Adas
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Monosacharide + NaOH yields red color???
Hello,
Today I was very surprised when I tried this experiment. I put a bit fructose with NaOH into a test tube and put some water in it.
But I was quite surprised when I got the red color. I heated it a little above a candle, the undissolved NaOH dissolved while making the water boil.
Very dark brown solution was observed.
The same worked with glucose, but with sucrose (table sugar) there was no reaction.
Note: In the test tube was a little red P coating (the red P comes from match boxes), which dissolved in the NaOH (or maybe it was just removed by the
boiling? I think red P doesn't react with boiling NaOH soln.). But the reaction worked even after the P was removed.
Also, the brown solution smelled somewhat milky (I can't explain better).
What the hell was that? I didn't find any infos. (Just some other tests for reducing sugars, but all of them used also Cu salts).
Thanks for your time.
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zoombafu
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How hot were you heating it? You could have 'burnt' the sugar. Also this should probably go in beginnings.
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ScienceSquirrel
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Thread Moved
9-3-2012 at 10:03 |
Adas
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Fructose has a melting point od 103°C. I doubt it would decompose at 100°C into RED color!
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bfesser
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<a href="http://en.wikipedia.org/wiki/Fructose#Dehydration" target="_blank">Fructose dehydration</a> <img src="../scipics/_wiki.png"
/> <em>may</em> yeild <a href="http://en.wikipedia.org/wiki/Hydroxymethylfurfural"
target="_blank">hydroxymethylfurfural</a> <img src="../scipics/_wiki.png" /> (HMF). HMF is purportedly yellow, and under strongly
alkaline conditions red wouldn't be surprising. It may also explain the smell you observed.
[edited to remove harshness, my apologies]
[Edited on 7/9/13 by bfesser]
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Mixell
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bfesser, don't be so harsh on him. Judging by the information he has provided, seems like it was all conducted in an aqueous medium, so the
temperature couldn't have risen well above 100C.
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Adas
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Yeah, it was in water, as I said in my first post 
And when you think it's HMF, why does it also work with glucose?
[Edited on 9-3-2012 by Adas]
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Ozone
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It is not HMF! That results from a sequence of dehydrations (E1) which are acid catalyzed.
When alkali is introduced into a mixture of carbohydrates, reverse aldol reactions occur (via tautomerization od adjacent hydroxyls). This leads to
scission through ene-diol intermediates and subsequent re-aldolization to yield (after dehydration) a complex mix of red-brown (eventually black)
conjugated products known as hexose alkaline degradation products or HADP. Eventually, a lowish molecular weight polymer can result. The mixture can
smell unpleasant, like sugarbeet molasses--beets are processed at pH 10 with Ca(OH)2.
This only works with monosaccharides with a reducing end. Non-reducing disaccharides such as sucrose are quite stable, reducing disaccharides such as
maltose are not.
Hope this helps,
O3
[Edited on 9-3-2012 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Ozone
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I should also add that glucose is quite capable of producing HMF... The rate (which is significantly smaller), however, is limited by the rate of
isomerism with fructose...and, apparently, mannose.
O3
[Edited on 10-3-2012 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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phlogiston
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Years ago, I got a similar result with Sucrose after dissolving it in 10% hydrochloric, and then adding little balls of solid NaOH to it. It resulted
in a very pretty dark red color.
I also do remember being able to extract the color from the aqueous solution with white spirit, but I never found out what causes the color.
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Ozone
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That is because the acid caused acid catalyzed inversion (hydrolysis of the glycosidic bond) of the sucrose to give one equivalent each of glucose and
fructose (both reducing monosaccharides). The addition of alkali to this mixture caused the formation the aforementioned HADP.
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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