The aldohexose D-(+) -galactose can be obtained by...

The aldohexose $\mathrm{D}-(+)$ -galactose can be obtained by hydrolysis of lactose, a disaccharide found in milk. When $\mathrm{D}-(+)$ -galactose is treated with nitric acid, it yields an optically inactive aldaric acid. When $\mathrm{D}-(+)$ - galactose is subjected to Ruff degradation, it yields D-(-)-lyxose (see Practice Problem 22.15 ). Using only these data, write the Fischer projection formula for $\mathrm{D}-(+)$ -galactose.

Key Concepts

Ruff Degradation

Ruff degradation is a chemical reaction that shortens aldoses by removing the terminal carbon, while preserving the configuration of the remaining carbons. The nature of the degradation product—the configuration and optical rotation of the resulting pentose—provides insight into the stereochemistry of the original hexose. This technique is useful in determining the arrangement of hydroxyl groups in monosaccharides.

Oxidative Degradation to Aldaric Acids and Optical Activity

When aldohexoses are oxidized with strong oxidizing agents like nitric acid, both the aldehyde group and the terminal primary alcohol are converted to carboxylic acids, producing an aldaric acid. In some cases, such as with D-(+)-galactose, the resulting aldaric acid is optically inactive, indicating internal symmetry (meso structure) in the oxidized molecule. This information is used to deduce aspects of the original sugar's stereochemistry.

Fischer Projections

Fischer projections are a two?dimensional representation of three?dimensional organic molecules, notably carbohydrates. They are especially useful for depicting the stereochemistry of sugars by positioning the chiral centers vertically along a carbon chain, with horizontal lines representing bonds projecting toward the observer. This convention simplifies the comparison of stereochemical configurations among similar molecules.

Aldohexoses

Aldohexoses are six?carbon sugars that contain an aldehyde group at one end. They exhibit multiple chiral centers whose configurations determine the sugar’s identity and properties. Understanding the stereochemistry of aldohexoses is fundamental, as small differences in the arrangement of –OH groups at specific carbons (typically C2–C5) can lead to distinct sugars with different biological roles.

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