The Vespiary
The Hive => Chemistry Discourse => Topic started by: PEYOTE on May 07, 2002, 02:29:00 PM
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Hi.
If you have a benzylic halide (such as variously substituted benzylic chloride) and you have to make a benzylic cyanide without using KCN or NaCN, is it possible to use K4Fe(CN)6 with a strong acid like H2SO4 in order to generate in the reaction mixture the CN- ion? I've seen some old post but I've not found a feasible way.
Thanks in advance
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Treating cyanoferrate with acid produces molecular HCN (which is not fully dissociated in acid solution as HCN is a relatively weak acid. HCN is a bad nucleophile, while CN- is a good one, especially in polar aprotic solvents (DMF, DMSO etc) or in a dual-phase reaction mixture with a PTC. To get cyanide ions instead of HCN, the solution needs to be basic, but adding base after reacting cyanoferrate with acid will just re-form the cyanoferrate.
Your best bet will be to react cyanoferrate with acid to give HCN gas which is bubbled through a NaOH solution to produce aqueous sodium cyanide and use that...
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...if I acidify with H3PO4, Fe will precipite to ferrous phosphate, insoluble, and then I can basify to form CN-.... isn't it a good idead, without using tubes, bubblinga and so on?
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That may work. But remember that acidified cyanide solutions are poisonous to the extreme - do not attempt without a GOOD fumehood, or do it outside.
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Add acid to the ferocyanide and let the gass bubble trough a solution of sodiumhydroxide to make NaCN(aq) in a CLOSED system. Iff you are unsure don't fuck around with HCN (when you can smell some almond's in the air it's time to go home :) )
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Yes, I know. A student in chemistry thesis here in my department was cleaning a waste of [Fe(CN)6]4- using HCl!!!!!!! He was a jerk, in fact he was moved to a compilative thesis instead of an experimental one, eh eh eh....
In the end, this is my idea:
a solution of (substituted) benzyl chloride and K4[Fe(CN)6] is first treated with H3PO4 => Fe3(PO4)2 precipitated and in soln there is Ar-CH2Cl and HCN. Then NaOH soln is added, CN- is formed and it will react with Ar-CH2Cl forming Ar-CH2CN.
It'd be preferred a closed flask, so HCN dispersion from reaction flask is avoided. I think that a PTC (like a quaternary ammonium salt) is useful, isnt it?
:)
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But are HCN solubel in H3PO4 ?? I think it just will gass off.
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Yes, almost surely. But for this I've choosed a closed flask... ;)
"Dark star crashes, pouring its light into ashes..."
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using a viscous soln, addign for example glycerol (but it can react) or PVP. In this way HCN will gass off more difficult, wont it?
"Dark star crashes, pouring its light into ashes..."
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Nope, the week acid HCN won't stay together with the strong acid. I still mean that gassing the HCN off is the only way.