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Author: Subject: Hydroiodic acid made in situ
chemrox
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[*] posted on 18-1-2007 at 23:37
Hydroiodic acid made in situ


A reducing technique that was around for a long time was brought into vogue by crank makers using it to reduce the benzillic hydroxyl on ephedrine. The popular method was to use HI and P together the P being there to regenerate the HI as it was consumed. Vogels gives the method for making HI from red P and I2 with H2S.

My question: can the HI be made with I2, P and S in water? I have a non crank related application for this reduction and prefer to avoid the stink, let alone notariety, of H2S. Especially since the H2S is needed in a rapid stream in Vogel's cite.

What would be the stoichiometry?

EbC: Corrected title spelling ... why? People might search for it!

[Edited on 24-1-2007 by chemoleo]
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[*] posted on 19-1-2007 at 03:49


The HI can be produced with I2 and red P in situ, for a matter of fact thats the standard methcooking procedure.
It cannot be made from H2S and I2 in situ for this method produces HI of to low a strength with must be redistilled to procure the constant boiling azeotrope of HI and water which has just enough strength for the projected reduction.




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[*] posted on 22-1-2007 at 14:46


I think we have too many discussion already on the hi +p + e saga. Please, let's not start this here too!
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[*] posted on 22-1-2007 at 15:07
I have little idea, but...


If this HI in situ isn't specifically for the reduction of ephidrine, then it may be that a lower strength of HI could be used. I thought the reason ephidrine needed high conc. HI<sub>(aq)</sub> was because of one or more of the usual suspects; steric hinderance, e<sup>-</sup>-negativity and markovnikov inhibition [is that even a term! what crap am I saying :P]. So yea, for a methcook's speedy reaction, high conc. HI is desired, but thats <u>not what the question was</u> if I read right.
I think lower concentrations produced <i>in situ</i> would suffice for speedy red. of <i>e.g.</i> eth-




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[*] posted on 22-1-2007 at 19:33


Quote:
My question: can the HI be made with I2, P and S in water? I have a non crank related application for this reduction and prefer to avoid the stink, let alone notariety, of H2S. Especially since the H2S is needed in a rapid stream in Vogel's cite.

I also have no interest in 'crank' but do know (thanks to the 'crank' cooks) that the easiest (read: most otc in the US) method for generating HI is refluxing an aqueous solution of H3PO3 with I2. I believe it is common practice to reflux the H3PO3, water and I2 to make the HI, and then to add whatever it is that is to be reduced. Then the mixture is refluxed for a given amount of time, usually several hours.




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Chris The Great
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[*] posted on 23-1-2007 at 00:07


Phosphoric acid and potassium iodide makes 57% HI in high yield, but apparently leaves behind some nasty solid residue as well. But it might be the best bet for otc (assuming you can get P), since HI works better for the reduction anyway by needing less phosphorus.
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[*] posted on 23-1-2007 at 09:14


If one were to attempt to produce 57% HI via the H3PO4 + KI method, be advised that the reaction runs at a rather high temperature, and MUST be done using a heating mantle as temperatures in the flask can reach in excess of 400deg. C, do not attempt this using an oil bath, as it has been known to end in broken glassware and the release of phosphine gas, a rather nasty substance
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[*] posted on 23-1-2007 at 09:25


Phosphoric acid will pretty much eat all your glassware at 400 deg C anyways. Not only that but the HI will be created in gas form so you have to condense it back into liquid with a very cold trap/ condensor and reconstitute with water to 57%



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[*] posted on 25-1-2007 at 07:34


The I2 and H2S method works, but it seems, that the endprocduct is not as strong as in situ generated HI, by the aforementioned method.

It was distilled and the residue left in the flask, when the destillat reached 95°C/ 220torr.

Nonetheless, I remember some very impressive holes in some cloth and eaten-up aluminium-foil from the rp/HI generated acid, that I´ve yet to see, from the hydrogensulfid produced method... (methinks it was a better working technique what didn´t produce any holes this time, but at least some newspaper from under the flask is a bit eaten-up) and the ph paper indicates a purple red colour so something <ph1.

[Edited on 25-1-2007 by nightflight]
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chemrox
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[*] posted on 30-1-2007 at 00:07


Quote:
Originally posted by Coldfinger
If one were to attempt to produce 57% HI via the H3PO4 + KI method, be advised that the reaction runs at a rather high temperature, and MUST be done using a heating mantle as temperatures in the flask can reach in excess of 400deg. C, do not attempt this using an oil bath, as it has been known to end in broken glassware and the release of phosphine gas, a rather nasty substance


I fried a heating mantle running this reduction and will use dot 3 brake fluid or silicone oil if I ever have occaision to use it again. High T is right!
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