ADDkid:
In the old days when HI reductions were common in industry, what was more than a hundred years ago, hydrochloric acid was used as solvent in those reactions. The need for pressure vessels and the corrosive nature of HCl and HI in addition to the availability of other starting materials caused that these reactions where replaced by other ways.
Solvents useful in HI reductions are:
- conc. HCl
- conc. HI
- conc. acetic acid
- conc. phosphoric acid (85%+ concentration)
In the old days fuming HI was the medium of choice, followed by HCl. GAA and H3PO4 where not cheaply available then.
The reaction as preferred here on this board - the "long wet reflux" uses HI as solvent, produced in the reaction "in situ". For reasons I dont know H3PO4 - phosphoric acid is rarly used although in certain parts of the world its use as solvent is quite common and it works very well and it has advantages. It is to be preferred over acetic acid because it lacks the obnoxious smell has an higher bp and is available OTC and easily concentrated so need arises.
Nicodem, I dont understand the copper-powder named by you. I know it is possible to use copper to scavenge free iodine from discolored HI solutions though. Can you enlighten me?
PI3 under mainly anhydrous conditions halogenates alcohols faster, at lower temperatures and more completely than HI does for HI halogenation is an equilibrium reaction, PI3 halogenation is not. In the HI halogenation the equilibrium is shifted by the followup HI reduction of the formed iodoephedrine. This says that rather high temperatures and time is needed. All the time HI concentration has to be kept high.
The "long wet reflux" is a good and practical answer. It might be improved by the addition of some phosphoric acid after my best of knowledge. As long the concentration of the H3PO4 is over 85% the amount of acid is not very critical.
Other answers would be, either the use of PI3 (produced beforehand or in situ) under anhydrous conditions followed by the addition of some aqueous HI, or the use of fuming HI in a pressure vessel. This is not so practical at least the fuming HI under pressure isnt.
ORG