Sciencemadness Discussion Board

Sluggish paraformaldehyde depolymerization?

turd - 1-9-2010 at 10:53

Howdy, it is well known that aqueous formaldehyde solutions can be obtained by thermal or acid catalyzed depolymerization of paraformaldehyde. So I made a quick experiment: 5 g paraformaldehyde, 10 ml H2O and 1 ml MeOH were put in a tightly closed Teflon container and heated overnight at 100°C. Result: a suspension with considerable amounts of solid. The experiment was repeated at 150°C. Result: a clear solution with lovely formaldehyde smell, though less intense than anticipated.

This is in stark contrast to Formaldehyde (http://library.sciencemadness.org/library/books/formaldehyde...) which says on p. 73:
Quote:
At reflux temperatures almost any desired concentration of formaldehyde can be obtained in one or two hours.
I prefer not to use high temperatures to avoid side reactions like the formation of methylal (could of course be avoided by not adding MeOH) and disproportionation to methanol and formic acid.

Am I doing something wrong?

Quantum_Dom - 1-9-2010 at 19:53

Im sorry to hear that. First of all the obvious question: was the paraformaldehyde home-made ? Was it free of impurities sensible to shift significantly the pH of an aqueous solution ? Notice that on page 76, it is stressed that solubility can be affected depending on how the paraformaldehyde was isolated, which makes sense as it will greatly influence its degree of hydration. Therefore, you might actually have used much less than 5 grams of effective H2CO which could explain the "dissapointing smell" you measured (good read BTW, thanks):
Quote:

According to Naujoks, an extremely soluble paraformaldehyde is precipitated when a water solution containing 55 to 65 per cent formaldehyde
and 10 to 15 per cent methanol is gradually cooled from 65 to 15°C. The
dried product, which is described as crystalline, is reported to contain
93.2 per cent formaldehyde.
........
Paraformaldehyde of low reactivity and solubility is preferred for some
uses and may be obtained by heating regular paraformaldehyde at 100°C10.
In general, the solubility of paraformaldehyde is a measure of its reactivity


Is there a reason why you didnt adjust the pH in order to bring lightly acidic or alkaline conditions ? IMHO the graph of the rate of dissolution versus pH of solution, which is essentially related to the rate of depolymerization, speaks for itself.

QD

[Edited on 3-9-2010 by Quantum_Dom]

turd - 18-9-2010 at 06:57

No, no, it's OTC technical grade and served me well so far in reactions that call for paraformaldehyde.

Another data point:
4 g paraformaldehyde, 6 g water in Teflon container, 36 h at 100°C. Opened while still warm - got a clear solution with very minor amounts of brown solid decomposition products at bottom (probably dirt from Teflon container). Added 1 ml MeOH to obtain ~36% formalin. On pH 1-14 paper only very slightly more acidic than distilled H2O (maybe pH 5-6? Hard to say.). I will store it a few weeks at RT and see how stable it is. Smell is quite strong, at least when warm, but not even close to say 30% aq. ammonia.

Maybe I cooled too much in the first experiment or this is some unreactive paraformaldehyde.

Rosco Bodine - 3-10-2010 at 16:39

Add some NaOH, a high pH, (not high heat) is what you need for depolymerization