Sciencemadness Discussion Board

Isolating dimethylhydantoin

Antwain - 17-11-2007 at 15:19

Some time ago I produced bromine by reduction of BCDMH with Na2SO3. I kept the residue for isolating dimethylhydantoin. I neutralised the acid produced in the reaction by adding NaHCO3 until no more fizzing occurred, but nothing is precipitating. Very little had crystallised before but I simply put this down to the formation of 'onium' salts in the acidic solution. Does anyone know why I am still getting no precipitate? DMH is rather insoluble and there should be the best part of 100g in a liter of solution. Is it possible that it was hydrolised or in some way destroyed by leaving it in acid solution for so long?

chemrox - 17-11-2007 at 22:37

Quote:
Originally posted by AntwainIs it possible that it was hydrolised or in some way destroyed by leaving it in acid solution for so long?


How long was it in acid solution? Also "dimethylhydantoin" is a little vague. Is it N,N' dimethylhydantoin, 1,5 0r 3,5 ?

You said you added base until fizzing stopped. Did you check the pH? Carbonyl compounds are complicated by the fact that under basic conditions they are electrophillic at at the carbonyl C and nucleophillic at the a-C. You might have deprotonated the a-C with excess base and made the enolate. You could try neutralizing the solution or making it slightly acidic and see what happens.

[Edited on 17-11-2007 by chemrox]

not_important - 17-11-2007 at 22:48

BCDMH was given as the origin, so it would be 5,5 dimethylhydantoin

chemrox - 17-11-2007 at 22:58

OIC-thanks N_I I missed that. That eliminates deprotanation at the a-C doesn't it? You could have the enolamine forming with two good resonance structures. Ugh. I've had all sorts of troubles with heterocyclic carbonyls in basic solutions



[Edited on 17-11-2007 by chemrox]

The_Davster - 17-11-2007 at 23:57

Well TCCA when it looses its chlorines ends up at cyanuric acid. Cyanuric acid can form salts.
Perhaps something similar happens when dimethylhydantoin is under basic conditions. Cyanuric acid forms salts, why not the lower right structure below doing the same?

Perhaps carefully acidifying?

[Edited on 18-11-2007 by The_Davster]

TCCA BCDMH.JPG - 14kB

not_important - 18-11-2007 at 01:04

According to http://www.cfsan.fda.gov/~acrobat2/fnea0308.pdf DMH has a solubility of 135 g/l in water at what I assume is 25 C. It's resistant to hydrolysis in the pH range 5 to 9, no info for outside that range.

Nicodem - 18-11-2007 at 01:52

It won't hydrolyze at room temperature even at pH 1.
There was no need to neutralize, but it won't hurt either as its pKa1 is about 9 so even if you added excess NaHCO3 it will barely increase any solubility by deprotonation. A more important thing would be to quench all the remaining oxidant species with sulfite or whatever.
Extraction is not wise due to the unfavorable partition coefficients. So you are left with either:
A.) evaporating the water out, extracting the solids with ethanol (or acetone, i-PrOH or what you have), evaporating the solvent and recrystallization from water;
B.) saturating with NaCl, waiting for 5,5-dimethylhydantoin to crystallize out, filtering and recrystallization.
Of course, method A would give you more product, but evaporating a liter of water down without using vacuum sucks a lot.

not_important - 18-11-2007 at 20:29

Quote:
Originally posted by Nicodem
...
A.) evaporating the water out, extracting the solids with ethanol (or acetone, i-PrOH or what you have), evaporating the solvent and recrystallization from water;
...


Do you have solubility information for solvents other than water?

leu - 19-11-2007 at 17:53

It's listed as very soluble in acetone, alcohol, benzene, chloroform and ether so it seems it's probably soluble in most every organic solvent except possibly the most highly non polar hydrocarbons :P

not_important - 19-11-2007 at 17:59

Sounds like evaporation, perhaps helped along by an aspirator, followed by extraction with organic solvent is the way to go.

Antwain - 20-11-2007 at 05:10

Thanks guys.
Quote:
Originally posted by Nicodem
It won't hydrolyze at room temperature even at pH 1.
There was no need to neutralize, but it won't hurt either as its pKa1 is about 9 so even if you added excess NaHCO3 it will barely increase any solubility by deprotonation. A more important thing would be to quench all the remaining oxidant species with sulfite or whatever.
Extraction is not wise due to the unfavorable partition coefficients. So you are left with either:
A.) evaporating the water out, extracting the solids with ethanol (or acetone, i-PrOH or what you have), evaporating the solvent and recrystallization from water;
B.) saturating with NaCl, waiting for 5,5-dimethylhydantoin to crystallize out, filtering and recrystallization.
Of course, method A would give you more product, but evaporating a liter of water down without using vacuum sucks a lot.


Yes I neutralised bromine with sulfite prior to adding bicarbonate. I also deliberately chose bicarbonate so that the pH wouldn't be too high, I knew about the rather acidic proton alpha to 2 carbonals.

I couldn't find the solubility of DMH in water, but I thought that it was far lower. That alone probably explains the solubility. Evaporating water is simple with a shallow dish and if it must be done quickly a fan blowing across the surface. Evaporating water under vacuum is tricker for me, as you can't use a solvent trap for fear of expansion and cracking, and letting it go through a pump is out of the question. Using a rotorvap is great, but I don't have one myself.

Nicodem - 20-11-2007 at 13:40

You can live without a rotavapor if you have a magnetic stirrer and an aspirator. You just connect your flask to the aspirator and heat the flask in a bath on the stirrer while stirring intensively. Works great for amateurs! Of course you can also set up a trap in between the flask and aspirator for cases where you expect the solvent bumping out.