Experiment 76

Objective

To identify the presence of aldehyde and ketone groups in a carbonyl compound using 2,4-dinitrophenylhydrazine.

Introduction

2,4-Dinitrophenylhydrazine (DNP) is a reagent that reacts with the carbonyl groups of aldehydes and ketones to form a hydrazone, producing an orange-red precipitate. This reagent is sensitive to shock and friction due to its explosive nature, so it is generally supplied wet to reduce hazard risks. DNP is widely used for qualitative testing of carbonyl groups associated with aldehydes and ketones.

Materials

Chemicals

●      2,4-Dinitrophenylhydrazine

●      Acetone (as a sample carbonyl compound)

●      Concentrated HCl

●      2 N HCl

Apparatus

●      Test tube

●      Test tube holder

●      Glass stirring rod

●      10 mL snap-cap vials

●      Beaker

Reagents

●      5–6 ml of acetone in a test tube

●      1–2 ml of HCl, added dropwise

●      3–4 ml of 2,4-dinitrophenylhydrazine solution

Procedure

1.    Place the acetone sample in a test tube and secure it in a test tube holder.

2.    Add HCl dropwise to the test tube.

3.    Add the 2,4-dinitrophenylhydrazine to the tube.

4.    Shake the mixture gently; heat if necessary to initiate precipitation.

5.    Observe the test tube for the formation of precipitate.

Observation

The appearance of an orange-red precipitate confirms the presence of an aldehyde or ketone group in the sample.

Questions

1.    What is 2,4-dinitrophenylhydrazine?

2.    How is the reagent solution prepared?

3.    What color precipitates are typically observed in the presence of an aldehyde or ketone?