Br orgo carbohydrates

[drh531]

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I guess I am not understaning about these tests..
on page 112 (orgo II) example 6.15
which of the following sugars would have a positive Tollen's test, yield the same osazone as D-gulose, and be optically active after treatment with nitric acid?
a) D-Allose
b) D-Altrose
c) D-Idose
d) D-Tallose

The answer is D but I just dont understand this..
HELP please~~~😱

 

[BB8730]

That's one of the many annoying mistakes in TBR - the correct answer should be C.

But in case you didn't understand the problem I'll help break it down. Aldoses result in a positive Tollen's test, so eliminate all sugars that are not aldoses. In this case, they're all aldoses, so none are eliminated. Next, eliminate all sugars that would not have the same osazone test as D-Gulose, so eliminate any sugar that is not a C-2 epimer of D-Gulose. Of the answer choices, D-Idose is the only C-2 epimer of D-Gulose, so the answer is C.

 

[drh531]

Thank you so much!!!! 🙂
quick question..
do we have to know/memorize all these stuctures (gulose, idose, talose etc etc)??

 

[Whiteshoes]

That's one of the many annoying mistakes in TBR - the correct answer should be C.

But in case you didn't understand the problem I'll help break it down. Aldoses result in a positive Tollen's test, so eliminate all sugars that are not aldoses. In this case, they're all aldoses, so none are eliminated. Next, eliminate all sugars that would not have the same osazone test as D-Gulose, so eliminate any sugar that is not a C-2 epimer of D-Gulose. Of the answer choices, D-Idose is the only C-2 epimer of D-Gulose, so the answer is C.

Did they give the drawings or do we have to memorize what all those sugars look like. so you knew that they were all adloses by the -ose ending and in order to figure out if a reagent has a similar osazone test to (any compound) you check to see if it is different with respect to the 2nd carbon ie (C-2 epimer)?

 

[BB8730]

Thank you so much!!!! 🙂
quick question..
do we have to know/memorize all these stuctures (gulose, idose, talose etc etc)??

I believe the only structure we need to know from memory is glucose, but don't quote me on that. I'm definitely not memorizing all those in that question.

Did they give the drawings or do we have to memorize what all those sugars look like. so you knew that they were all adloses by the -ose ending and in order to figure out if a reagent has a similar osazone test to (any compound) you check to see if it is different with respect to the 2nd carbon ie (C-2 epimer)?

They gave drawings. The -ose ending doesn't mean aldose - ketoses end in -ose as well. You have to look at the structure to see the aldehyde/ketone. Yes, the osazone test tests for C-2 epimers.

 

[Lunasly]

Sorry to bump an old thread, but this is still a mistake in TBR OC 2014 edition in case anyone is still wondering if they corrected for it. 🙁