3
$\begingroup$

I boiled a solution of glycerol and water at about 160 degree Celsius and got a brown liquid much less in volume (boiled off the entire water I think; also not so sure about the accurate temperature).

Before mixing glycerol with water it was a colourless liquid, now it is golden brown! Why would that be? Also, I smelled burnt sugar during the last couple of minutes of boiling. So I'd like to know what that gas was. So, do I have glycerin in my hand or any other byproducts?

Improve this question
$\endgroup$

2 Answers 2

Reset to default
10
$\begingroup$

If your mixture reached 160 degrees C then you must have boiled off all the water. If there was some water left, it would stay at around 100 degrees C and all (most of) the heat would be used to evaporate water.

The boiling point of glycerol is about 290 degrees C and so you didn't boil that off. I disagree with @Skeptical Chymist about you making the caramel (for the sake of god, don't eat it to test it out). Not all that gets brown is caramel and there were no sugars there I presume. If i had to guess I'd say that at 160 C you've managed to completely obliterate any complicated organic structures and what's left is glycerol, charcoaled bits of any organic impurities that you happened to have in the starting material and possibly some acrolein which is a common product of dehydration of glycerol (you can recognize it by it's smell).

Overall; it's hard to tell what one gets, when they heat the hell out of something.

Improve this answer
$\endgroup$
3
  • 3
    $\begingroup$ Apparently the process is worthy of a master's thesis. I disagree slightly with the 100 degree comment - don't forget intermolecular forces and boiling point elevation. $\endgroup$
    – bobthechemist
    Commented Jul 17, 2013 at 17:15
  • 1
    $\begingroup$ @bobthechemist I never said that it's not an interesting topic, but also: compare the burner design from the thesis you linked to heating something in a pot. It is much less controlled and thus all sorts of stuff can happen. (By the way, I enjoyed reading the thesis. Thank you.) About the boiling point elevation: surely not by 60 degrees? (I don't have a vary good intuition on the topic so I admit: I don't know) $\endgroup$
    – MarcinKonowalczyk
    Commented Jul 18, 2013 at 10:26
  • $\begingroup$ never intended to comment on your judgement of the topic, rather that I was surprised that someone wrote a thesis on it. I agree, a 60 degree elevation due to intermolecular forces is unreasonable. Further, since glycerol decomposes just below its boiling point, I suspect you are right in that contaminants in the glycerol are the culprit. $\endgroup$
    – bobthechemist
    Commented Jul 18, 2013 at 14:09
6
$\begingroup$

When heated to high temperatures, glycerol decomposes through dehydration to yield acrolein.

$$\ce{C3H3(OH)3 -> 2H2O + C3H4O}$$

Acrolein is a highly reactive unsaturated aldehyde. It can polymerize all by itself into a yellowish material, and can probably react with any other impurities or materials in your sample, which is my guess for what caused the brown gunk.

One point of uncertainty is the temperature at which acrolein can form. Wikipedia says decomposition starts at 280 °C, which is much hotter than the temperature you reported.

So even if there is not large-scale conversion of glycerol to acrolein, the lower temperatures you used maybe might have been enough for low levels of acrolein to form, which could react and cause browning.

A lot depends on how pure your water and your glycerol were.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ In support of the line of thought «formation of acrolein», the synthesis of quinolines by the Skraup reaction (an aniline and glycerol as an easier to carry protected form of acrolein, some Bronsted acid like polymeric $\ce{H3PO4}$) basically is boiling off water, too. (Somehow the question was shown today as «active», despite its age.) $\endgroup$
    – Buttonwood
    Commented Oct 13, 2021 at 9:44

Not the answer you're looking for? Browse other questions tagged or ask your own question.