chemoleo - 15-9-2009 at 15:31
The compounds in question are
R-O-N=CR1R2 and
R-NH-N=CR1R2
Is there a quick way to determine whether the N=C double bond is present or whether it has been fully reduced, i.e. by Na-cyanoborohydride?
I've read that the double bond is not very stable, particularly in acids, so I'd prefer to reduce that - but how can I quickly verify that reduction
was successful?
Thanks
DJF90 - 15-9-2009 at 16:27
Being carbonyl derivatives formed in an acid catalysed equilibrium, the addition of dilute acid should yield the carbonyl and the hydroxylamine or
hydrazine salt respectively. If the C=N bond has been reduced then you should not be able to get such an acid catalysed reaction, for which you can
test for by forming the 2,4-dinitrophenylhydrazone of the carbonyl compound produced. Assuming the compounds you have arent brightly coloured
(although its possible they might be), the bright yellow-orange crystalline precipitate should be visible, indicating the presence of aldehyde or
ketone and hence the presence of the C=N bond in your substrates.
chemoleo - 16-9-2009 at 02:55
Yah, except DNPH is not good enough because concentrations aren't in the millimolar range, but much less than that. Also, DNPH derivatives are used in
the first place to create these compounds!
You can't think of anything that reacts specifically and directly the imine C=N bond, but not with its reduced form?
Klute - 16-9-2009 at 03:09
FeCl3 might give a specific color, forming a complexe. Try it out on your imine-species and on a secondary amine to compare colors.
Also, using the ferrous ammonium sulfate test for nitro compounds might work here, the double bond of the imine species oxidizing Fe2+ to Fe3+ and
precipitating Fe(OH)3. Check with the secondary amine to be sure n o other functionities give a false positive.
[Edited on 16-9-2009 by Klute]
panziandi - 16-9-2009 at 03:14
Klute that is a really good idea! That would be an incredibly simple and convenient spot test!
Klute - 16-9-2009 at 03:28
He he, well having 500g of hydrated FeCl3, i always try to use it as much as I can! It's a pretty versatil test, but the results aren't always easy to
decifer, a specific function doesn't always give a specific color, and there are often false positives from other functionalities.. It's ashamed this
former-routine test isn't well documented now days.. there are only few mentions in old articles about it use. It would be nice to have a documented
review detailing all the possible applications..
[Edited on 16-9-2009 by Klute]
chemoleo - 16-9-2009 at 16:28
Hmm, what conditions would you recommend to ensure reduction with NaBH3CN?
I tried aq conditions (it is meant to be reasonably stable in H2O) at pH 8, but mass spectrometry did not show (admittedly it's only 2 Da of a
difference) any difference at all - and I'm pretty sure I should have been able to see that (i.e. by the shift in the MS peak). A 10 x quantitiy of
NaBH3CN was used wrt the quantity of hydrazone/oxime available (i.e. 1 mM of NaBH3CN/0.1 mM hydrazone/oxime)
I know solvent reductions are advantageous (less hydrolysis of the reducing agent), but for the time being I'd like to keep it aqueous. Any
suggestions?
By the way, what does a tiny accidental whiff of this stuff feel like? It made me mighty dizzy! 
I'll be mighty careful in the future - thank your very much but no thanks to HCN poisoning!
[Edited on 17-9-2009 by chemoleo]
Klute - 17-9-2009 at 02:33
You're talking of NaBH3CN, right? I had the exact same accident! It really got me panicked as I thought I was experiencing a rather high HCN
poisoining, askign myself how much I could have inhaled.. Hopefully after takign fresh air for 10min things went better and that was that.. Now I
always have aprension when opening the bottle..
What could be the best way to avoid accumulation/generation of HCN? Having a cup of NaOH in the bottle, or putting the closed bottle in a baggie with
NaOH, or Ca(OCl)2? ANy ideas?
chemoleo - 17-9-2009 at 14:14
That's interesting - seems it's not such an uncommon accident! And it doesn't say anything about inhalation hazard on that bottle!
In fact I became so woozy (and worried) that I walked over to my colleague - so that he'd see me if I collapsed!
Anyway, any ideas on aqueous NaBH3CN reduction?
Klute - 17-9-2009 at 14:30
Well, maybe you could try buffering the solution, to a very slightly acid pH, using acetic acid triethylamine, or a phosphate buffer, these seem to be
the most common in NaBH3CN reductions.. But I'm surprised a un-buffered aq. reduction doesn' afford any product.. Maybe your imino-species are
getting hydrolyzed before any reduction proceeds?