6.1.2(d)-(e) - Carbonyl identification tests

The evidence chain

Qualitative analysis identifies what is present from observations. For carbonyl compounds, OCR expects a chain of evidence:

The first test tells you whether the compound is an aldehyde or ketone. The Tollens' test then tells you whether that carbonyl compound is specifically an aldehyde. If 2,4-DNP is positive but Tollens' reagent gives no visible change, the compound is a ketone.

The wording matters. "Carbonyl compound" in this lesson means aldehyde or ketone. A carboxylic acid or ester also contains a $\ce{C=O}$ bond, but it is not the positive inference from 2,4-DNP in this OCR test.

2,4-DNP detects carbonyl compounds

2,4-dinitrophenylhydrazine is commonly used as Brady's reagent. Add it to a small sample of the organic compound. If an aldehyde or ketone carbonyl group is present, a yellow, orange or red precipitate forms.

The precipitate is a solid derivative of the original carbonyl compound. OCR requires you to know that the derivative can be used for identification, but not its structure and not the reaction equation.

Worked example: detecting a carbonyl group

An unknown compound is tested with 2,4-DNP. A bright orange precipitate forms.

The reagent is 2,4-DNP and the observation is a coloured precipitate. The inference is that the unknown contains an aldehyde or ketone carbonyl group. This test alone does not tell you which aldehyde or ketone it is, and it does not distinguish aldehyde from ketone.

Using the derivative to identify the compound

To identify the particular carbonyl compound:

1. Form the 2,4-DNP derivative.
2. Purify the solid derivative if required.
3. Measure the melting point of the solid derivative.
4. Compare that melting point with known values in a data book or database.

For a measured melting range, heat the purified, dry solid in a melting-point apparatus slowly near the expected melting temperature and record the start and finish of melting. A narrow range supports purity; impurities can lower and broaden the range, making identification less reliable.

The melting point is used because different carbonyl compounds form derivatives with different melting points. The colour of the precipitate is not enough for identification.

Worked example: spotting an exam trap

A student writes, "Add 2,4-DNP. If an orange precipitate forms, measure the boiling point of the precipitate and compare it with a database."

The reagent and observation are acceptable, but the identification method is wrong. OCR expects the melting point of the solid derivative, not a boiling point and not chromatography, for this specific 2,4-DNP identification step.

Obtaining a reliable derivative melting point

Collect the precipitate by filtration. Recrystallise it from the minimum hot suitable solvent, cool to form crystals, filter, wash with a little cold solvent and dry. Load a small amount into a melting-point capillary. Rapid heating near melting can overshoot the true temperature; slow heating gives a more reliable start-to-finish range. Compare the range with reference values measured for the same derivative, using the reference uncertainty rather than claiming that any numerical match proves identity.

Worked example: using a derivative melting range

A dry, recrystallised 2,4-DNP derivative melts at $163.5\text{–}164.5^\circ\mathrm{C}$. Reference derivatives of compounds P, Q and R melt at 112, 164 and 196 degrees Celsius. The range matches Q, supporting Q as the starting carbonyl compound; the value belongs to the derivative, not the original liquid. This conclusion is strengthened by a narrow range and independent evidence. A broad range from an impure derivative would not justify choosing a single compound just because the top of its range matched a table value.

Tollens' reagent detects aldehydes

Tollens' reagent is ammoniacal silver nitrate. It is used after, or alongside, evidence that a carbonyl compound is present, because it distinguishes aldehydes from ketones.

Use a fresh portion of the original organic sample for Tollens' reagent, not the isolated 2,4-DNP derivative. For an aldehyde, warm this sample gently with Tollens' reagent. A positive result is a silver mirror on the inside of the test tube, or a grey/black precipitate of silver. A ketone gives no visible change under the conditions of this test.