Preparation of Manganese(III)acetate


Manganese(III)acetate from KMnO4

Manganese(III)acetate can be made from any water-soluble manganese(II) salt, or from the corresponding acid soluble hydroxide (Mn(OH)2) and oxide (MnO) by precipitating it as the carbonate salt, and then boiling it in acetic acid to form manganese(II)acetate, which is finally oxidized to manganese(III)acetate by potassium permanganate. Potassium permanganate can also be used as a starting material itself, by reducing it with hexamine or formaldehyde solution. All manganese(II) salts are suceptible to oxidation to manganese(IV) by oxygen, so try to exclude air as conveniently possible from these compounds and their solutions.

Theory of the reaction, by Rhodium:

At first, the Hexamine is hydrolyzed by HCl to ammonium chloride and formaldehyde:

C6H12N4 + 4 HCl => 6 HCHO + 4 NH4Cl

The KMnO4 is then reduced to Mn2+ in acidic solution, when the formaldehyde is oxidized to the acid:

2 KMnO4 + 5 HCHO + 6 HCl => 2 MnCl2 + 2 KCl + 5 HCOOH + 3 H2O

Or alternatively by the formaldehyde being oxidized all the way to carbon dioxide:

4 KMnO4 + 5 HCHO + 12 HCl => 4 MnCl2 + 4 KCl + 5 CO2 + 16 H2O

The MnCl2 in the solution is then precipitated as the water-insoluble Manganese(II)carbonate:

MnCl2 + 2 NaHCO3 => 2 NaCl + MnCO3 + CO2 or MnCl2 + Na2CO3 => 2 NaCl + MnCO3

The carbonate is then converted to the Manganese(II)acetate

MnCO3 + 2 AcOH => Mn(OAc)2 + CO2 + H2O

As the last step, potassium permanganate oxidizes Manganese(II)acetate to Manganese(II)acetate

4 Mn(OAc)2 + 5 HOAc + KMnO4 => 4 Mn(OAc)3 + KOAc + MnO2

Thus, it can be concluded that either hexamine or formaldehyde can be used for the reduction, and that either sodium bicarbonate (baking soda, NaHCO3) or sodium carbonate (washing soda, Na2CO3), if you balance the proportions of the reactants appropriately.


Procedure, by Antoncho:

15 g KMnO4 was dissolved in some hot water and combined w/a soln of 15 g hexamine. Nothing happened. Then conc HCl was added in portions w/swirling until mixtr acidic. As he did it, the mxtr gradually turned completely clear and the stench of formaldehyde became almost unbearable . Then SWIM put the soln in a 5 liter jar and mixed it w/a molar excess of saturated NaHCO3 (you can calculate it if you want - SWIM just took a large xcess). White precipitate (MnCO3) forms. The jar was filled w/water to wash it. DO NOT use NaOH for precip'ing manganese - Mn(OH)2 instantly oxidizes when exposed to air, and the carbonate is pretty stable.

When all settled, the water was decanted into another jar (to collect the still remaining suspended MnCO3 later) and the precip't washed in the same manner once again, MnCO3 in the other jar washed later too. Then all combined, water decanted off, precip't vac. filtered - do not dry it - there's no need for that, and it WILL oxidize somewhat during drying. Just scrape it off the filter and dump into some GAA. Now, guess what, there was no fizzling and bubbling at this stage as SWIM expected, in fact, to dissolve all the MnCO3 it took a 2 hour reflux. At this stage the soln was a pleasant pink color, just as it should have been.

The GAA was boiled off overnight at 130C in an oil bath and when SWIM woke up he found that the bottom of the flask was covered w/some white (not pink) crystals - well, he thought, that must be unhydrous Mn(AcO)2 (the pink stuff is a tetrahydrate) - and so it turned out to be. SWIM dosn't remember the exact weight (21 g?) but the yield was quantative.

Now SWIM went on to dissolving his salt in GAA, 250 ml (SWIM thought you'd have to 1st evap. GAA and then dissolve in it again as the former one has water in it - but he was wrong as you'll see. Well, he needed to weigh it anyway. Just skip this evap'n/dissolution step, it's probably not needed) - the shit doesn't dissolve! even w/reflux! Probably, SWIM thought, the unhydrous stuff doesn't, so he added a theoretical amount of water, well, a little more in fact, about 7 ml to the mxtr and w/some boiling most of it went in the soln, some was still left at the bottom - it was OK, as it turned.

To this boiling soln SWIM added about 5,4 g (~1/4 molar equivalent) of KMnO4, in, say, 6 portions. During and after each addition the mixture was vigorously stirred with a glass rod. The KMnO4 must be prior to that ground as fine as possible - it's not hard at all. The mixture turned dark and opaque. Boiling was continued for some 10 min, then the flask put into the fridge (16C) and 3 ml water added to it to induce crystallization. The walls of the flask were periodically furiously scratched (on the inside) with a glass rod and after like 10 hours - voila! - a heavy crop of dark brown, as they should be, crystals precipitated. (If it doesn't, you can add 3 ml more water to it and w/some scratching it'll happen in an hour).

The mixture was filtered, the mother liquor still very dark (put 3 ml water into it and let stand for a week - all of your prouct will crystallize and the liquor will turn colorless, this time the crystals will be not dark brown, but of rust/cinnamon powder color, just like the dry stuff), crystals sucked as dry as possible and dried for ~3 days in a dessiccator over CaO. Dry Mn(AcO)3 is a very, very fine rust colored powder, it's unbelieveable how much it increases in volume comparing to the initial KMnO4! Still the weight is consistent - SWIM got about 22 g from the 1st crop, and the 2nd one is still in the flask, probably will be ~5-7 g - the original article states the yield of 85%, fits w/SWIM's result perfectly.


Manganese(III)acetate (dihydrate) [1]

In a 5000ml, 4-necked round-bottom flask fitted with a stirrer, condenser and a thermometer, 428.7 g of Mn(OAc)2.4H2O in 3000ml of glacial acetic acid was heated to 110C. Ground KMnO4 (68.2 g) was added in small portions through the condenser over a 20 min. period, while the temperature was maintained at 110C. The reaction mixture was heated an additional 20 min., cooled, poured into water (750 mL), and left to crystallize overnight. The solid was filtered off, washed with ether and air dried. Yield: 486 g (82%) of Mn(OAc)3.2H2O.

Manganese(III)acetate (dihydrate) [2]

303g Anhydrous Mn(OAc)2 in 4400g glacial acetic acid was refluxed for 2h. 68.2g KMnO4 was added and the solution further refluxed for 45 min. The solution was allowed to cool while 750ml water was added dropwise. After 3 days standing, the precipitate was filtered off and recrystallized from a mixture of 2100ml acetic acid and 425ml water to afford very pure Mn(OAc)3.2H2O. [No yield given]

Manganese(III)acetate (anhydrous) [1]

A mixture of Mn(NO3)2.6H2O (60 g) and acetic anhydride (225 mL) was stirred vigorously in a 4-l. beaker and heated cautiously until the cloudy mixture suddenly turned clear. The heating was stopped and a vigorous rxn followed during which there was a copious evolution of NO2. The product was allowed to crystallize overnight, filtered, washed with acetic anhydride, and dried over P2O5. The anhydrous Mn(OAc)3 thus obtained titrated as 90-98% Mn(OAc)3. Yield: 70-90%.


References

[1] J. Am. Chem. Soc., 51, 138-145 (1969)
[2] J. Am. Chem. Soc., 88, 5743 (1966)