Polverone - 29-5-2002 at 11:04
As several of my chemistry books mention, so-called "reducing" sugars will reduce a solution containing copper(II) compounds under alkaline conditions
to Cu2O. So last night I mixed 50 g of CuSO4*5H20 with fructose, added water, heated, and added NaOH solution. As expected, I obtained a precipitate
of Cu2O. Since the precipitate was fine, I wanted to clean it by repeatedly adding water, waiting for it to settle, and decanting. The solution I was
decanting was dark brown and had a very strong sugary/candy smell, one that I found very familiar yet couldn't exactly place. Since the books I have
on hand didn't mention, does anybody know what the fructose (or other reducing sugar) is oxidized to when the copper is reduced? I am wondering, in
particular, if the fructose is oxidized to a single compound or to a spectrum of compounds (as if I had heated it in the air).
Nick - 30-5-2002 at 04:38
Reducing sugars are reducing, IIRC, because their straight-chain forms are aldehydes, so I guess it'd be oxidised to fructic acid.
PHILOU Zrealone - 10-4-2003 at 12:57
HO-CH2-CHOH-CHOH-CHOH-CHOH-CH=O is an example of suggar (aldose)!
You see the aldehydic part and the polyol part! In certain suggar (cetoses) the ceto carbon is not at the end of the chain and is thus a ceton; the
cetose are non reducing suggars!
Also if you have a polyose often the C=O makes hemiacetals, acetals (in the case of aldoses) hemicetals and cetals (in the case of cetoses); if the
anomeric carbons (holding the aldehyde group or ceto group) of the two oses are linked that way there is no free reducing group and no reducing
abilities!
The tiniest aldose would be HOCH2-CH=O and the tiniest cetose HOCH2-CO-CH2OH!
As expected hydroxyacetal displays suggar reducing properties!
The reaction can be extrapolated to the aldohexose I have typed in the first line:
2HOCH2-CH=O + 2Cu(OH)2 -OH(-)-> 2HOCH2-CO2(-) + Cu2O + 3H2O
The result is a polyolcarboacid!But only for aldoses and reducing polyoses (thus displaying a free aldehyde or hemiacetal!).