PEYOTE (Hive Bee)
05-07-02 14:29
No 305743
      in-situ formation of CN- from ferrocyanide  Bookmark   


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

(Chief Bee)
05-07-02 14:42
No 305747
      cyanoferrate a bad nucleophile  Bookmark   

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...
(Hive Bee)
05-07-02 14:45
No 305750
      but if I....  Bookmark   

...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?
(Chief Bee)
05-07-02 15:02
No 305752
      That may work. But remember that acidified ...  Bookmark   

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.
(Hive Bee)
05-07-02 16:38
No 305774
      HCN  Bookmark   

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 smile)
(Hive Bee)
05-07-02 18:08
No 305803
      Yes, I know. A student in chemistry thesis here ...  Bookmark   

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?

(Hive Bee)
05-07-02 19:01
No 305830
      But are HCN solubel in H3PO4 ??  Bookmark   

But are HCN solubel in H3PO4 ?? I think it just will gass off.
(Hive Bee)
05-07-02 19:44
No 305841
      Yes, almost surely. But for this I've choosed a ...  Bookmark   

Yes, almost surely. But for this I've choosed a closed flask...wink

"Dark star crashes, pouring its light into ashes..."
(Hive Bee)
05-09-02 22:47
No 306797
      Another trick may bee...  Bookmark   

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..."
(Hive Bee)
05-10-02 00:14
No 306822
      Sol. off HCN  Bookmark   

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.