Best Method
Through experimentation, it was determined that the best method for extracting the MeNH2 from the aqueous solution is to raise the temperature of the solution while stirring. Gas is produced immediately upon stirring at standard temperature, and pressure and the solution begins to boil at 60 ºC. Copious amounts of MeNH2(g) can be obtained by gradually increasing the temperature of the solution between 60 ºC and 80 ºC at normal pressure. A reflux condenser and a gas washing tube filled with anhydrous MgSO4 to pre-dry the gas and 3A molecular sieve to provide a final drying are sufficient to remove any water vapor. (Note, use of NaOH to dry the gas is not recommended for the following reason: NaOH will form a hard moist cake at the vapor/NaOH interface. This thin cake will eventually impede gas flow, raising line pressure, and causing a joint to pop or explode--escaping toxic noxious MeNH2 fumes will quickly render the workplace uninhabitable. This is a fact based on experience, not idle speculation, so avoid the posted suggestion to use NaOH to dry MeNH2 gas: that suggestion is unsound and although a person can “get by” in the short term, in the long term it will eventually lead to catastrophe.)
As the temperature of the solution increases to 80 ºC, water vapor is observed condensing in the lower half of the reflux condenser. After a time at the same temperature, the production of MeNH2(g) begins to decrease. At this point, the partially spent solution is allowed to cool and is pumped out of the reactor flask (a peristaltic pump is ideal) and into a plastic (HDPE) carboy for further treatment to recover the remaining MeNH2. The reactor is reloaded with fresh 40 % solution and stirring and heating are applied, as above, until the temperature reaches 80 ºC and gas production diminishes, whereupon this partially spent solution is added to the contents of the carboy, and the reactor reloaded. In this fashion, a substantial amount of 40 % solution can be processed, without having to take the gas apparatus apart, nor expose the work area to MeNH2 fumes, nor with any solid residue remaining in the boiling flask.
The partially spent solution can then be further treated to obtain practically 100% of the remaining MeNH2. This is accomplished by addition of muriatic acid according to the reaction MeNH2(aq) + HCl(aq) = MeNH2•HCl(aq). The reactor should be maintained in an ice bath during acid addition because there is substantial heat generated by acid addition. Upon neutralization, the MeNH2•HCl solution is brought to a boil, the water and any MeNH2 vapor recovered by condensation, and the dry MeNH2•HCl can then be reacted with saturated NaOH solution to generate MeNH2(g) according to the reaction:
MeNH2•HCl(s) + NaOH(aq) = MeNH2 (g) + NaCl(aq)+ H2O
There is an advantage obtained by first boiling off the major part of the MeNH2(g), prior to acid addition, since substantially less acid is consumed and substantially less hydrochloride salt is produced, therefore less NaOH is needed to convert the hydrochloride to a gas, etc., and less mess and hassle overall. In fact, a person could produce all the MeNH2 he or she might need by simple stirring and boiling of the initial 40 % solution, easily unloading and reloading the reactor, and save the task of reacting the spent solution with muriatic acid for some later date.
Absorption of MeNH2 in MeOH
It is assumed that members reading this post understand that one of the reasons for generating MeNH2 gas is to absorb that gas in cold, stirred MeOH. By weighing the MeOH before and after gas dissolution, the amount of MeNH2 recovered can be calculated, which is required for additional syntheses. Please note that some people suggested using a dispersion tube when absorbing MeNH2(g) into MeOH. That advice is not warranted and is unsound because it will increase line pressure on the system, which could lead to catastrophe. Do not follow those suggestions-do not use a dispersion tube. MeNH2(g) is readily absorbed in cold MeOH. The b.p. of MeNH2 is -6 ºC, so a salt water/ice bath is sufficient to condense vapors. Besides, MeNH2(g) is absorbed in MeOH at just about any temperature short of hot. 1/2” OD polyethylene tubing from the hardware store without any sort of dispersion device on the end of the tubing is sufficient for this purpose.
Suck-back control
Suck-back occurs when the amount of gas being generated is insufficient to compensate for the amount of gas being absorbed. When the production of gas diminishes, suck-back will occur. The suck-back of MeOH with MeNH2 gas can be rapid and violent. Continual suck-back indicates that it is time to replace the spent solution in the reactor with fresh 40 % solution. Suck-back is controlled by stop-cocks to relieve line pressure. However, a trap must be installed between the gas drying tube and the receiver for those inevitable times when the operator looks the other way and valuable MeNH2/MeOH solution is sucked into the system. The trap must be larger than the volume of MeOH in the receiver, so that nothing is lost and nothing can reach the reactor. If MeOH were ever to be sucked back into the hot reactor where the temperature is above the b.p. of MeOH, the resulting explosion of glassware will get everybody’s attention. But be assured that this is not possible with the described set-up.
