Question

In a diabetic some glucose is converted to sorbitol, an alcohol that accumulates in the eye and can cause cataracts. Draw the open-chain structure of sorbitol, which is identical to that of D-glucose except that the aldehyde group has been reduced to an alcohol group. Can sorbitol form a five- or six- membered cyclic hemiacetal? Explain why or why not. (Hint: The open-chain structure of glucose is found in Section 20.1.)

Short answer

Sorbitol cannot form a cyclic hemiacetal as it lacks a carbonyl group needed to react with an alcohol group.

Step-by-step solution

Understand the Open-Chain Structure of D-Glucose

D-glucose is an aldohexose, which means it has six carbon atoms and an aldehyde functional group at the first carbon. The carbon chain is in the order: carbon-1 (an aldehyde group), carbon-2 with a hydroxyl group on the right, carbon-3 with a hydroxyl group on the left, carbon-4 and carbon-5 both with hydroxyl groups on the right, and carbon-6 with a primary alcohol on the right.

Convert Aldehyde to Alcohol in Sorbitol

To convert the D-glucose to sorbitol, you reduce the aldehyde group at carbon-1 to an alcohol. This means the carbon that was originally an aldehyde now has an additional hydrogen, making it an alcohol (CH2OH group). Thus, sorbitol has CH2OH groups at both ends of the carbon chain.

Analyze the Formation of Cyclic Hemiacetal

A hemiacetal is formed when an alcohol group reacts with a carbonyl group, either an aldehyde or ketone. In the case of glucose, it reacts internally to form cyclic structures. However, in sorbitol, the original aldehyde group at carbon-1 has been reduced to alcohol, meaning there is no carbonyl group to facilitate the reaction with an alcohol group to form a ring.

Conclusion on Cyclic Formation Potential

Since sorbitol's structure lacks a carbonyl group, it cannot undergo internal nucleophilic additions to form a stable five- or six-membered cyclic hemiacetal. Both the functional groups required to form a hemiacetal are alcohols in sorbitol, therefore it cannot form the cyclic structure like glucose.

Key concepts

Understanding D-glucose Conversion

D-glucose is a type of sugar known as an aldohexose, characterized by having six carbon atoms and an aldehyde functional group. This structure is the basis for understanding how glucose transforms into other compounds. In the body, particularly under certain conditions like diabetes, glucose converts into sorbitol. This conversion involves changing the aldehyde functional group at the first carbon into an alcohol group.

The aldehyde group at carbon-1 is reduced, meaning it gains hydrogen atoms to become an alcohol group, resulting in sorbitol. This compound maintains the original chain of carbons found in D-glucose but has CH2OH groups at both terminal carbons. Once the aldehyde is reduced, sorbitol can no longer participate in reactions typical for glucose, especially those involving the aldehyde group.

Cyclic Hemiacetal Formation

Cyclic hemiacetals are structures that form when alcohol groups react with carbonyl groups (aldehydes or ketones) within the same molecule. For sugars, cyclic hemiacetal formation is an important characteristic because it allows sugars to exist in more stable ring forms.

In glucose, the internal reaction between the carbonyl group at carbon-1 and a hydroxyl group typically at carbon-4 or carbon-5 allows the sugar to form either a five-membered (furanose) or six-membered (pyranose) ring. This transformation is a reversible process and essential in the biochemistry of sugars.

However, sorbitol lacks this potential due to the absence of a reactive carbonyl group, which is necessary for hemiacetal ring formation.

Exploring Sugar Alcohols

Sugar alcohols, such as sorbitol, are derived from sugars but feature altered structures due to specific reductions. These compounds are polyols, meaning they have multiple hydroxyl (OH) groups, making them alcohols by nature.

The conversion of glucose into sorbitol transforms it into a sugar alcohol. Sorbitol and other sugar alcohols are widely used in food processing due to their sweetness, though they are not absorbed in the same way as sugars by the body. They are often found in products catering to diabetic diets as they have a minimal impact on blood sugar levels.

Unlike their sugar counterparts, sugar alcohols are less likely to undergo reactions typical of sugars, such as the formation of cyclic hemiacetals, emphasizing their role as stable compounds.

Principles of Aldehyde Reduction

Aldehyde reduction is a chemical process wherein an aldehyde group is converted into an alcohol group. This transformation involves adding hydrogen ( H_2 ) to the aldehyde. Bio-catalysis or chemical agents can facilitate this conversion in lab settings or within the body.

In the context of sorbitol, the reduction process occurs at the aldehyde functional group of glucose, leading to the formation of an additional alcohol group at carbon-1. This means that both the first and the last carbons in the chain are alcohol groups, adjusting the compound's reactivity and inability to form a cyclic hemiacetal.

Aldehyde reduction is crucial for forming various sugar alcohols, contributing significantly to food science, medicine, and biochemistry, understanding reactivity, stability, and functional applications.