[divexo]

Hi, i've been researching for an hour, looking through all my notes and i absolutely have no idea how to identify the nonreducing sugar out of the below (see attachment), which one is the nonreducing sugar & why?

thank you so much

[LVJ]

A reducing sugar has a carbonyl group in the open-chain form, i.e. a non-reducing sugar doesn't. So in order to find out which one it is, try to think what it would look like if the sugars were open-chained. Drawing out reactions mechanisms would probably help.

[AWK]

A reducing sugar has a carbonyl group in the open-chain form, i.e. a non-reducing sugar doesn't.

aldehyde carbonyl !

[LVJ]

Both aldehyde and ketone carbonyls, since ketoses can tautomerize to aldoses. Anyway, if the sugar has a hemiacetal group in the cyclic form, it is reducing. If it contains an acetal, then it's not.

[divexo]

Thank you all.
So i see that the 'I' has an acetal, so does that mean that I is the nonreducing sugar  ??

[AWK]

Both aldehyde and ketone carbonyls, since ketoses can tautomerize to aldoses. Anyway, if the sugar has a hemiacetal group in the cyclic form, it is reducing. If it contains an acetal, then it's not.

But not during test for reducing sugars.

[divexo]

So... Is the answer the first, 'I'?

[Doc Oc]

No.  I isn't an acetal, it's a hemiacetal.

[Dan]

But not during test for reducing sugars.

LVJ is correct, ketoses will test positive. Fructose, for example, is classified as a reducing sugar.

Divexo: Hemiacetals and hemiketals are in equilibrium with their corresponding open chain aldehyde and ketone forms. It is these open chain forms that react with an oxidising agent, ketoses achieve this via tautomerism to aldoses. Acetals (or ketals) will test negative unless first hydrolysed (acid catalysed) to hemiacetals (or hemiketals).