Sciencemadness Discussion Board

Help with Oxime preparation

SunriseSunset - 7-4-2016 at 12:37

I'm wondering how much Sodium carbonate monohydrate would be used to prepare an oxime from hydroxylamine hydrochloride in a water and methyl alcohol medium. The reason why I ask this question is because Sodium Carbonate has two parts on it which could deprotonate the Hydroxylammonium chloride into free hydroxylamine base.

Sodium Acetate is commonly used for this sort of reaction. Normal ratio of sodium acetate is just a slight molar excess over the NH2OH·HCl.

But with Na2CO3*H2O, would the molar ratio still be the same or different? please explain why. Thank you.

Here's what I found to reference.
Quote:

Mix: 7.9g Na2CO3 or 20.4g NaOAc·3H2O (150mmol)
15ml H2O
10.3g (NH2OH)2·H2SO4 or 8.7g NH2OH·HCl (125mmol)
Stir bar

Stir, then add: 100 mmol of a Ketone/aldehyde
75ml MeOH or 75mL EtOH
Then do the 1.5hr reflux, cool in fridge, and suction filter



According to that reference, I noticed that with Sodium carbonate monohydrate, the molar ratio is roughly 2-part ketone/aldehyde, 1-part sodium carbonate. Given their math.

[Edited on 7-4-2016 by SunriseSunset]

[Edited on 8-4-2016 by SunriseSunset]

SunriseSunset - 7-4-2016 at 12:45

From what I read and as well with some experience I've had trying this method out before is that the oxime (if even formed properly), doesn't always precipitate easily from the H2O&Alcoholic solution.

I've wanted to try this again but try out this one improvement.

The idea is once the reflux is over and temperature drops down to 20~25*C, pull a mild vacuum to distill away the alcohol bp(64.7°C) and traces of volatile hydroxylamine base bp(58°C).

The logic behind this improvement is so that there's less alcohol in the post-rxn flask that would prevent the oxime from precipitating; once cooled in an ice bath.

However, my major worry is that incase it didn't indeed react properly and there's still a lot of hydroxylamine freebase that will be pulled out and into the vacuum pump, it may cause an explosion due to the temps that some pumps run at. Maybe that's me being overly worried though. Because it would be a pretty small amount. I'd like to test with only 5 mmol ketone/aldehyde substrate rxns.

Any advice or links to new references is highly appreciated!!!!!!! Take care everyone.

One other random reference:
Quote:

Org. Synth. 1935, 15, 85DOI: 10.15227/orgsyn.015.0085

In a 1-l. round-bottomed flask, 83 g. (0.5 mole) of veratraldehyde (p. 619) is dissolved in 200 cc. of warm 95 per cent alcohol, and a warm solution of 42 g. (0.6 mole) of hydroxylamine hydrochloride (Org. Syn. Coll. Vol. I, 1941, 318) in 50 cc. of water is added. The two solutions are mixed thoroughly, and a solution of 30 g. (0.75 mole) of sodium hydroxide in 40 cc. of water is introduced. After the mixture has stood for two and one-half hours at room temperature, 250 g. of crushed ice is added and the solution is saturated with carbon dioxide. This causes the separation of the aldoxime as an oil which solidifies on standing overnight in an ice chest (Note 1). The crystalline oxime is filtered with suction, washed thoroughly with water, and allowed to dry in the air. The yield of oxime is 88–89 g. (97–98 per cent of the theoretical amount).


Does anyone know what the purpose of saturating with carbon dioxide is? Like how does it help cause separation of the aldoxime? I'm thinking about trying that if it works.

[Edited on 7-4-2016 by SunriseSunset]

Darkstar - 7-4-2016 at 17:36

Quote: Originally posted by SunriseSunset  
Does anyone know what the purpose of saturating with carbon dioxide is? Like how does it help cause separation of the aldoxime? I'm thinking about trying that if it works.


Probably to neutralize the excess NaOH in order to lower the pH so that the oxime becomes less soluble. I believe oximes are more soluble in strongly basic solutions because the hydroxyl hydrogen is slightly acidic and can be deprotonated, giving a much more soluble anionic species.

As far as the neutralization part, the CO2 first reacts with water to give carbonic acid:

CO2 + H2O ⇌ H2CO3

The carbonic acid then reacts with the sodium hydroxide to give sodium carbonate and water:

H2CO3 + 2 NaOH ⇌ Na2CO3 + 2 H2O

The sodium carbonate can then react with more carbonic acid to give the slightly less basic sodium bicarbonate:

H2CO3 + Na2CO3 ⇌ 2 NaHCO3

SunriseSunset - 7-4-2016 at 19:30

Thanks that's very useful to know broken down like that.

I think that's the exact problem I had when I tried it before. I did everything via the NaOH & alcohol method, except I didn't bubble Carbon dioxide into it to saturate it. So it remained as a viscous yolk colored oil. And when it was on a watch glass, it looked like the oil was constantly mobile, and would never evaporate. I could sort of tell that the small amount of excess NaOH was preventing the water from evaporating and having any precipitate form. I just called it over from that point.

The hydroxylamine hydrochloride I have has probably expired. it has a pretty obvious yellow tint all through out and minor darker areas of orange.. :( I may try to recrystallize it in water. Perhaps any pointers?

Says shelf life for hydroxylamine HCl is about 8 months and I know I've had it for a year in a sealed bottle at room temp. It might help during purification if I knew what some of the degradation products might be, in order to recycle the hydroxylamine*HCl and purify those out. Do you think it's necessary, or might it still be good for use? Anyone have any experience with that?

Would the breakdown byproducts of the salt be soluble in water in order to purify what I can from the bottle?

[Edited on 8-4-2016 by SunriseSunset]