10-12-01 05:07
No 223472
For academic reasons, would one of the chemists explain the reaction mechanism when 40% MeAM/H20 is dripped over NaOH? Why is MeAM gas liberated? As I understand basic chemistry, NaOH ionizes in water to form Na+ and OH-, why would that cause MeAM to be expelled from water?
With an eye to improving the yield of MeAM gas, what procedure should one follow? How would one increase yields of MeAM gas, beyond gentle heating of the flask containing NaOH? I have considered an experiment for academic reasons, whereby a 1:2 w/w ratio of 40%meAM to NaOH is used. Would the knowledgable bee care to comment on ratios and yields as well? Stirring seems obvious, but HOW does one stir such a mess?
The equipment at hand for such an experiment include a 2L equalizing sep funnel with 24/40 fitting connnected to a claisen adapter inserted into a 3L reaction flask (the two-piece reaction flask with ground glass dome lid for easy disposal of waste) the gas being carried through PE tubing from the claisen (a 24/40 teflon stirrer bearing has been employed to provide the connection for the 1/2" NPT X 1/2" compression fitting to the 24/40 on the claisen) through a SS gas drying apparatus charged with MgSO4, all fittings being SS compression fittings--a ball valve is located on the drying apparatus to relieve negative pressure thus preventing "suck-back", when the NaOH loses umph. I assume everybody knows where the MeAM gas is being directed and why suck-back would occur.
All of this equipment would be under a good fume hood. Leaks detected with pH paper strips. And, I hope, enough lab sense not to get in trouble.
Always appreciative of the Hive's collective knowledge.
Regards
Argox.
(Chief Bee, Ex Hive Addict)
10-12-01 05:48
No 223495
NaOH is a much stronger base than MeNH2, so it rips the proton from the MeNH2*HCl, forming water (and NaCl). The formation of water is essentially irreversible.
And as free MeNH2 (without the hydrochloride group) is a gas, it evaporates.
(Old P2P Cook)
10-12-01 05:59
No 223496
explain the reaction mechanism when 40% MeAM/H20 is dripped over NaOH? Why is MeAM gas liberated?
It is that the NaOH has such a strong affinity for water that it forces the methylamine out of solution. A form of "salting out".
With an eye to improving the yield of MeAM gas, what procedure should one follow? How would one increase yields of MeAM gas, beyond gentle heating of the flask containing NaOH?
Only, perhaps by erducing the amount of water per given amount of methylamine. Perhaps by making the methylamine into the hydrochloride you could get a water solution containing a higher ration of methylamine to water.
The 1:2 ratio of methylamine solution to NaOH seems good. A lot of NaOH is good I wouldn't use any less NaOH. If you have the large pellets of NaOH then stirring really isn't necesssary, just some gentle heating.
(Stranger)
10-12-01 22:56
No 223755
Thank you for reply. I did not realize that 40% w/w MeAM in water was produced by dissolving MeAM.HCl in water. I had assumed that it was from the absoption of MeAM gas in water just as aqua ammonia is produced. But are you sure of your information? On further reflection, why would industrial producers first make MeAM.HCl (consuming HCl in prodigious quantity and raising costs) and then dissolve said hydrochloride in water, when for a lot less money they could just absorb MeAM gas in water? If you were correct, then I would observe several things, which in fact I DO NOT observe, principally:
1. The 40% w/w MeAM (derived from MeAM.HCl dissovled in water would be neutral or acid to pH and would consist of water, the MeAM ion, the chloride ion, and the hydrogen ion. In fact, the 40% MeAM is highly alkaline, which leads me to believe that the only ions present are the MeAM ion and the hydroxyl ion.
So can you confirm your reply?
It seems terbium is more on target, but is this really the explaination, a greater affinity for water on the part of NaOH?
Thanks for your time in answering.
