Textbook Question
In an aqueous solution, D-glucose exists in equilibrium with two six-membered ring compounds. Draw the structures of these compounds.
28. Carbohydrates
Monosaccharides - Cyclization
4:39 minutesProblem 69
Textbook Question
Predict whether D-altrose exists preferentially as a pyranose or a furanose. (Hint: In the most stable arrangement for a five-membered ring, all the adjacent substituents are trans.)
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Analyze the structure of d-altrose: d-altrose is an aldohexose, meaning it contains six carbon atoms and an aldehyde functional group. It has four chiral centers, and its stereochemistry determines the orientation of its hydroxyl groups.
Understand the formation of pyranose and furanose rings: Pyranose rings are six-membered rings (five carbons and one oxygen), while furanose rings are five-membered rings (four carbons and one oxygen). Both forms result from the intramolecular reaction between the carbonyl group (C=O) and a hydroxyl group (-OH) on the sugar.
Consider the stability of the furanose form: In a five-membered furanose ring, the most stable arrangement occurs when adjacent substituents are trans to each other. Examine the stereochemistry of d-altrose to determine if this arrangement is possible and favorable.
Evaluate the stability of the pyranose form: In a six-membered pyranose ring, stability is influenced by the ability to adopt a chair conformation, minimizing steric hindrance and placing bulky groups in equatorial positions. Analyze the stereochemistry of d-altrose to determine if this conformation is more favorable than the furanose form.
Compare the two forms: Based on the stereochemical analysis and the stability considerations for both pyranose and furanose forms, predict which form is preferentially adopted by d-altrose. Remember that the most stable form will minimize steric hindrance and maximize favorable interactions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Pyranose and Furanose Forms
Pyranose and furanose are two cyclic forms of monosaccharides. Pyranose is a six-membered ring structure, while furanose is a five-membered ring. The stability of these forms is influenced by steric interactions and the orientation of substituents around the ring. Understanding these structures is crucial for predicting the preferred form of sugars like d-altrose.
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Chair and Boat Conformations
In the context of pyranose rings, the chair and boat conformations describe the spatial arrangement of atoms in the ring. The chair conformation is generally more stable due to minimized steric strain and torsional strain, allowing substituents to be positioned equatorially. This concept is essential for understanding how substituent orientation affects the stability of cyclic sugars.
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Trans and Cis Substituent Orientation
The terms trans and cis refer to the relative positioning of substituents on a ring structure. In a trans arrangement, substituents are positioned on opposite sides, which often leads to lower steric hindrance and greater stability. This concept is particularly relevant when predicting the preferred form of d-altrose, as the hint suggests that the most stable arrangement favors trans orientations.
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