Benedict's test, reducing sugars?

[vlct0ria]

There are a couple Destroyer OChem questions about Benedict's test and reducing sugars...I never learned that stuff in orgo I & II, and I am kind of confused by what Benedict's tests for (ie. what a positive result reveals), and what characteristics make a sugar a reducing sugar. If anyone can clarify this in the simplest possible terms, I would really appreciate it!

Also- I didn't see anything about carbohydrates in the DAT content specifications for OC, but its covered in KBB and Destroyer...does that mean its fair game? Has anyone actually gotten questions on sugars/carbs during the real DAT?

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[txchexmex]

I believe a reducing sugar is something that can form an aldehyde or ketone when opened up. A reducing sugar also can mutarotate.

For carbohydrates, studying the KBB and the questions on destroyer should be sufficient for the test. I think I had one question on carbs for the real DAT.

 

[Shinpe]

Refer to this thread for a list of some ochem tests I saw while studying:

http://forums.studentdoctor.net/showthread.php?t=650702

A reducing sugar is one that can be oxidized (thus being a reducing agent). As you'd read in Kaplan's BB, aldose and ketose sugars isomerize between linear and ring form. In linear form, aldoses have an aldehyde group which can be oxidized to carboxylic acids (by reagents such as those in benedict's test, tollen's, etc), making them reducing sugars. It seems that ketose sugars can also become aldehydes in their linear form with a series of shifts of that carbonyl group to one end, so they're considered reducing sugars also.

So which ones are not? Thos that can't open up their ring (I think) like sucrose because the hemiacetals are now converted to acetals. This doesnt necessarilly mean disaccharides as maltose and lactose are still reducing because one of the two rings still has that hemiacetal.

Hope that helps. ;p