Regards,
Argox
(Synaptic Cleft Mountaineer)
10-12-01 23:49
No 223786
Argox: I was talking shit above, I was tired and thought you were talking about using NaOH to get methylamine out of its hydrochloride salt.
Commercial 40% MeNH2 is indeed made by dissolving the gas in water directly.
(Stranger)
10-13-01 00:49
No 223810
Rhodium--you are a prince among men. Next time I go to Scanland, I'll buy you a beer, or whatever you drink.
Anyway, is that it? No more help on extracting as much MeAM from the 40%solution as is humanly possible, given that it isn't exactly easy to find. Any help appreciated.
(Synaptic Cleft Mountaineer)
10-13-01 00:53
No 223813
I believe that the most efficient method of turning 40% methylamine into dry gas (but not the easiest) would be to turn it into methylamine hydrochloride (by careful HCl addition with cooling), followed by evaporation of the water, and then add this as a concentrated solution onto solid NaOH. The methylamine will then be evolved as a gas.
(Hive Bee)
10-13-01 02:17
No 223842
SWIM has tried various methods to make anhydrous MeNH2; the results are as follows:
40% aq. MeNH2 + solid NaOH
SWIM could not get this to work with either NaOH or KOH, even with heating. I don't know why. Reagents were all lab grade. However, I understand this method should work.
Heating 40% aq. MeNH2
By simply heating 40% aq. MeNH2 soln., the gas is pushed out of solution. However, quite a bit of the water will also come, so it is important to dry the gas by passing it through solid NaOH with this method.
aq. MeNH2.HCl + solid NaOH
This method worked well - make a concentrated solution of MeNH2.HCl in H2O by neutralising 40% aq. MeNH2 with aq. HCl, then boiling until you get a saturated solution. Then drip this onto solid NaOH, stirring occasionally. You will get lots of MeNH2 gas. When gas evolution stops, heat the flask gently, and it will start again.
solid MeNH2.HCl + solid NaOH + H2O
This method works the best; mix thoroughly equimolar amounts of MeNH2.HCl and NaOH. Then add a few drops of water. Lots of MeNH2 gas will bee produced. You won't need to add much more water, as water is produced by the reaction. However, if you just add a few drops at the beginning and do not use external cooling, all water will boil off and the reaction will stop again. At some stage, you will probably need to cool the rxn with an ice-bath to prevent it becoming too vigorous. Its all a question of how much water there is. When MeNH2 evolution has stopped altogether, heat it gently.
If totally anhydrous MeNH2 is required, pass the MeNH2 generated through these procedures through solid NaOH first.
Have fun, and remember if you play with MeNH2 gas, your clothes will stink of fish for days
. And don't smoke at the same time...Sweet dreamz,
IudexK
(Hive Addict)
10-13-01 19:00
No 224041
The problem that occurs with aq. MeAm dripped onto solid base is heat evolution. It will pump out a lot of steam/water vapor. Dripping a concentrated solution of a base onto the MeAm.HCl avoids this problem.
4:20
(Old P2P Cook)
10-13-01 22:24
No 224089
The problem that occurs with aq. MeAm dripped onto solid base is heat evolution.
This is how I always used to do it. I would then pass the gas through a second flask filled with NaOH pellets to provide additional drying. I felt that the generated heat was useful for driving off the methylamine.
(Stranger)
10-13-01 23:24
No 224115
Thanks for the very measured responses. I don't recall that this subject has ever been discussed in so much detail, the FAQs are pretty lean when it comes to comparing the different ways of generating MeNH2.
IudexK2--thank you very much for the suggestions. SWIA wants to try the method whereby 40%MeAM is neutralized with HCl and the excess water driven off. SWIA would appreciate tips, such as:
1. How much heat is generated by the addition of muriatic acid to aqueous MeNH2? How bad is the evolution of MeNH2 gas during the addition? SWIA imagines cooling the aqueous MeNH2 beforehand and putting the flask containing the MeNH2 in an ice bath during acid addition. Any other tips? Mag stirring seems obvious, right?
