Aldehydes and ketones: properties, reactions, tests
Aldehydes and Ketones
What are they?
Aldehydes and ketones are simple organic compounds that contain a carbonyl group $\ce{C=O}$. The difference lies in the atoms attached to the carbonyl carbon:
- Aldehyde: one side is a hydrogen atom – $\ce{R-CHO}$ (think of it as a “friendly” carbonyl that likes to share). 🍰
- Ketone: both sides are alkyl or aryl groups – $\ce{R-CO-R'}$ (a more “independent” carbonyl that keeps its own company). 🔬
Structural Features
| Compound | Key Feature | Analogy |
|---|---|---|
| Aldehyde | One hydrogen attached to the carbonyl carbon. | Like a student who always brings a snack (H) to class. |
| Ketone | Two carbon groups attached to the carbonyl carbon. | Like a student who brings two different books to share. |
Physical Properties
Because of the polar $\ce{C=O}$ bond, both aldehydes and ketones are generally soluble in water (especially the lower ones) and have higher boiling points than alkanes of similar size. They also give a characteristic smell – many aldehydes smell fruity or sweet. 🍓
Chemical Properties
The carbonyl carbon is electrophilic (electron‑poor) and the oxygen is nucleophilic (electron‑rich). This makes them reactive toward nucleophiles and oxidising agents. Below are the most common reactions.
Common Reactions
1. Oxidation
- Aldehyde → Carboxylic Acid (strong oxidisers, e.g., $\ce{KMnO4}$, $\ce{NaOCl}$). ⚡️
- Ketone → No reaction (most mild oxidisers). ❌
2. Nucleophilic Addition
- Hydroboration–Oxidation → Primary alcohol from aldehyde, secondary alcohol from ketone.
- Grignard Reaction → Alcohol after work‑up. Example: $\ce{R-MgBr + R'CHO → R-R'CH(OH)MgBr}$.
- Reduction (NaBH4 or LiAlH4) → Alcohols.
3. Condensation Reactions
- Fischer–Speier Aldol Condensation – two aldehydes/ketones join to form β‑hydroxy carbonyls.
- Schiff Base Formation – aldehyde + amine → imine.
Common Tests
These quick tests help you identify aldehydes and ketones in the lab.
Aldehyde Test – 2,4‑Dinitrophenylhydrazine (2,4‑DNPH)
Add a few drops of 2,4‑DNPH solution to the sample. A yellow, orange, or red precipitate indicates an aldehyde (or ketone). The colour depends on the substituents. 🧪
Tollens’ Test (Silver Mirror)
Aldehydes reduce the diamminesilver(I) ion to metallic silver, forming a silver mirror on the test tube. Ketones do not react. 🔍
Fehling’s Test
Aldehydes reduce the blue copper(II) complex to red copper(I) oxide, giving a brick‑red precipitate. Ketones remain blue. 🔴
Quick Reference Table
| Test | Result for Aldehyde | Result for Ketone |
|---|---|---|
| 2,4‑DNPH | Yellow–red precipitate | Yellow–red precipitate (usually lighter) |
| Tollens’ | Silver mirror forms | No mirror |
| Fehling’s | Brick‑red precipitate | Blue solution remains |
Take‑Home Message
Aldehydes are like the “open‑handed” friends who are easily oxidised and give a silver mirror, while ketones are the “reserved” friends that stay put under most oxidising conditions. Both share the same polar carbonyl core, making them versatile in building more complex molecules – just like how a good foundation can support a whole building. 🏗️
Revision
Log in to practice.