Oxidation to the aldonic acid: practice and mechanism
Bromine water only oxidises the aldehyde group (C1) to COOH. The primary CH₂OH group at the other end remains; an open-chain aldonic acid forms.
Example
Bromine water is a mild oxidant: it only oxidises the aldehyde group (C1) to the carboxylic acid. The primary OH group at the other end of the chain survives. Glucose gives gluconic acid.
Mechanism step by step
- 1
The cyclic hemiacetal is in equilibrium with the open-chain aldehyde form (below 1 % for glucose).
- 2
Bromine (or HOBr) oxidises the aldehyde group of the open form; in water this goes via the hydrate.
- 3
The aldonic acid forms; the primary OH at C6 survives because Br₂/H₂O is too mild.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That is the open aldehyde form, not the oxidation product. Br₂ oxidises the aldehyde further to the carboxylic acid.
Often confused with: Oxidation to the aldaric acid · Reduction to the sugar alcohol
Frequently asked questions
What is a reducing sugar?
A sugar with a free hemiacetal that can open to the aldehyde form. It reduces Br₂, Cu²⁺ (Fehling) or Ag⁺ (Tollens) and is itself oxidised to the aldonic acid.
Do ketoses such as fructose react too?
Not with bromine water (no aldehyde). Fehling and Tollens still react, because in base fructose isomerises via an enediol to an aldose.