Aldehydes and ketones sit at a key junction in organic synthesis. They can be prepared from alcohols and further oxidized or reduced to form other functional groups. This lesson aggregates all the reactions of carbonyls, maps out their synthetic pathways, and introduces the chemical analysis techniques used to identify specific, unknown carbonyl compounds.
🔑 Key Principle
Primary alcohols can be oxidized to aldehydes (by distillation) and then to carboxylic acids (under reflux). Secondary alcohols are oxidized to ketones (under reflux). Both oxidation pathways use acidified potassium dichromate(VI) and can be reversed using the reducing agent \(\text{NaBH}_4\). Aldehydes and ketones also undergo nucleophilic addition with \(\text{HCN}\) to extend the carbon chain.
1. Carbonyl Reaction Map
The flowchart below details the interconversion pathways between alcohols, carbonyls, carboxylic acids, and hydroxynitriles. Review the reagents and reaction conditions required for each step:
2. Summary of Carbonyl Reactions
| Reaction Type | Reactant(s) | Reagents & Conditions | Organic Product(s) | Key Observations |
|---|---|---|---|---|
| Oxidation | Aldehyde | \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\), heat under reflux | Carboxylic Acid | Orange solution turns green |
| Oxidation | Ketone | \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\), heat under reflux | No reaction | Solution remains orange |
| Reduction | Aldehyde | \(\text{NaBH}_4\) in aqueous solution | Primary Alcohol | None (colorless reactants and products) |
| Reduction | Ketone | \(\text{NaBH}_4\) in aqueous solution | Secondary Alcohol | None (colorless reactants and products) |
| Nucleophilic Addition | Aldehyde or Ketone | \(\text{HCN}\) with \(\text{KCN}\) catalyst, room temp | Hydroxynitrile | None (extension of carbon chain) |
3. Analytical Identification: 2,4-DNPH
While Tollens' and Fehling's tests allow us to distinguish between aldehydes and ketones, they do not tell us the exact identity of the compound (e.g. distinguishing propanal from butanal). To identify a specific carbonyl compound, chemists use 2,4-dinitrophenylhydrazine (2,4-DNPH), also known as Brady's reagent.
A yellow-orange chemical solution. It undergoes a condensation reaction with the carbonyl group (\(\text{C=O}\)) of aldehydes and ketones, forming a bright yellow, orange, or red crystalline precipitate.
The solid hydrazone product of the 2,4-DNPH reaction. Because different carbonyl compounds yield crystals with unique, sharp melting points, measuring this melting point allows for precise identification.
Steps to Identify a Carbonyl Compound experimentally:
- Precipitation: Add 2,4-DNPH solution to the unknown carbonyl compound. A bright orange/yellow precipitate of 2,4-dinitrophenylhydrazone (a derivative) will form.
- Filtration: Filter the impure solid precipitate under reduced pressure (using a Buchner funnel) to separate it from the solution.
- Recrystallisation: Purify the crude solid by recrystallisation. Dissolve the solid in a minimum volume of hot solvent, filter hot to remove insoluble impurities, allow the filtrate to cool slowly to form pure crystals, and filter again.
- Drying: Dry the purified crystals thoroughly in a desiccator or warm oven.
- Melting Point Determination: Measure the melting point of the dry, pure crystals using a melting point apparatus. The melting point will be a sharp value.
- Database Comparison: Compare the measured melting point against a table of known melting points of 2,4-DNPH derivatives to identify the starting aldehyde or ketone.
A common exam question asks you to outline how to identify a carbonyl compound. You must write out the practical steps:
1. React with 2,4-DNPH to form a precipitate.
2. Filter, recrystallise (using minimum hot solvent), and dry the crystals.
3. Measure the melting point of the crystals.
4. Compare the melting point with known values in database tables.
Note that 2,4-DNPH reacts only with aldehydes and ketones, not with carboxylic acids or esters, despite them containing a \(\text{C=O}\) bond.
Describe a chemical scheme using simple test-tube reactions to identify each compound.
Solution:
Step 1: Test with Acidified Potassium Dichromate(VI).
- Add \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\) to a sample of each compound and warm gently in a water bath.
- Propan-1-ol (primary alcohol) and propanal (aldehyde): The orange solution turns green (positive oxidation result).
- Propanone (ketone): The solution remains orange (no reaction). This bottle is now identified as propanone.
Step 2: Distinguish between the remaining two bottles (propan-1-ol and propanal).
- Add Tollens' reagent to a sample from each of the remaining two bottles and warm gently in a water bath.
- Propanal (aldehyde): A silver mirror forms on the walls of the test tube. This bottle is now identified as propanal.
- Propan-1-ol (alcohol): No change; the solution remains clear and colorless. This bottle is now identified as propan-1-ol.
Alternative Step 2: Add 2,4-DNPH to both. Propanal gives a bright orange precipitate, while propan-1-ol does not react.
Include all reagents, conditions, and balanced equations for each step.
Solution:
To convert propan-1-ol (three carbons) into 2-hydroxybutanenitrile (four carbons), we must first oxidize the alcohol to an aldehyde, and then add HCN to extend the chain.
Step 1: Oxidation of propan-1-ol to propanal.
- Reagents: Acidified potassium dichromate(VI) solution, \(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\).
- Conditions: Heat gently and distil off the product (propanal) immediately to prevent further oxidation to propanoic acid.
- Equation:
\[ \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{[O]} \rightarrow \text{CH}_3\text{CH}_2\text{CHO} + \text{H}_2\text{O} \]
Step 2: Nucleophilic addition of HCN to propanal.
- Reagents: \(\text{HCN}\) with \(\text{KCN}\) catalyst.
- Conditions: Room temperature and pressure.
- Equation:
\[ \text{CH}_3\text{CH}_2\text{CHO} + \text{HCN} \rightarrow \text{CH}_3\text{CH}_2\text{CH(OH)CN} \]