KiloMentor is always on the hunt for immiscible solvent pairs that could be used for liquid-liquid extractions of mixtures of organic solutes. Acetic acid is sufficiently inexpensive to be used as a solvent and it will dissolve many more organic compounds than will water. Acetic acid/water mixtures will have intermediate capabilities and make the partitioning of substrates between hexane and acetic acid/water somewhat tuneable.
There seems to be some disagreement among online compilations of miscibility/immiscibility data about the relationships between various saturated hydrocarbon solvents, an example being hexane and acetic acid. Some tables declare that acetic acid is immiscible with pentane and hexane but miscible with the longer chain homologs, while other places suggest the opposite. It seems likely that acetic acid is immiscible with at least some molecular weight saturated hydrocarbon and this is sufficient for my proposed purpose whichever is true since substances of both very short and longer chains (or rings) are available in the price range to be used as solvents.
I expect that the inconsistency relates to the extent of drying of the acetic acid. It probably only requires a very small amount of water to change the properties of acetic acid since in its purest form it would probably consist of doubly hydrogen-bonded dimers while some water would disturb that.
Whatever the truth may be, we can be confident that at least acetic acid mixed with some small proportion of water will cleanly separate from hexane and other pure hydrocarbons.
Hexane, a low molecular weight hydrocarbon, will be able to dissolve materials that are substantially hydrophobic. Mixtures with acetic acid/water may be ‘tuned’ to separate organic solutes with some polar functionalities.
Substrates that can exhibit either acidic or basic properties in water are not the substrates of concern here. These already can be switched back and forth between aqueous and organic phases by adjusting the pH. It is the neutral substrates for which we want to find new methods of separation.
Hydrogen bond acceptors would be expected to have some favorable interaction with acetic acid. Among these would be ethers, phosphine oxides, N-alkylated amides, alcohols, esters, nitriles, ureas, and ketones. As the proportion of a structure that is hydrophobic increases so the solubility in the saturated hydrocarbon phase is likely to trend.
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