Author Topic: Easy HCl salt preciptiation of amines via sieves  (Read 1510 times)

0 Members and 1 Guest are viewing this topic.


  • Guest
Easy HCl salt preciptiation of amines via sieves
« on: September 28, 2004, 12:05:00 AM »

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...


References & notes:

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.


  • Guest
If one just wanted to convert his freebase to...
« Reply #1 on: September 28, 2004, 12:35:00 PM »
If one just wanted to convert his freebase to hcl would it be fine if you just left the IPA out and once the sieves have removed the water wouldnt you just be left with 0.1 mole of meth hcl suspended in 150ml of acetone?

I guess that way is no different to gassing, but instead of using a salt to remove the water from the hcl and passing the gas through the solution you just use the sieves and do it all in one neat little package... i like it  ;D

two thumbs up


  • Guest
IPA is just for reXtalization/sieve retrievement
« Reply #2 on: September 28, 2004, 06:22:00 PM »
"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)"

Then you add your sieves, then the salt precipitates.

Up to this point, no IPA is needed!
The only purpose for adding it is removing the hydrochloride from the sieves AND recrystallizing it in one step, in a smooth way.

If you plan on separating the sieves and amine salt by other means (by hand maybe!?  :) ) and don't want to reXtalize your stuff anyway, you're fine without IPA...

(besides: two thumbs up, from me too!)



  • Guest
oww owww owww mr cotterrrr....
« Reply #3 on: September 29, 2004, 06:25:00 AM »
oww owww owww mr cotterrrr.... i just came up with an idea...

Lets just say you have your amine freebase in your NP, we'll say xylene in this case.
Now add equal amounts of acetone so when the hcl is added it will all be miscible.
Add your 30% hcl in molar ratios and the acetone/xylene/hcl/water will all be one layer.
Now mix/stir/shake this till you are certain all the amine has taken its salt.
Then all you would have to do is boil the solution at 100C until the acetone and water was all gone.
Filter off and wash with acetone  ;)


  • Guest
will work - but...
« Reply #4 on: September 29, 2004, 07:41:00 AM »
Bandil had this idea, too...

Post 513755

(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)