Hi!
Allthough the last described method for preparing anhydrous HCl salts of amines by azeotropical distillation worked nicely, I thought it was time for some additional lab notes in this field.
Molecular sieves generally dislike strong acids such as HCl and cousins1. From empirical knowledge anhydrous HCl in IPA prepared this way gives the IPA a very dirty look from the somewhat dissolved sieves. It's really no good for preparing those really nicely glistering crystals. To overcome this issue, the following method can be used to produce über dry amine salts in non-polar solvents each time! The following example is of course only hypothetical in nature, but can be extended to many legal applications 8)
The following reagents solvents where combined without any fuzz in a 250 mL erlenmeyer flask:
14.9 g methamphetamine freebase (0.1 mole)
150 mL acetone
12.33 g 30% aqeous HCl (0.1 mole)
The mixture prepared this way, contains 12.33 g - 3.7 g = 8.64 g H2O. To soak this up, a 25% excess of 4A molecular sieves2 are added in one portion. This corresponds to 60 g's assuming 18% water absorbtion capability. The whole mess is carefully agitated for 30 secs, causing a instant precipitation of methamphetamine.HCl. The nifty thing about using sieves is that they retain the water even at the boiling point of acetone and IPA.
To separate the stuff, the mixture is lowered into boiling water (cover the flask with a little foil to prevent water vapour from the bath entering). Add IPA one mL at a time, constantly agitating and heating the flask such that it continuosly boils lighty. Once 20-30 mL's IPA has been added, the solution is clear. Quicly filter the hot solution without using filter paper. Simply use a large gridded funnel. Pour the sieves back into the original erlenmeyer flask and boil it for a few seconds with some fresh IPA. Filter this again, and combine with the first filtrate. Save the sieves for further runs. Add more fresh acetone to the IPA/acetone/meth mix, while boiling it, untill you get that perfect dual recrystallization mixture we all love 8) .
Crystallize as usuall and obtain the methamphetamine hydrochloride in 95-97% yield :)
Quite a lot of sieves to use, but they are not harmed this way, as the acid is buffered by the amine. The sieves can be reused ad infinitum, so it's really a cheap setup. Much faster and easier than the azeotropical scheme if you ask me...
Regards
Bandil
References & notes:
[1]:
https://www.thevespiary.org/rhodium/Rhodium/chemistry/equipment/dryingagent.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/equipment/dryingagent.html)
[2]:
The sieves where reused from similar reactions, oven dried for one hour at 250°C and then nuked for 1 min in the microwave immediately prior to use.
Bandil had this idea, too...
Post 513755 (https://www.thevespiary.org/talk/index.php?topic=10164.msg51375500#msg51375500)
(Bandil: "Easy preparation of HCl salt w/o gassing", Newbee Forum)
(and you won't need to add acetone before boiling off the solvent, but afterwards, to induce crystalization)
indole_amine