Question

Glucose and mannose can be interconverted (in low yield) by treatment with dilute aqueous NaOH. Propose a mechanism.

Short answer

The reaction mechanism involves base-catalyzed enolate formation, hydroxide migration, and tautomerism, resulting in the conversion of glucose to mannose.

Step-by-step solution

Understanding the Reaction Condition

This reaction occurs in a basic environment provided by dilute aqueous NaOH, which suggests that a base catalyzed isomerization can take place. This involves an enolate formation followed by rearrangement.

Formation of Enolate

In the presence of NaOH, glucose can lose a proton from the alpha carbon adjacent to the carbonyl group to form an enolate ion. This is the key intermediate in base-catalyzed isomerizations.

Keto-Enol Tautomerism

The enolate ion can quickly equilibrate with its corresponding enol form. This enol form is not stable, but it sets up the possibility for rearrangement to other sugars like mannose.

Migration of Hydroxide

In the enol form, the position of the hydroxyl group can shift, allowing the conversion of glucose to mannose. This involves the migration of the -OH group to a different position enabled by the enolate's reactivity.

Reformation of Aldose

After migration, the enol can tautomerize back to the keto form, this time resulting in mannose instead of glucose. This reforming step culminates the conversion mechanism.

Key concepts

Base-Catalyzed Reactions

In a chemical reaction, a base catalyst is responsible for increasing the rate of the reaction without being consumed in the process. Base-catalyzed reactions are essential in organic chemistry, as they involve bases that facilitate the formation of reactive intermediates.

A classic example is the isomerization of sugars, as seen in the conversion of glucose to mannose. This process uses a dilute aqueous solution of Sodium Hydroxide (NaOH) as the catalyst.

• The base environment creates conditions where certain chemical groups are more reactive.
• The reaction involves the deprotonation of certain atoms, aiding in the rearrangement of molecular structures.

Base-catalyzed reactions specifically enable mechanisms like tautomerism, significantly impacting the structure and function of the molecules involved.

Enolate Chemistry

Enolate chemistry revolves around the formation and reactions of enolate ions, which are crucial intermediates in many organic reaction mechanisms. These ions are formed when a base removes an acidic hydrogen atom adjacent to a carbonyl functional group.

In the glucose to mannose isomerization, base-catalysis allows glucose to form an enolate ion by losing a proton. This reaction highlights several features of enolate ions:

• They have a resonance-stabilized structure, which allows both the keto and enol forms to be readily accessible.
• Enolates are highly reactive, facilitating subsequent rearrangements in a chemical reaction, such as migration of functional groups.

Understanding enolate formation is key to predicting reaction outcomes in various base-catalyzed organic transformations.

Tautomerism

Tautomerism is a chemical phenomenon where compounds known as tautomers easily convert to one another by simple shifts of a proton and a double bond. This dynamic equilibrium plays a significant role in organic reactions such as isomerization.

During the sugar interconversion between glucose and mannose, tautomerism occurs as the enolate ion formed equilibrates with its enol form. This forms an essential step that allows the rearrangement events required for interconversion.

• The keto form (glucose) is converted to an enol intermediate.
• The enol then allows for repositioning of functional groups like hydroxyl groups, leading to new sugar forms like mannose.

Tautomerism enables flexibility in molecular structures, which is particularly important in the variability of natural substances like carbohydrates.

Reaction Mechanism

Understanding a reaction mechanism is vital for grasping how a set of reactions occurs from start to finish. The reaction mechanism for the interconversion of glucose and mannose reveals the steps involved in this sugar transformation process.

Here's how it happens:

• A base like NaOH initiates the formation of an enolate ion by removing a proton from glucose.
• The enolate converts rapidly to an unstable enol form, setting up the stage for rearrangement.
• Functional groups, like the -OH group, migrate, allowing glucose to transition to mannose in the enol form.
• Finally, the enol tautomerizes back to a keto form, resulting in mannose.

This journey through intermediate states, involving enolate formation and tautomerism, encapsulates the fascinating world of molecular changes within sugar chemistry.