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Identifying reducing / non-reducing sugar been confusing me for a while now , I know that reducing sugar contain aldehyde or ketone group . It's easy to identify them in monosaccharides but this becomes confusing in case of disaccharides! enter image description here

I fail to see carbonyl group here, How can I identify reducing / non-reducing sugar by looking at structure in picture? can anyone help me out?

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Many sugars are drawn in the cyclic, closed form where the carbonyl group has been converted to a hemiacetal.

enter image description here

Once you realize that a hemiacetal can equilibrate with a carbonyl (e.g. it is a carbonyl in disguise), identification of reducing sugars becomes easier.

Here is an example from Wikipedia, it is Maltose, the same as your third compound

enter image description here

See the aldehyde in the open isomer drawn on the right; see how it can close to form a hemiacetal on the left.

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  • $\begingroup$ The way I always heard what that (in a sugar) was that the anomeric C atom was the one bound to two oxygens. And if one of those two oxygens was part of an OH group, then the sugar was a reducing sugar. $\endgroup$ – Curt F. Mar 18 '15 at 8:47
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The right most c of maltose contains one OH bond and one OR bond and one H and one R. So it is a reducing sugar. R is any alkyl group. And this is the condition for reducing sugar.

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  • $\begingroup$ Actually, this does answer the question with the exception of not going into detail with respect to hemiacetal formation and breakdown. $\endgroup$ – Jan Nov 19 '17 at 14:44
  • $\begingroup$ You are right, it does answer the question. But I believe, an answer is not complete without explanation. So, to me, just stating that this is how it is and not explaining why does not satisfactorily answer the question. $\endgroup$ – basseur Nov 20 '17 at 10:21

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