Chemical Tests for Aldehydes & Ketones

Overview

This document outlines the procedures for 2 tests used to identify aldehydes and ketones:

• 2,4-DNP Test
• Tollen’s Test

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2,4-DNP Test for Aldehydes and Ketones

Standards

• Cyclohexanone
• Benzophenone
• Benzaldehyde

Procedure

1. Add a solution of 1 or 2 drops or 30 mg of the unknown compound in 2 mL of 95% ethanol to 3 mL of 2,4-dinitrophenylhydrazine reagent.
2. Shake vigorously.
3. If no precipitate forms immediately, allow the solution to stand for 15 minutes.

• Note: The 2,4-dinitrophenylhydrazine reagent will already be prepared.

Positive Test

• Formation of a precipitate indicates a positive test.

Complications

• Some ketones yield oils that will not solidify.
• Some allylic alcohols may be oxidized by the reagent to aldehydes, resulting in a positive test.
• Unpurified alcohols may contain aldehyde or ketone impurities.

Tollen’s Test for Aldehyde

Standards

• Cyclohexanone
• Benzaldehyde

Procedure

1. Add one drop or a few crystals of the unknown to 1 mL of freshly prepared Tollens reagent.
2. Gentle heating can be employed if no reaction is observed immediately.

Tollens Reagent Preparation

• Clean a test tube with 3M sodium hydroxide.
• Place 2 mL of 0.2 M silver nitrate solution in the test tube.
• Add a drop of 3M sodium hydroxide.
• Add 2.8% ammonia solution drop by drop, with constant shaking, until almost all silver oxide precipitate dissolves. Do not use more than 3 mL of ammonia.
• Dilute the entire solution to a final volume of 10 mL with water.

Positive Test

• Formation of a silver mirror or a black precipitate indicates a positive test.

Complications

• The test tube must be clean and oil-free for a silver mirror to be observed.
• Easily oxidized compounds (e.g., aromatic amines and some phenols) may yield false positives.

Cleaning Up

• Place all solutions used in this experiment in an appropriate waste container.