Ketohexoses form from aldohexoses. Suggest a mechanism for the acid-catalyzed version of this reaction. What type of reaction is this? [You’ve seen it before.]


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Verified step by step guidance

1. Identify the starting material: Begin with an aldohexose, which is a six-carbon sugar with an aldehyde group at the first carbon.
2. Protonation of the carbonyl: In an acid-catalyzed reaction, the carbonyl oxygen of the aldehyde group is protonated, increasing the electrophilicity of the carbonyl carbon.
3. Formation of an enediol intermediate: The protonated carbonyl group facilitates the formation of an enediol intermediate through a keto-enol tautomerization process.
4. Rearrangement to form a ketone: The enediol intermediate undergoes a rearrangement where the hydroxyl group at the second carbon is converted into a carbonyl group, forming a ketohexose.
5. Recognize the reaction type: This is an isomerization reaction, specifically an aldose-ketose isomerization, which is a common transformation in carbohydrate chemistry.

Key Concepts

Aldohexoses and Ketohexoses

Aldohexoses are six-carbon sugars with an aldehyde group, while ketohexoses contain a ketone group. The conversion from aldohexoses to ketohexoses involves the rearrangement of functional groups, specifically the migration of the carbonyl group from the end of the chain to the second carbon. This transformation is essential in carbohydrate chemistry and plays a significant role in metabolic pathways.

Acid-Catalyzed Mechanism

An acid-catalyzed mechanism involves the use of an acid to facilitate a chemical reaction, often by protonating a reactant to make it more electrophilic. In the case of converting aldohexoses to ketohexoses, the acid can protonate the hydroxyl group, leading to the formation of a more reactive intermediate. This mechanism typically involves steps such as protonation, rearrangement, and deprotonation.

Rearrangement Reaction

A rearrangement reaction is a type of chemical reaction where the structure of a molecule is reorganized to form a new compound. In the context of converting aldohexoses to ketohexoses, this reaction involves the migration of the carbonyl group and the reorganization of the carbon skeleton. Such reactions are common in organic chemistry and are crucial for understanding carbohydrate transformations.