Concentration of HI vs. yield

[borolithium]

Most RP/HI reactions use the azeotrope of hydriodic acid, with a constant boiling point of 127 C and a specific gravity of 1.72.

Most of the HI I have been offered for sale has a specific gravity of 1.65, which is barely 50%. I have found that yields have been around 55% w/w, increasing to around 65%-70% when the weaker part of the acid is distilled off.

Recently, some dreams I had involved using homemade HI which had a specific gravity of 1.85, due to the way it was produced, by dissolving as much HI gas as possible under heavy cooling, and found the yields jump to around 75% w/w.

I am seeing a trend here. Has anyone tried using fuming HI with a higher specific gravity under cold reflux cooling, and have they found that their yields increased proportionally, or is there a point at which the high concentration of the HI has a negative impact on yield?

Generally speaking I use a 2.2 ml of HI for every gram of ephed, and about .25 grams of red p.

[Rhodium]

I am seeing a trend here. Has anyone tried using fuming HI with a higher specific gravity under cold reflux cooling, and have they found that their yields increased proportionally, or is there a point at which the high concentration of the HI has a negative impact on yield?

If you heat hydriodic acid with a concentration exceeding 57%, the excess HI will evaporate out of your reaction vessel as a gas (the condenser will not stop it) until the concentration has reached 57%. Thus 57% HI is the best concentration to use, anything exceeding that is just wasteful.

[biotechdude]

As Rhodium said, you may start with fuming D1.9 HI...but it will drop back to 57% (and lower) as soon as it is exposed to air (like an open condenser).

So the theoretical solution is to use a balloon to keep all the fuming HI contained in the reaction vessel.  HOWEVER, as the reaction proceeds, the HI concentration drops.... 

Thats why a P-based recycler is used to regenerate the HI.  BUT, a P-based recycler would struggle to maintain the fuming D1.9 HI...

So in short, fuming D1.9 HI is great but difficult to maintain in this state for the duration of a reaction (and hence measure if a yield increase occurred).  Thats why D1.7 57% HI is used for the reaction; as it can be contained in the (open or closed) reaction vessel and the recycling agent can efficiently maintain it (at 57%).

[borolithium]

I have most certainly dreamed of an increase in yield as a result of the concentrated HI over the azeotrope. Is it possible that some of this extra product occured in the early stages of the boil, or is there not enough time for this? The reaction will fume gaseous HI throughout the entire boil. Is it possible that the extra gas allowed 57% to be maintained longer in the reaction, thereby increasing the yield?

I would think that during heating, a significant amount of the iodine would be tied up as iodoephedrine and free iodine, thereby reducing the concentration of the acid during the course of the reaction. If a super saturated acid is used initially over the azeotrope, a slightly higher concentration should be maintained for longer, thereby accounting for the increase in yield.

Am I out to lunch on this one? I only see my dreams and wonder why I dream better yields.