Question

When glucose (Problem \(19-52\) ) is treated with \(\mathrm{NaBH}_{4}\), reaction occurs to yield sorbitol, a polyalcohol commonly used as a food additive. Show how this reduction occurs.

Short answer

The aldehyde group in glucose is reduced to an alcohol by NaBH₄, forming sorbitol.

Step-by-step solution

Understanding the Reactants and Products

Glucose is an aldohexose with the molecular formula C₆H₁₂O₆. It contains an aldehyde group at one end. Sodium borohydride ( NaBH₄ ) is a reducing agent that selectively reduces aldehydes to primary alcohols. The product, sorbitol, is a polyalcohol, meaning all OH groups remain, with the aldehyde group reduced to an alcohol.

Identify Functional Groups in Glucose

Glucose contains an aldehyde group at the end of the molecule, specifically at the carbon-1 position, and hydroxyl groups at other positions. The aldehyde group is reactive and will be targeted by NaBH₄.

Reduction Reaction Mechanism Explanation

NaBH₄ functions by providing a hydride ion (H⁻) that attacks the carbonyl carbon of the aldehyde group, reducing it to a hydroxyl group. The chemical reduction can be represented by the conversion of the C=O group to a CH₂OH group.

Outline the Chemical Equation for the Reduction

The chemical reaction for this reduction can be written as: Glucose (C₆H₁₂O₆) + NaBH₄ + H₂O → Sorbitol (C₆H₁₄O₆). The NaBH₄ provides the hydride ion to reduce the aldehyde group to a hydroxyl group, resulting in sorbitol.

Visualize the Structural Changes

Initially, the glucose structure has 5 hydroxyl groups (OH) and 1 aldehyde group (CHO). After reduction, the aldehyde group (CHO) transforms into an additional hydroxyl group (CH₂OH). The whole molecule now has a polyalcohol structure, sorbitol.

Conclude the Process and Verify

Verify that the major change in the structure occurs at the aldehyde group, which becomes an alcohol, turning the molecule into sorbitol. Ensure that all other hydroxyl groups remain unchanged to confirm the conversion.

Key concepts

Aldohexose

Glucose is a prime example of an aldohexose, which is a type of carbohydrate. Aldohexoses are characterized by having six carbon atoms and an aldehyde group. In glucose, the aldehyde group appears at the first carbon of the molecule.
This aldehyde group plays a crucial role in chemical reactions, as it is highly reactive. During a reduction reaction, such as when glucose is treated with sodium borohydride, the aldehyde group is specifically targeted and transformed.
Understanding the structure of aldohexoses is important, as it influences how the sugar behaves chemically and biologically.

• Six carbon chain
• Contains an aldehyde group
• Important for its reactions

Sodium Borohydride

Sodium borohydride, or \text{NaBH}_{4}, is a common reducing agent in chemistry. It is particularly useful due to its ability to selectively reduce aldehydes and ketones to their corresponding alcohols.
In the context of glucose reduction, sodium borohydride supplies a hydride ion, \(H^{-}\), which plays a key role in turning the aldehyde group of glucose into an alcohol group, thus forming sorbitol.
Its selective reduction capability makes it indispensable in laboratory settings for specific transformations without affecting other parts of the molecule.

• Efficient reducing agent
• Target specificity for aldehydes
• Enables conversion to alcohols

Polyhydric Alcohol

Sorbitol is an example of a polyhydric alcohol, or polyol, which means it contains multiple hydroxyl groups (OH) within its structure. It is the product of glucose reduction using sodium borohydride.
As opposed to the aldohexose form of glucose that has a reactive aldehyde group, sorbitol is stable and has six hydroxyl groups - one for each carbon. This increased number of hydroxyl groups makes sorbitol a good humectant, which is why it's used in food products, cosmetics, and even pharmaceuticals.

• Contains multiple OH groups
• Stable compared to aldohexoses
• Used widely in industries

Reduction Reaction

A reduction reaction in chemistry involves the gain of electrons or hydrogen atoms, leading to a decrease in oxidation state. In the case of glucose being reduced, the aldehyde group ( ext{CHO}) is converted to an alcohol group ( ext{CH}_2 ext{OH}).
Sodium borohydride acts as a source of hydride ions (H⁻), which are crucial for this transformation. The hydride ion donates a pair of electrons to the carbon atom in the aldehyde group, facilitating the formation of the alcohol group.
Reduction reactions like this are essential tools for chemists, allowing the modification of molecules in a controlled manner.

• Involves gain of electrons/hydrogen
• Changes functional group from aldehyde to alcohol
• Controlled method for modifying molecules

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical reactions of those molecules. In glucose reduction, the primary functional groups of interest are the aldehyde and hydroxyl groups.
The aldehyde group in glucose is highly reactive and is the primary target during the reduction process. Sodium borohydride reduces this aldehyde group into a hydroxyl functional group, resulting in sorbitol, which has an additional OH group.
Understanding functional groups is essential in predicting how chemical reactions will proceed and what products will be formed.

• Determine chemical reactivity
• Aldehyde group is key in reduction
• Transformation to hydroxyl group in sorbitol