Reflux under vacuum to increase yields.

kinesthetic1 · Mon Feb 14, 2005 5:44 am

Tweaking the Hypo work-up in OZ...
I can imagine there's more than just a handful of us who'll admit they regularly spend time lost in deep thought, analyzing ways in which to perfect our processes and refine the reactions that we do Wink

Here are a few methods born out of those analytical daydreams which by increasing both the quality/yield of final product have proven their worth and come to be 'standard protocols' I now use in my work-ups . Gone are the days (hopefully) where the industry standard was to work at a ratio of 1:1:1 , refluxing the required 4hrs, and then steam distilling a.k.a 'banging the whole lot over' until nothing remains in the reaction flask but one unholy lye-locked mess...
The goal is to get a complete conversion of your pseudoephedrine to methedrine, to do this requires an environment rich in Hydrioidic acid. Working the two precursors that create this environment ie: Hypophosphoric acid and Iodine in an excess ratio to the amount of pseudo being used is tip --1 Exclamation

Try 5:5:1 (5partsHI: 5partsI2: 1partPseudoephedrine) or really overindulge yourself and go a 10:10:1 -I've only gone this OTT when there's been a real lack of time for refluxing. Reflux for the longest time possible on the lowest temp at which reflux can be maintained is tip--2 Exclamation

This avoid any possible high temp decomposition of your goodies. Maintaining reflux over extended periods of time at temperatures lower than would normally be used, is made possible by tip--3 Exclamation

applying vacuum to the reaction. (A 60mL syringe , a one way flow valve and a length of PVC tubing are the basic groundlevel pieces of equiptment required for this) The first thing I noticed is how much less ice was needed for the water to stay feezing cold in the condenser- and 'topping up' the ice went from having to be done every 30mins, to once every 10hrs.
No expensive equipment is required or used. Maybe this will give a few ideas to anyone out there who is working toward perfecting their technique regardless of what 'lab aquisitions' they do or don't have.

Guest · Fri Feb 18, 2005 3:08 pm

Instead of refluxing under reduced pressure one could of course also stirr* the reaction at the wanted temperature.
This would be preferable as I believe and mixing would be better too. Preferable because one actually wants to stay all the compounds IN the reaction and not to drive them out by boiling (in special the HI).

*Stirring says here to mix the soup, not to stirr it up and bring air into it.

It is a widespread misinterpretation that something has to boil/bubble or something else has to be visible for a reaction taking place.

/ORG

ApprenticeCook · DILLIGAF

org is right.... the idea that if you dont see something nothing is happening is really hard to de-program from people when your helping them with chemistry woes as thats whats taught at schools in oz.... its a pain... oh well... its a pain but it is easily done....

-AC

loki · guinea pig

i can't see how reduced pressure would in any way help the reaction, in fact the obverse - because it would implicitly pull more out of the liquid phase where it does its reaction thing.

having watched an e/rp/i run in a simple test tube with nothing special other than a fan to keep the top cool, and having witnessed what would clearly was a 80% yield... i dunno, maybe it helped that the chemist stirred it??? I know if I were running a hypo/HI rxn i would think that it would improve reaction efficiency to lower the temperature and stir the thing the whole time. i don't know who started promulgating the idea that the RXN should be done without any agitition, i recall vague reference to such somewhere but i can't recall... but it is dead wrong, agitation always helps in any reaction. what is heat added to the process in any case, if it were not for it's micro-kinetic agitation. keeping everything in constant flux is key to ensuring complete reaction, and possibly it shortens the time required too. mixing is only contraindicated in the case where one deliberately wants to keep it calm so it doesn't spew forth from the top of the apparatus, and even then, cooling is preferable to not mixing as heteregenous substances have a tendency to fractionate in a solution if left to settle for long enough.

ApprenticeCook · DILLIGAF

Indeed stirring can help any reaction but the vacuum treatment of some reactions is a good idea for yields.
Try a safrole isomerisation with KOH without hitting it with the vacuum. yields suffer but its not because the reaction requires it... simply to remove water from KOH crystals as water content hinders the process.
More often than not extra equiptment in a reaction is to monitor / maintain the enviroment it is occuring in and not to deal with the reaction itself...

-AC