Predict the products obtained when D-galactose reacts with each reagent.
(h) NaBH₄

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Identify the functional groups present in D-galactose. D-galactose is an aldose sugar, meaning it contains an aldehyde group (-CHO) at the anomeric carbon and multiple hydroxyl (-OH) groups.

Understand the role of the reagent NaBH4 (sodium borohydride). NaBH4 is a reducing agent that specifically reduces aldehydes and ketones to their corresponding alcohols.

Focus on the aldehyde group in D-galactose. The aldehyde group (-CHO) will be reduced by NaBH4 to a primary alcohol (-CH2OH).

Write the structure of the product. After reduction, the aldehyde group at the anomeric carbon of D-galactose is converted into a primary alcohol, resulting in a sugar alcohol known as D-galactitol (also called dulcitol). The rest of the molecule remains unchanged.

Verify the stereochemistry of the product. Since NaBH4 does not affect the stereochemistry of the other chiral centers in D-galactose, the stereochemistry of the hydroxyl groups on the sugar remains the same in the product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Reduction Reactions

Reduction reactions involve the gain of electrons or the addition of hydrogen to a molecule, often resulting in the conversion of carbonyl groups (C=O) to alcohols (C-OH). In the case of d-galactose, a reducing sugar, the aldehyde group can be reduced to an alcohol when treated with reducing agents like NaBH4.

D-Galactose Structure

D-galactose is a six-carbon aldose sugar with an aldehyde functional group at one end. Its structure includes multiple hydroxyl (-OH) groups, which are crucial for its reactivity. Understanding its stereochemistry and functional groups is essential for predicting the products of its reactions.

Stereochemistry

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the case of d-galactose, the specific configuration of its hydroxyl groups influences the outcome of its reactions, including the formation of specific alcohol products upon reduction.

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