Ketones generally do not react with potassium dichromate (VI) under normal conditions, producing no visible change in the solution.
Understanding Ketones and Their Reactivity
Ketones are organic compounds characterized by a carbonyl functional group (C=O) located within a carbon chain, meaning the carbon atom of the carbonyl group is bonded to two other carbon atoms. This structural arrangement significantly influences their chemical reactivity, particularly concerning oxidation.
The Lack of Reaction with Potassium Dichromate
Potassium dichromate (VI) is a strong oxidizing agent, typically appearing orange in solution. When exposed to an oxidizable substance, it gets reduced, often changing color to green as the chromium(VI) ions (Cr₂O₇²⁻) are converted to chromium(III) ions (Cr³⁺).
For a carbonyl compound to be easily oxidized by mild oxidizing agents like potassium dichromate, it typically requires a hydrogen atom directly attached to the carbonyl carbon. Aldehydes possess this structural feature, allowing them to be oxidized to carboxylic acids.
Ketones, however, lack a hydrogen atom on the carbonyl carbon. Instead, the carbonyl carbon is bonded exclusively to other carbon atoms. This makes ketones highly resistant to oxidation by moderate oxidizing agents such as potassium dichromate (VI). Consequently, when ketones are mixed with potassium dichromate (VI), there is no visible reaction; the orange color of the dichromate solution persists.
Distinguishing Between Aldehydes and Ketones
The difference in reactivity with potassium dichromate provides a straightforward chemical test to differentiate between aldehydes and ketones. Other tests also leverage this difference in oxidizability.
Common Chemical Tests for Aldehydes and Ketones
| Reagent / Functional Group | Aldehydes (RCHO) | Ketones (RCOR') |
|---|---|---|
| Potassium Dichromate (VI) | Orange to Green (Oxidation occurs) | No visible reaction (Solution remains orange) |
| Fehling's Solution | Blue to Brick-Red Precipitate (Cu₂O formed) | No reaction (Solution remains blue) |
| Tollens' Reagent | Silver Mirror (Ag deposited) | No reaction |
| Brady's Reagent (2,4-DNPH) | Orange/Yellow Precipitate (Both react, not distinguishing) | Orange/Yellow Precipitate (Both react, not distinguishing) |
Chemical Principles of Oxidation Resistance
- Aldehydes: The presence of the C-H bond on the carbonyl carbon allows for the facile oxidation of the aldehyde to a carboxylic acid without breaking carbon-carbon bonds. The hydrogen atom is removed, and an oxygen atom is inserted.
R-CHO + [O] → R-COOH - Ketones: To oxidize a ketone, carbon-carbon bonds must be broken, which requires much harsher conditions (e.g., strong oxidizing agents, high temperatures) and typically leads to a mixture of shorter-chain carboxylic acids. This is why ketones are considered resistant to mild oxidation.
Practical Insights
In a laboratory setting, this differential reactivity is crucial for identifying unknown carbonyl compounds. If an unknown compound gives a positive test with 2,4-dinitrophenylhydrazine (indicating a carbonyl group) but shows no reaction with potassium dichromate or Fehling's solution, it is likely a ketone.
For further reading on organic chemistry reactions, you can consult reliable academic resources like Organic Chemistry by Clayden, Greeves, and Warren, or visit educational platforms such as Khan Academy Organic Chemistry.
