Organic Chemistry : Help with Organic Carbohydrates
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Example Question #11 : Help With Organic Carbohydrates
Which of the following correctly describes a reducing sugar?
None of these
A reducing sugar has an acetal/ketal group when in its cyclic form, and is able to oxidize other chemicals (while itself being reduced).
A reducing sugar has a hemiacetal/hemiketal group when in its cyclic form, and is able to reduce other chemicals (while itself being oxidized).
A reducing sugar has a hemiacetal/hemiketal group when in its cyclic form, and is able to oxidize other chemicals (while itself being reduced).
A reducing sugar has an acetal/ketal group when in its cyclic form, and is able to reduce other chemicals (while itself being oxidized).
A reducing sugar has a hemiacetal/hemiketal group when in its cyclic form, and is able to reduce other chemicals (while itself being oxidized).
A reducing sugar contains a hemiacetal/hemiketal group which means that in its open chain form it contains a ketone/aldehyde group. Sugars containing a free aldehyde group can be oxidized to a carboxylic acid, while sugars containing a free ketone group must be tautomerized to an aldehyde group through an ene-diol intermediate (shown below), and this can be oxidized to a carboxylic acid.
Reducing sugars are detectable with the formation of either a precipitate or a solution color change after addition of Tollens' Reagent, Benedict's solution, or Fehling's solution.
Example Question #11 : Help With Organic Carbohydrates
Alpha-D-glucopyranose:
Glucose (pictured) is defined as which of the following?
Triose phosphate
Aldopentose
Ketohexose
Aldohexose
Ketopentose
Aldohexose
Glucose in its open chain form (shown below), has a free aldehyde group (an aldose), and it contains six carbons (a hexose). Together, glucose is shown to be an aldohexose.
Example Question #12 : Help With Organic Carbohydrates
Alpha-D-glucopyranose
Which of the labelled carbon atoms is the anomeric carbon?
C
A
B
D
E
A
The anomeric carbon is formed from the original carbonyl carbon in the straight-chain form of the molecule being attacked by a hydroxyl group to form a hemiacetal. This is seen as a carbon that is bonded to two oxygen atoms. In the case of glucose, the carbon labelled A is a hemiacetal, and is considered to be the anomeric carbon.
Example Question #13 : Help With Organic Carbohydrates
Shown below is the structure of a sugar molecule.
Which of the following is the most appropriate classification of this sugar?
Ketoheptose
Aldoheptose
Aldohexose
Ketohexose
Aldoheptose
In this question, we're given the structure of a sugar molecule, and we're asked to identify which answer choice represents the correct identification of this molecule.
To answer this question, there are two things we need to look at. For one, we need to determine whether it is an aldehyde sugar (aldose) or a ketone sugar (ketose). The first carbon atom in the molecule (shown at the very top in the image) is shown as 
Next, we need to look at the number of carbon atoms in the molecule. In the image shown in the question, there are a total of seven carbon atoms. Thus, this sugar would be classified as a heptose sugar (seven carbons) rather than a hexose sugar (six carbons).
Putting these two pieces of information together, we know that the sugar is both an aldose and a heptose. Therefore, this sugar is an aldoheptose.
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