Final question. One mol of PMK reacts with one mol of MeNH2 during red. amin, correct? Some MeNH2 will react with MeOH, but not much with the cooling applied, correct? So why do the red amin write-ups (hydrogenation and NaBH4) call for such as excess of MeNH2? What is the lower limit of MeNH2 that can be used in the NaBH4 red. amin.?
Again, thanks for the time spent replying.
Regards
Argox
(Hive Bee)
10-13-01 23:59
No 224132
To neutralise 40% MeNH2 with HCl, do the following:
Measure out equimolar amounts of 40% MeNH2 and hydrochloric acid into separate containers and put them in a freezer for for hours. Take them both outside, and mix them together. Its actually easier to do it fast and then put a plate or something over the reaction flask than to do it slowly and be suffocated by the clouds of white vapour (MeNH2.HCl) given off. Swirl them around thoroughly, and check the pH. Adjust the pH until it is just above 7. This is to make sure your solution is not acidic, which would greatly interfere with some reactions. You can easily tell if the pH is above 7 by smelling it - if it smells of MeNH2, it is above 7. If it isn't, add 40% aq. MeNH2 dropwise while swirling until it is. You now have a solution of MeNH2.HCl in H2O, with a little tiny bit of MeNH2. Now you can either boil it down to get a saturated solution of MeNH2.HCl, or boil all the water off to get MeNH2.HCl crystals. Once you boil it, virtually all the free MeNH2 you used will be driven off. If you decide to boil off all the water to get MeNH2.HCl xtals, don't carry on heating after the water has all evaporated - the MeNH2.HCl will melt and looks just like water, except the "steam" coming off will look more smoky. This is your MeNH2.HCl boiling away. MeNH2.HCl is extrememly hygroscopic, so while leaving to cool, cover with something.
Although only 1mole of MeNH2 is theoretically needed in borohydride reduction, an excess is used to give higher yields. Much of the excess MeNH2 can often bee recovered from some reactions.
Sweet dreamz,
IudexK
(Old P2P Cook)
10-14-01 00:47
No 224151
Final question. One mol of PMK reacts with one mol of MeNH2 during red. amin, correct? ... So why do the red amin write-ups (hydrogenation and NaBH4) call for such as excess of MeNH2? What is the lower limit of MeNH2 that can be used in the NaBH4 red. amin.?
As you say, the theoretical molar ratio is 1:1. The actual ratio that you use depends on which is more precious to you - P2P or MeNH2. Most people consider P2P to be far more precious than MeNH2 so they use excess methylamine to drive the reaction equlibrium towards the imine. My feeling is that a 1.2X to 2X excess of methylamine is the correct range and if P2P is substantially more precious to you than methylamine then probably more towards the 2X excess of methylamine.
(Stranger)
10-14-01 00:55
No 224156
Thx IudexK2. SWIA will use pH paper rather than nose, and a hood rather than outside.
How does one recover MeNH2 from reaction products?
Hey terbium, thx also. Would you agree with adding aq. HCl to aq. MeNH2 SLOWLY in stirred reaction flask with equalizing addition funnel under a hood rather than outside, and maybe a reflux condenser or some sort of gas washing apparatus to control fumes? As opposed to adding acid to base quickly and outside.
(Old P2P Cook)
10-14-01 01:06
No 224165
Would you agree with adding aq. HCl to aq. MeNH2 SLOWLY in stirred reaction flask with equalizing addition funnel under a hood rather than outside, and maybe a reflux condenser
Yeah, that should work nicely.
I have never seen the need for a fume hood when working with methylamine - at most, just a box fan in a window. After all, do you need a fume hood when you use household ammonia to strip floor polish off a vinyl floor? IMHO, methylamine is no more stinky than household ammonia. I doubt that the inexperienced nose would even readily recognize the small difference in smell.