic acids) will not be extracted out of the hydrocarbon phase and so will be protected from significant alkaline hydrolysis.
Hypothetical Examples
An example of a preparation that might be improved using this methodology can be found in Organic Synthesis Col. Vol. 1 pg. 60 Anthrone Synthesis. The anthrone is finally crystallized from 3:1 benzene and petroleum ether. It is reported that about 12 g of the 3:1 mixture is required for each gram of anthrone. The yield percent recovery is 62/82.5. An effort is made in this preparation to recycle mother liquors and this reuses about 2/3 of the liquid. The anthrone is much more soluble in benzene than in the petroleum ether antisolvent. It would be interesting to see how the purification would proceed with heptanes as antisolvent and one of these hydroxyl benzoate esters as the solvent with dissolution at the reflux temperature of heptanes.
Another opportunity to use this technology seems to be presented by the bromination of anthracene to 9, 10-dibromo anthracene. A process is described in Organic Synthesis Col. Vol. 1 pg. 207. This procedure uses carbon tetrachloride as solvent. This would be unacceptable in scale-up to-day since carbon tetrachloride is a recognized carcinogen. It might work to brominates anthracene with bromine in heptanes. The dibrominated product is likely to be poorly soluble in heptanes and anthracene itself would only be somewhat better. In the heated reaction mixture, the anthracene would probably dissolve enough to allow the reaction to proceed. At the end, the crude dibromoanthracene would be precipitating. To recrystallize and recover the solvent one of our salicylates could be added with heating to get a solution; the combination then could be filtered hot; dilute aqueous alkaline added to hydrolyze and extract the hydroxyl benzoate ester. Since the 9, 10-dibromoanthracene would then be insoluble both in the aqueous and the hydrocarbon phases it should crystallize.
Other Possible applications
Other compounds from Organic Synthesis that could benefit from purification from a two-phase mixture of aq. alkali and high boiling hydrocarbon solvent: desoxybenzoin pg. 156: desyl chloride pg. 159; dibenzalacetone pg. 167; ethyl 2,3-dibromo-3-phenylpropionate pg. 270; m-nitroacetophenone pg. 434; Organic Synthesis Col. Vol. III acenaphthenequinone pg. 1; acenaphthenol-7 pg. 3.
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