Polyethylene glycols are “complexant” phase-transfer catalysts that act by complexing with cations such as sodium ion and potassium ion through the polar non-bonding electrons on the multiple oxygen atoms. Since sodium cation and potassium cation are part of a salt, the complex [PEG-NaX] moves as one and can distribute between aqueous and organic phases.
PEG 2000 has a solubility of about 60% in water at 20 deg C, but can also dissolve in organic solvents due to the many ethylene groups on the molecule (an average of 90 carbon atoms). PEG’s are on the FDA’s GRAS list (compound Generally Recognized As Safe) which suggests lower toxicity than quaternary ammonium phase-transfer catalysts.
A question sometimes asked about high molecular weight PEG’s is ‘how many alkali metal cations bind to one molecule of PEG?” This is important to attempt to estimate about how much PEG 2000 might be required to be an effective phase-transfer catalyst. The basis for the question is that it takes about 5-6 oxygen atoms in a crown ether, that is essentially a cyclic PEG, to complex with one Na cation or potassium cation. There are about 45 oxygen atoms on average in molecules/oligomers of PEG 2000! So, PEG 2000 might be efficient or might be very inefficient. The answer is that sometimes only one NaX or KX on pair can be transferred by one high MW PEG molecule and sometimes several NaX or KX ion pairs can be transferred by a single high MW PEG molecule. Each case is different based on the application, the solvent used, the identity of X (the anion we are trying to transfer and react) and possibly other factors. The easiest way to find out is to compare the performance of PEG 2000 with the performance of PEG 400 in the actual PTC system.
If you are looking to consider less expensive alternatives to quaternary ammonium phase-transfer catalysts and/or that contain no nitrogen atoms, contact Marc Halpern of PTC Organics to explore how we might work together to achieve your company’s process development goals.
