Sciencemadness Discussion Board

Formic Acid Decomposition

tekkado - 19-4-2017 at 21:45

Today I set up for preparing some formic acid from glycerol and oxalic acid. Following from nurdrages youtube guide; heating 100ml of glycerol with 100gm of oxalic acid with stirring up to the 105oC range. (I havent got used to my hotplate yet and over shot the mark. It was taking forever to get upto 100oC at the stillhead but there was distillate coming over.) I tuned up the temp a bit and got into the right temp range but then it got upto 120oC at the still head and I noticed that the reaction mixture had turned black and was effervesing. Some drops would fall from the thermometer and react with the mixture.
At this point I ended the reaction and collected what I had. I'm not sure of what formic acid smells like but it had a strong solvent type smell and there was ~90mls collected.
My question is should I be more patient and let what distillate that is coming over continue to come over before raising the temp again?
Im assuming I was too eager to complete the reaction and decomposed the reactants?

clearly_not_atara - 20-4-2017 at 00:13

I do not think it's wise to smell a mixture of acids from which you are distilling formic acid. Carbon monoxide is the decomposition product. In your case, decomposition of formic acid was not too much because you are still alive. Be thankful.

However other components of the reaction mixture can decompose. Allyl alcohol is a major product. This smells bad.

tekkado - 20-4-2017 at 00:43

I was in a ventilated area but could pick on some smells.
Would storing the crude mixture present any problems reacting with each other in the bottle?

Deathunter88 - 20-4-2017 at 07:52

Quote: Originally posted by tekkado  
I was in a ventilated area but could pick on some smells.
Would storing the crude mixture present any problems reacting with each other in the bottle?


The problem with carbon monoxide is that even in a well-ventilated area it's far too easy to inhale a fatal dose without realizing at first (when it is being produced chemically).

PirateDocBrown - 20-4-2017 at 08:03

CO has no smell at all.

HCO2H has a very sharp acid smell.

Alice - 20-4-2017 at 08:25

Interaction of Glycerol and Oxalic Acid.

Acrolein is another minor product.

S.C. Wack - 20-4-2017 at 15:55

Quote: Originally posted by tekkado  
but then it got upto 120oC at the still head and I noticed that the reaction mixture had turned black


It's toilet cleaner now. You may want to look up the maximum boiling azeotrope of formic acid. 120 is the maximum temperature for in the flask. It shouldn't get dark...it's a very recyclable reaction, the same batch of glycerol can be heated over and over with infinite kgs of oxalic acid. There is surely a thread here with the correct stepwise mode of operation.

[Edited on 21-4-2017 by S.C. Wack]

Boffis - 21-4-2017 at 06:08

Forgive me, but isn't this the synthesis of allyl alcohol? Check out Org Syn Col. Vol 1 page 42. Only a small part of the oxalic acid is converted to formic acid the rest comes of as allyl formate with the allyl alcohol. I have seen no reference to significant CO being produced in this reaction though it may be at the higher temperature used at the end of the formic acid stripping stage when CO2 is evolved and there is break down of the glycerol esters to give the allyl derivatives.

Alice - 21-4-2017 at 07:37

@Boffis, from the paper I've cited before:
Quote:
ALTHOUGH
the interaction of glycerol and oxalic acid is no longer
employed to prepare formic acid on a large scale, it is still the
simplest process for obtaining a small quantity in the laboratory,
and is a practical exercise habitually performed by students. The
more complicated decomposition which takes place when the initial
product is heated to a higher temperature is by far the most
convenient source of allyl alcohol, and is invariably used for its
preparation.


http://pubs.rsc.org/en/content/articlelanding/1914/ct/ct9140...

tekkado - 21-4-2017 at 13:25

Quote: Originally posted by S.C. Wack  

It's toilet cleaner now.

Like actually useful? Or is that expression to flush it Haha :D

S.C. Wack - 21-4-2017 at 13:53

The distillate might have sterilizing properties...IME black means finely divided carbon and a mess for a couple flushes.

So here's part of the Mann and Saunders entry btw. I didn't follow this but it was something like that. They and others make lead formate, but I was able to concentrate a fair amount with only distillation IIRC. I also never dehydrated USP glycerin. The oxalic acid would be the better option for working on the water content.

"Since glycerol is a very hygroscopic substance, it may be necessary to ensure that the sample used is anhydrous. For this purpose, place about 70 ml. in a porcelain evaporating-basin, and heat it carefully over a gauze (preferably in a fume-cupboard), stirring it steadily with a thermometer until the temperature is 175-180°: then maintain this temperature for a further 5 minutes. Allow the glycerol to cool, but while it is still warm (i.e., before it becomes viscous) pour 50 ml (63 g.) into a 250 ml distilling-flask containing 40 g. of powdered crystalline oxalic acid. Fit a thermometer in the flask so that the bulb is completely immersed in the glycerol mixture, and then fit a water-condenser to the flask. Heat the mixture carefully over a gauze so that the temperature rises to 110-120°, and then adjust the heating so that the temperature remains within these limits. A vigorous effervescence of carbon dioxide occurs, and the aqueous formic acid begins slowly to distil over. When the effervescence tends to subside, remove the Bunsen flame and allow the temperature to fall to 70-80°: then add a further 40 g. of powdered oxalic acid, and continue the heating as before. Ultimately 25-30 ml. of distillate are obtained, the total period of heating being about 1 hour."

tekkado - 22-4-2017 at 01:15

Ive read the above procedure before and following on from the procedure I poorly followed on Nurdrages channel could the above be applied to it? I.e. 100ml glycerol, dry it then add 100gm oxalic acid?

S.C. Wack - 22-4-2017 at 12:30

Not exactly no. It is not sensible to dry glycerol that isn't wet and breaks down thermally, then add a dihydrate to it; the addition must be made over and over. After doing that a few times only then you'll get good formic acid with every fresh addition. Formic does not have a solvent smell and you'll know it when you get it. Actually you won't know it it will be unfamiliar, but you'll be made to see; your nose will let you know.