Unit 22: Carbohydrates
Every earlier tool in one molecule: a Fischer projection's stereocenters, a hemiacetal closing into a ring, and a full acetal linking two rings together.
6 topics · 19 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Fischer Projections of Monosaccharides
- How it worksHow does a Fischer projection stack every stereocenter of an open-chain sugar into one clean, readable vertical column?
- Worked exampleFind the number of stereocenters in open-chain glucose and the maximum number of stereoisomers that implies.
- Going furtherWhat happens to a sugar's classification, aldose or ketose, based on where its carbonyl group sits in the chain?
Your turn1 of 2
An open-chain aldohexose sugar has 5 stereocenters along its chain. What is the maximum number of distinct stereoisomers this many independent stereocenters allows?D and L Sugars
- How it worksHow does the hydroxyl's position on the bottom stereocenter, farthest from the carbonyl, decide whether a sugar is D or L?
- Worked exampleFind whether a Fischer projection of glyceraldehyde with the hydroxyl on the right represents D or L glyceraldehyde.
- Common mistakeWhy doesn't D or L tell you anything about whether a sugar actually rotates light clockwise or counterclockwise?
Your turn1 of 2
In a Fischer projection of glyceraldehyde, the hydroxyl group on the single stereocenter is drawn on the right. Does this represent D or L glyceraldehyde?Cyclic Hemiacetal Formation and Anomers
- How it worksHow does the hydroxyl on carbon 5 of glucose reach around and attack its own carbonyl carbon to form a cyclic hemiacetal?
- Worked exampleFind whether the new anomeric hydroxyl points up or down when glucose cyclizes to form its alpha anomer.
- Common mistakeWhy aren't the alpha and beta anomers of glucose just two names for the exact same molecule?
- Ties togetherHow does this intramolecular hemiacetal formation reuse the exact same hemiacetal chemistry from the carbonyl unit?
Your turn1 of 2
When glucose cyclizes to form its alpha anomer, does the new anomeric hydroxyl point up or down in the standard Haworth-derived convention?Mutarotation
- How it worksHow does the cyclic hemiacetal ring open back up and re-close to interconvert the alpha and beta anomers in solution?
- Worked exampleFind the final equilibrium mix of alpha and beta glucose anomers, roughly 36 percent to 64 percent, once mutarotation finishes.
- Going furtherWhat happens to the observed optical rotation of a glucose solution as it moves from freshly dissolved to fully equilibrated?
Your turn1 of 2
At mutarotation equilibrium, a sugar exists as 41 percent alpha anomer, with specific rotation 125 degrees, and the rest as beta anomer, with specific rotation 38 degrees. What is the resulting weighted-average observed specific rotation?Glycosidic Bond Formation
- How it worksHow does an anomeric carbon's hydroxyl react with another sugar's hydroxyl under acid catalysis to form the glycosidic bond?
- Worked exampleFind the type of glycosidic bond, alpha-1,4 or beta-1,4, that links the two glucose units in maltose.
- Ties togetherHow does forming a glycosidic bond as a full acetal explain why it stays stable to base the way a protected acetal does?
Your turn1 of 2
What type of glycosidic bond links the two glucose units in maltose?Reducing Sugars and Oxidation Tests
- How it worksHow does a free anomeric hydroxyl let a sugar ring open back into its aldehyde form and get oxidized by a mild oxidant?
- Worked exampleFind whether maltose or sucrose gives a positive Tollens' test, based on which one has a free anomeric carbon.
- Going furtherWhat happens to a disaccharide's ability to reduce Tollens' reagent once both its anomeric carbons are tied up in a glycosidic bond?
Your turn1 of 2
Which gives a positive Tollens' test, maltose or sucrose, based on which one has a free anomeric carbon?