Identifying Aldehydes and Ketones (OCR A-Level Chemistry A): Revision Notes

Identifying Aldehydes and Ketones

Introduction to detecting carbonyl compounds

Aldehydes and ketones both contain the carbonyl functional group (C=O), which makes them chemically similar. However, there are specific tests that can help you identify whether an unknown compound contains a carbonyl group, and further tests can distinguish between aldehydes and ketones.

Two main reagents are used in this identification process:

• 2,4-dinitrophenylhydrazine (2,4-DNP) - detects the presence of any carbonyl group
• Tollens' reagent - specifically identifies aldehydes only

Testing for carbonyl groups using 2,4-DNP

What is Brady's reagent?

Brady's reagent is a solution that contains 2,4-dinitrophenylhydrazine (commonly abbreviated as 2,4-DNP) dissolved in methanol and sulfuric acid. This reagent appears as a pale orange solution and is used to detect the presence of carbonyl groups in both aldehydes and ketones.

Practical procedure for the 2,4-DNP test

Follow these steps to test for a carbonyl group:

1. Add approximately 5 cm depth of 2,4-dinitrophenylhydrazine solution to a clean test tube (this is in excess)
2. Using a dropping pipette, carefully add three drops of the unknown compound to the test tube
3. Leave the test tube to stand and observe
4. If no crystals form initially, add a few drops of sulfuric acid to encourage precipitation
5. A yellow or orange precipitate indicates a positive result for the presence of an aldehyde or ketone

The formation of a coloured precipitate confirms that the unknown compound contains a carbonyl group. The precipitate formed is called a 2,4-dinitrophenylhydrazone derivative.

Understanding the chemical reaction

The reaction between 2,4-DNP and carbonyl compounds is a type of condensation reaction, meaning that two molecules join together with the elimination of a small molecule - in this case, water.

The equation for the reaction can be written as:

$\ce{R_1R_2C=O + H2N-NH-C6H3(NO2)2 -> R_1R_2C=N-NH-C6H3(NO2)2 + H2O}$

where $\ce{R_1}$ and $\ce{R_2}$ represent hydrogen atoms or alkyl groups. For aldehydes, one of these will be a hydrogen atom, while for ketones, both will be carbon chains.

Distinguishing between aldehydes and ketones

Once you've confirmed that a compound contains a carbonyl group using 2,4-DNP, you can carry out a further test to determine whether it's specifically an aldehyde or a ketone. This test uses Tollens' reagent.

What is Tollens' reagent?

Tollens' reagent is a solution of silver nitrate ($\ce{AgNO3}$) in aqueous ammonia. The silver is present as silver(I) ions, $\ce{Ag+}$(aq), which act as an oxidising agent.

Preparing Tollens' reagent

To prepare fresh Tollens' reagent:

1. Add 3 cm depth of aqueous silver nitrate, $\ce{AgNO3}$(aq), to a clean test tube
2. Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide, $\ce{Ag2O}$, forms
3. Add dilute ammonia solution gradually until the brown precipitate just dissolves
4. The resulting clear, colourless solution is Tollens' reagent and should be used immediately

Testing for aldehydes with Tollens' reagent

Follow this procedure to test whether a carbonyl compound is an aldehyde:

1. Pour about 2 cm depth of the unknown solution into a clean test tube
2. Add an equal volume of freshly prepared Tollens' reagent
3. Place the test tube in a beaker of warm water heated to approximately 50°C
4. Leave for 10-15 minutes and observe whether a silver mirror forms on the inside of the test tube

Results:

• If the unknown compound is an aldehyde, a silver mirror forms on the inside of the test tube
• If the unknown compound is a ketone, no reaction occurs and no silver mirror forms

The redox chemistry of Tollens' test

Tollens' reagent works because aldehydes can be oxidised to carboxylic acids, whereas ketones are resistant to oxidation under mild conditions. The test involves a coupled redox reaction.

Reduction half-equation: $\ce{Ag+(aq) + e- -> Ag(s)}$

The silver ions are reduced to metallic silver, which deposits on the glass surface as a shiny mirror.

Oxidation half-equation: $\ce{R-CHO + [O] -> R-COOH}$

The aldehyde is oxidised to a carboxylic acid. The symbol [O] represents the oxidising agent (which provides the oxygen).

Identifying specific carbonyl compounds by melting point

After confirming that an unknown compound contains a carbonyl group (using 2,4-DNP), you can identify the specific aldehyde or ketone by determining the melting point of its 2,4-dinitrophenylhydrazone derivative.

The recrystallisation and melting point procedure

1. Filter the yellow/orange precipitate formed in the 2,4-DNP test to separate it from the solution
2. Recrystallise the solid to produce pure crystals of the derivative
3. Measure and record the melting point of the purified 2,4-dinitrophenylhydrazone
4. Compare the measured melting point to values in a database or data table to identify the original carbonyl compound

Limitations of melting point identification

Not all carbonyl compounds can be reliably identified using only melting point data. Some compounds may have very similar melting points for their derivatives, making it difficult to distinguish between them.

Common exam mistakes to avoid

Summary