Ch. 23 - Carbohydrates and Nucleic Acids

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 23 - Carbohydrates and Nucleic Acids

Problem 14

Chapter 23, Problem 14

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.

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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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Key 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.

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.

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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Textbook Question

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of α-D-galactopyranose is +150.7°, and that of the β anomer is +52.8°. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2°. Determine the percentages of the two anomers present at equilibrium.

Textbook Question

Draw and name the products of bromine water oxidation of

(a) D-mannose

(b) D-galactose

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Textbook Question

The carbonyl group in D-galactose may be isomerized from C1 to C2 by brief treatment with dilute base (by the enediol rearrangement). The product is the C4 epimer of fructose. Draw the furanose structure of the product.

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Textbook Question

Two sugars, A and B, are known to be glucose and galactose, but it is not certain which one is which. On treatment with nitric acid, A gives an optically inactive aldaric acid, while B gives an optically active aldaric acid. Which sugar is glucose, and which is galactose?

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Textbook Question

Draw and name the products of nitric acid oxidation of

(a) D-mannose

(b) D-galactose

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Textbook Question

Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.

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