Occasional Al[Hg] PN2P reduction issues

64bandil · Busy Bee

Hi!

I've used the Al[Hg] reduction of many nitropropenes with great success. I have however had quite a few failures which i've been unable to explain. Sometimes the exothermic hydrogen evolution simply cease to occur. It seems that the reaction never really kicks in. I always use the "original" amount of solvents, havn't tried the modified F-amp posted by Tetra yet.

It had happened once with MDP2NP and three times with 4-FP2NP. I've tried amalgamating in-situ, and prior to(shulgin style). I've also upped the acetic acid ratios quite a bit after the reaction has failed to set in, but it doesn't seem to help. Even refluxing the whole shebang for 24 hours doesn't make the foil dissolve.

Does the nitropropene substrate have anything to say about the hydrogen evolution? I always thought the hydrogen was evolved directly from the acid's decomposition of the Al foil?

I use regular kitchen foil, folded 4-5 times and ripped to shreds. 80% acetic acid and denaturated ethanol...

Anyone have some bright ideas?

thanks a bunch
- Bandil

icecool · Insistent Chemist

Make finer Al and add for every gram of Al 1mg of Hg salt.
So for 25g Al use 250mg HgCl2 and add it in 4 portions.
And first add 20ml of a 100ml solution of it and see what happens since it can react very vigorously and ofcourse because the Al is so small.
I put the Al foil normal thickness in the coffeebean grinder to little shreds.
And it hasn't been folded over or anything so quite small pieces.

joe_aldehyde · huxleys associate

i thought the hydrogen evolution takes place as Hg(2+) gets reduced into Hg(0) by Al(0) to Al(3+). Aluminium alone would of course decompose as well if put into acidic solution.

have you ever tried adding extra Hg(2+) to the reaction to resume?

when amalgamating in situ, do you just add your Hg(2+) solution and start adding the substrate right away?

64bandil · Busy Bee

Yeah - ive tried adding more mercury (LOTS more).

When i do it in situ, I put the Al + mercury sulfate in a large flask. Then EtOH + Acetic acid with substrate dissolved in it is added in one portion... I have been a little sloppy, with shredding the Al fine enough. It might have been a little "lumped"...

Thanks mates!

joe_aldehyde · huxleys associate

well, there's 3 variables: Al, Hg and AcOH.

anyways, i've been told, just like icecool says, that the size of the Al flakes is vital for the reaction speed. small flakes, high speed - but be sure to babysit your rxn all the time with an ice bath handy, or do like i'll do next time - 4L flask Smile

icecool · Insistent Chemist

Isn't the reaction flask size going to alter your yields.
I've heard if you do a 5g reduction in a 6 liter RBF you will get shitty yields.
If you do it in a 500ml flask you get high yields.
What are normal yields though for a reduction of 5g P2NP...

joe_aldehyde · huxleys associate

thats right, icecool - depends on the dilution of the reaction mixture. if of course all your p2np sticks to the flask walls and doesn't get reduced because your lump of Al sits in the bottom, you won't most likely get anything out of it. proper flask size matters. just use your brain! i wouldn't put 200ml MeOH with Al/Hg in a 4L flask just because it wouldn't foam and overshoot.