Yesterday's entry on crown ethers demonstrated a way to do reactions with a mixture of polar and nonpolar substrates. These crowns allowed for ions to be brought into the nonpolar solvent. Another approach is to just throw up your hands and mix oil and water, so to speak: reactions can occur with two phases.
Not very well, though.
One way to help this along is with phase transfer catalysts, usually quaternary ammonium salts. A QAC, ammonium compound, quat, or whatever you want to call it has four substitents on the nitrogen, (nitrogen usually carries 3) so it has a positive charge. This makes it interact strongly with water.
If we pick our substituents wisely and have long hydrocarbon tails, or aromatic rings, or things like this, we can have something that interacts well with water (because of the charge) and with nonpolar solvents (because of the fatty tails). The QAC allows polar and nonpolar reactants to come together and live happily.
Where does it live? At the interface of the organic/aqueous mixture (that would be right where vinegar meets oil in salad dressing, e.g.). Here, it's thought that they work by shuttling things back and forth between phases.
Using a phase-transfer catalyst puts you in a rather unique situation; rather than just having your reactants evenly dissolved throughout the solution, you are limited by how quickly things can cross the phase interface. For this reason, any phase-transfer-catalysed reaction has to be stirred vigorously.
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Quaternary ammonium compounds with relatively short aliphatic R-groups, such as tetraethylammonium and tetrabutylammonium are used as pharmacological agents that block the ion conduction pathway of potassium channel proteins. Their hydrophilicity allows them access to the aqueous ion conduction pathway of the channel, and their hydrophobicity allows them to interact stably with hydrophobic amino acid residues that point into the ion conduction pathway, thus blocking conduction.