Sciencemadness Discussion Board

Hydrazine soln. + Time = ... Maple Syrup?!

Rhodanide - 6-10-2016 at 08:41

Hi all, a bit over a year ago, I got my order of 250g of Hydrazine Sufate from Ukraine. (I still have a good 200g still unused) I used about 50 grams to make a solution of Hydrazine in Ethanol. Over time, it darkened, and continued to get more and more syrupy. Now it looks identical to Grade-A dark amber maple syrup. Now, what the hell would cause it to do this? The container it's in is plastic, and shows absolutely no signs of compromisation, or corrosion of any kind. The only impurities that were in there would be excess NaOH (I used too much in the reaction), and that's about it.
It's quite strange, and I'll add a picture when I can find one.

Rhodanide - 6-10-2016 at 08:47

Here it is after 3 months.

https://postimg.org/image/wdxiggmvp/

chemplayer... - 6-10-2016 at 20:26

Could this be slow oxidation of the alcohol to acetaldehyde (via air) and then subsequent condensation with the hydrazine? In addition if the alcohol also oxidises to aldehyde slowly, could the alkali conditions cause aldehyde self-condensation to form other ketones and then condensation with hydrazine etc.?

Tdep - 6-10-2016 at 20:37

Is the plastic PET or HDPE? Because if it is PET it may be pulling out some of the terephthalic acid (as strong bases tend to do) and that could be making some new polymers in the solution.

I do like chemplayers idea though, the colour does look like the aldehyde condensation reaction I've seen, they seem to go a red

Rhodanide - 10-10-2016 at 07:55

Quote: Originally posted by chemplayer...  
Could this be slow oxidation of the alcohol to acetaldehyde (via air) and then subsequent condensation with the hydrazine? In addition if the alcohol also oxidises to aldehyde slowly, could the alkali conditions cause aldehyde self-condensation to form other ketones and then condensation with hydrazine etc.?


Hey man, good to see you're still here.
I'll look to see what type of plastic the container is. It's an old 29% NH3 bottle, and I'll check to see whether it's PET or HDPE.
I also noticed that there's a negative pressure inside the container, because the sides are being gently sucked in. I'm currently busy, but will be back to see what material it is later. I'll let you know.

Rhodanide - 10-10-2016 at 07:58

Quote: Originally posted by Tdep  
Is the plastic PET or HDPE? Because if it is PET it may be pulling out some of the terephthalic acid (as strong bases tend to do) and that could be making some new polymers in the solution.

I do like chemplayers idea though, the colour does look like the aldehyde condensation reaction I've seen, they seem to go a red


It's probably HDPE, because of the consistency of the bottle.
There's also a strange smell, aside the N2H4 and denat. Alcohol. I can't quite put my finger on it, but it's there. Probably acetaldehyde like you and CP were saying.

Darkstar - 15-10-2016 at 22:09

Quote: Originally posted by chemplayer...  
Could this be slow oxidation of the alcohol to acetaldehyde (via air) and then subsequent condensation with the hydrazine?


Not only that, but if excess NaOH really is present, then it may even be the hydrazine itself that's causing the ethanol to be slowly oxidized to acetaldehyde. Hydrazine is already far more susceptible to autoxidation than ethanol is due to its much lower bond-dissociation energies. Ethanol's alpha-carbon C-H bonds require around 88 kcal/mol of energy to cleave homolytically, while hydrazine's N-H bonds only require around 77 kcal/mol. And then there's also the fact that alkali hydroxides are known to significantly accelerate the oxidation of hydrazines by free O2 in general.

So it could be that the excess NaOH in solution is inadvertently accelerating the oxidation of ethanol to acetaldehyde by first accelerating the autoxidation of hydrazine to diazene. This would inevitably lead to the generation of radical hydrazine intermediates that could then react with the nearby ethanol molecules to give the corresponding ethanol radicals, which, in turn, could then go on to react with other nearby radicals to give acetaldehyde through radical disproportionation. (either via direct disproportionation to acetaldehyde, or through a vinyl alcohol intermediate that then tautomerizes to acetaldehyde)

Edit: In addition to radical disproportionation, I suppose the ethanol radicals could also be oxidized by combining with the initial hydroperoxyl radicals generated by the abstraction of hydrogen from hydrazine, forming an organic hydroperoxide that gives acetaldehyde on elimination of hydrogen peroxide. And since the oxidation of hydrazine to diazene likely also produces hydrogen peroxide, the ethanol radicals could also react with any hydroxyl radicals around to form a geminal diol that then gives acetaldehyde on elimination of water.

[Edited on 10-16-2016 by Darkstar]