Textbook Question
Classify the following monosaccharides. (Examples: D-aldohexose, L-ketotetrose.)
28. Carbohydrates
Monosaccharide
3:19 minutesProblem 42
Textbook Question
A student isolated a monosaccharide and determined that it had a molecular weight of 150. Much to his surprise, he found that it was not optically active. What is the structure of the monosaccharide?
Verified step by step guidance1
Determine the molecular formula of the monosaccharide: Monosaccharides generally follow the formula \( C_nH_{2n}O_n \). Given the molecular weight of 150, calculate \( n \) by solving \( 12n + 2n + 16n = 150 \), where 12, 2, and 16 are the atomic weights of carbon, hydrogen, and oxygen, respectively.
Analyze the optical activity: The lack of optical activity suggests that the monosaccharide is achiral. This means the molecule either lacks a chiral center or is a meso compound (a molecule with chiral centers but an internal plane of symmetry).
Consider the possible structures: For a monosaccharide with \( n \) carbons, identify possible achiral structures. For example, a meso compound like D-glucose or D-mannose in their open-chain forms could be candidates if they exhibit symmetry.
Verify the structure: Check if the proposed structure satisfies the molecular formula and the condition of being optically inactive. For example, a meso compound such as D-glucose in its cyclic form (pyranose or furanose) could exhibit internal symmetry, making it optically inactive.
Conclude the structure: Based on the molecular weight, achirality, and structural analysis, identify the specific monosaccharide that fits all criteria. Ensure the structure is consistent with the chemical and physical properties described in the problem.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Monosaccharide Structure
Monosaccharides are the simplest form of carbohydrates, consisting of single sugar units. They typically have a backbone of carbon atoms, with hydroxyl groups (-OH) attached, and a carbonyl group (C=O) that can be either an aldehyde or a ketone. The structure can be linear or cyclic, with cyclic forms being more common in solution.
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Optical Activity
Optical activity refers to the ability of a compound to rotate plane-polarized light, which is a characteristic of chiral molecules. A molecule is chiral if it has non-superimposable mirror images, typically due to the presence of one or more asymmetric carbon atoms. If a monosaccharide is not optically active, it suggests that it may have a symmetrical structure, leading to no net rotation of light.
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Molecular Weight and Composition
The molecular weight of a compound is the sum of the atomic weights of all atoms in its molecular formula. For monosaccharides, the general formula is (CH2O)n, where n is the number of carbon atoms. A molecular weight of 150 suggests a monosaccharide with a specific composition, likely indicating a structure with five carbons (C5H10O5) or six carbons (C6H12O6) that may be symmetrical, thus explaining the lack of optical activity.
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