Organic Chemistry : Help with Organic Carbohydrates
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Example Questions
Example Question #1 : Help With Organic Carbohydrates
Chemists and biochemists have many ways of representing sugars. Glucose, the most common hexose, is shown below in various linear and cyclic projections. Using the linear and cyclic projection of your choice, can you indicate which colored oxygen in the linear form corresponds to the circled hemiacetal oxygen once the cyclization reaction is complete?
Blue
Purple
Green
Yellow
Red
Purple
This answer, regardless of your preference of projection type, is easiest to obtain using arrow pushing for the cyclization reaction to keep track of each carbon and oxygen:
The purple carbon in the linear projection ends in the circled hemiacetal position.
Example Question #2 : Identifying Monosaccharides
Which of the following structures represents the anomeric alpha ring structure of D-glucose? 
When converting a linear sugar to its ring form, a bond is formed between the oxygen attached to carbon 5 and the carbon at position 1. All hydroxyl groups that are not attached to the carbon in position 1 and are oriented to the right end up trans to the 
If the hydroxyl group attached to carbon 1 ends up trans to the 
The alpha ring structure of D-glucose bonds the carbon 1 hydroxyl group trans to the carbon 5 
Example Question #3 : Identifying Monosaccharides
Which of the following structures represents the anomeric alpha ring structure of D-galactose?
When converting a linear sugar to its ring form, a bond is formed between the oxygen attached to carbon 5 and the carbon at position 1. All hydroxyl groups that are not attached to the carbon in position 1 and are oriented to the right end up trans to the
If the hydroxyl group attached to carbon 1 ends up trans to the
The alpha ring structure of D-galactose bonds the carbon 1 hydroxyl group trans to the carbon 5
Example Question #4 : Identifying Monosaccharides
Which of the following ring structures represents the anomeric alpha ring structure of D-mannose?
When converting a linear sugar to its ring form, a bond is formed between the oxygen attached to carbon 5 and the carbon at position 1. All hydroxyl groups that are not attached to the carbon in position 1 and are oriented to the right end up trans to the
If the hydroxyl group attached to carbon 1 ends up trans to the
The alpha ring structure of D-mannose bonds the carbon 1 hydroxyl group trans to the carbon 5
Example Question #2 : Help With Organic Carbohydrates
Identify the aldose pictured, including its alpha or beta designation.
The structure pictured is mannose because the hydroxyl groups at carbons 2, 3, and 4 are situated cis, cis, and trans (respectively) to the 
The mannose pictured is in alpha form because the hydroxyl group at carbon 1 is trans to the 
Example Question #3 : Help With Organic Carbohydrates
The Fischer projection pictured is a form of glucose. The carbon labeled "x" is the chiral carbon farthest away from carbon 1 and the hydroxyl group connected to carbon "x" is on the right. This fact designates that the glucose as what configuration?
Alpha
L
D
Beta
Pyranose
D
The chiral carbon farthest away from carbon 1 is designated as "D" if its hydroxyl group is on the right side in the Fischer projection. In other words, this is D-glucose because the hyroxyl group on carbon "x" is oriented to the right.
Example Question #4 : Help With Organic Carbohydrates
What is the name of the aldose pictured in this Fischer projection?
L-xylose
L-lyxose
L-fructose
D-ribose
D-arabinose
D-ribose
The structure is D-ribose because it is a five-carbon aldose with the hydroxyl groups on carbons 2, 3, and 4 all on the right in the Fischer projection.
Example Question #3 : Help With Organic Carbohydrates
Which of the following statements is true regarding carbohydrates?
The anomeric carbon is the site of attachment from one monosaccharide to another
Aldoses are more common in nature than ketoses
In nature, carbohydrates are usually found with a "D" conformation
All of these
All of these
All of these statements are true. A carbohydrate is said to have a "D" conformation in its acyclic form when the alcohol group on the carbohydrate's top stereocenter is on the right side in a Fischer projection. Most carbohydrates that we deal with in organic chemistry are aldoses, which means that they contain an aldehyde. The anomeric carbon is the site of attachment from one monosaccharide to another, and can be used to create polysaccharides.
Example Question #5 : Help With Organic Carbohydrates
What is the name of the following carbohydrate?
Alpha-L-galactopyranose
Alpha-L-glucopyranose
Alpha-D-galactopyranose
Beta-D-galactofuranose
Beta-D-galactofuranose
Alpha-D-galactopyranose
Stereochemistry from second to fifth carbon is R, S, S, R, which indicates D-galactose. The Haworth structure is a six-membered ring, so the molecule is in its pyranose form. The molecule has its anomeric hydroxyl group pointing down, so it's the alpha anomer.
Example Question #6 : Help With Organic Carbohydrates
What is the name of the following molecule?
Alpha-D-glucopyranose
Alpha-D-glucofuranose
Beta-D-galactofuranose
Alpha-L-galactopyranose
Beta-L-glucofuranose
Alpha-D-glucofuranose
Stereochemistry from second to fifth carbon is R, S, R, R, which indicates D-glucose. The Haworth structure is a five-membered ring, so the molecule is in its furanose form. The molecule has its anomeric hydroxyl group pointing down, so it's the alpha anomer.
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