Sciencemadness Discussion Board - NaOH + lactic acid = strange smell

NaOH + lactic acid = strange smell

Adas - 11-1-2013 at 05:19

Hello guys,

I was trying to decarboxylate some lactic acid with NaOH. Here is what I did:

I put few pearls of NaOH into a test tube. Then I added some 80% lactic acid (food grade, brown-colored like vinegar). The NaOH has just been wetted by it. Then I heated the mix on a candle flame. It started boiling, everything dissolved, and a clear liquid appeared on the sides of the test tube. Don't know whether it was just water distilled off or ethanol, but when it touched the hotter part of the test tube, it boiled instantly. Then I added some water to cool it down. There was a little "poof" due to the superheated NaOH solution. After this, I observed strange smell, which I know pretty well, but can not identify. I think it smelled like dung-water or something like that.

What do you think, what caused the smell? Some kind of lactide formed? Or did the NaOH react with the coloring substance? Does anyone have time to reproduce this little experiment? (Of course without putting water into the superheated liquid

)

Thanks for your time.

vmelkon - 11-1-2013 at 07:11

You said "it started to boil". Your lactic acid was a solution in water?
I think for decarboxylation to occur, water must be eliminated. The required temperature is probably above 200 °C or 300 °C.
The product should be ethanol. One way to test it is to measure density, boiling point, smelling it and also adding it to K2Cr2O7 which should become green.

I don't have any lactic acid, so I can't do that experiment.

Adas - 11-1-2013 at 07:23

Quote: Originally posted by vmelkon

You said "it started to boil". Your lactic acid was a solution in water?
I think for decarboxylation to occur, water must be eliminated. The required temperature is probably above 200 °C or 300 °C.
The product should be ethanol. One way to test it is to measure density, boiling point, smelling it and also adding it to K2Cr2O7 which should become green.

I don't have any lactic acid, so I can't do that experiment.

Yes, as I stated it is a 80% solution. I smelled it but I could only smell that strange smell I described. But if what you say is true, that it needs 200-300°C, then no decarboxylation could have occured. I want to know what caused that strange smell, though. It doesn't have any list of additives so I can only wonder...

morry-cule - 11-1-2013 at 14:01

Perhaps it underwent a base catalyzed dehydrogenation and formed acrylic acid.

Ozone - 11-1-2013 at 14:38

That would be dehydration, and it is catalyzed by acid (to create water as a leaving group). My guess would be that you got CO2 and ethanol. It is possible that other stuff (impurities) in your lactic acid may have been degrading (or reacting with ethanol) to create strange-smelling compounds.

O3

Nicodem - 12-1-2013 at 03:15

Quote: Originally posted by Adas

What do you think, what caused the smell? Some kind of lactide formed?

In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.
Anyway, the mixture of pyrolysis products of sodium lactate is expectedly quite a complex crap. The first google hit would have told you that, if you would've taken a minute to search for the literature. Ethanol obviously cannot form, because there is no mechanistic pathway for its formation. There are however plenty of pathways for the poly-condensation products and their decomposition/decarboxylation/disproportionation products:

http://legacy.library.ucsf.edu/documentStore/b/k/j/bkj35e00/...

At lower temperatures, such as your melt of sodium lactate, lower molecular weight crap probably prevails. In any case, crap commonly smells like dung, so that I would dare to say that the results of your uncontrolled experiment are in accordance to the theory of organic reactions.

Adas - 12-1-2013 at 07:52

Quote: Originally posted by Nicodem

I searched on Google but didn't seem to find anything useful.

Thanks for the replies, guys

DieForelle - 15-1-2013 at 08:46

"Then I added some 80% lactic acid (food grade, brown-colored like vinegar)"
"it is possible that other stuff (impurities) in your lactic acid may have been degrading"

The two solutions of 85% Lactic Acid from 2 different vendors (a lab supplier and a wine making supplier) I've seen in my life were a light straw color, not brown.

[Edited on 15-1-2013 by DieForelle]

Adas - 15-1-2013 at 10:39

Quote: Originally posted by DieForelle

"Then I added some 80% lactic acid (food grade, brown-colored like vinegar)"
"it is possible that other stuff (impurities) in your lactic acid may have been degrading"

The two solutions of 85% Lactic Acid from 2 different vendors (a lab supplier and a wine making supplier) I've seen in my life were a light straw color, not brown.

[Edited on 15-1-2013 by DieForelle]

Yes, I meant that, I am sorry.