Oxidation to the aldaric acid: practice and mechanism
Nitric acid oxidises both chain ends: C1 (aldehyde) and C6 (CH₂OH) become COOH. An open-chain dicarboxylic acid forms, the aldaric acid.
Example
Nitric acid is stronger than bromine water: it oxidises both chain ends, the aldehyde group (C1) and the primary CH₂OH group (C6). A dicarboxylic acid forms, the aldaric acid (glucaric acid from glucose, meso-galactaric acid from galactose).
Mechanism step by step
- 1
The cyclic hemiacetal is in equilibrium with the open-chain aldehyde form (below 1 % for glucose).
- 2
Nitric acid oxidises the aldehyde group at C1 to the carboxylic acid.
- 3
The primary CH₂OH group at C6 is also oxidised via the aldehyde to the carboxylic acid: the aldaric acid forms.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be a uronic acid (only C6 oxidised, ring intact). It forms enzymatically in biochemistry; HNO₃ also oxidises the aldehyde group at C1.
Often confused with: Oxidation to the aldonic acid · Reduction to the sugar alcohol
Frequently asked questions
What was aldaric acid formation used for?
To determine configuration (Fischer): if the aldaric acid is meso (optically inactive), both chain halves must mirror each other. Galactose gives a meso acid, glucose does not.
How does it differ from a uronic acid?
In a uronic acid only C6 is oxidised and C1 remains a hemiacetal. Glucuronic acid forms enzymatically in the body and serves for detoxification.