Textbook Question
Predict the products obtained when D-galactose reacts with each reagent.
(e) H2, Ni
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28. Carbohydrates
Monosaccharides - Reduction (Alditols)
2:28 minutesProblem 14
Textbook Question
When D-glucose is reduced with sodium borohydride, optically active glucitol results. When optically active D-galactose is reduced, however, the product is optically inactive. Explain this loss of optical activity.
Verified step by step guidance1
Understand the reaction: Sodium borohydride (NaBH₄) is a reducing agent that reduces the aldehyde group (-CHO) in aldoses like D-glucose and D-galactose to a primary alcohol (-CH₂OH), forming sugar alcohols (alditols).
Analyze the structure of D-glucose: D-glucose is reduced to D-glucitol (also known as sorbitol), which retains optical activity because it has multiple chiral centers and no internal symmetry.
Analyze the structure of D-galactose: D-galactose is reduced to galactitol. However, galactitol is a meso compound, meaning it has an internal plane of symmetry and is optically inactive despite having chiral centers.
Explain the loss of optical activity: The reduction of D-galactose results in a product (galactitol) that is a meso compound. Meso compounds are optically inactive because their mirror images are superimposable due to the internal plane of symmetry.
Conclude the reasoning: The difference in optical activity between the products of D-glucose and D-galactose reduction arises from the structural differences in their sugar alcohols. D-glucitol lacks an internal plane of symmetry, while galactitol possesses one, leading to optical inactivity in the latter.
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2mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Optical Activity
Optical activity refers to the ability of a compound to rotate the plane of polarized light. This property is characteristic of chiral molecules, which have non-superimposable mirror images. The degree and direction of rotation depend on the specific arrangement of atoms in the molecule, making it crucial for distinguishing between different stereoisomers.
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Chirality and Stereoisomers
Chirality is a property of a molecule that has at least one chiral center, typically a carbon atom bonded to four different substituents. Stereoisomers are compounds that have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms. The presence of chirality in a molecule leads to the existence of enantiomers, which are mirror images of each other and can exhibit different optical activities.
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Reduction Reactions
Reduction reactions involve the gain of electrons or hydrogen by a molecule, often resulting in the conversion of carbonyl groups (like aldehydes or ketones) to alcohols. In the case of D-glucose and D-galactose, the reduction with sodium borohydride alters their structures. The specific stereochemistry of the resulting alcohol can influence whether the product retains optical activity, as seen with glucitol being optically active and the product from D-galactose being optically inactive.
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