Free Science Lessons: Identifying Aldehydes and Ketones with 2,4-Dinitrophenylhydrazine

Introduction

• Objective: Understand how to use 2,4-dinitrophenylhydrazine (2,4-DNP) to identify aldehydes and ketones.
• Context: Part of a series on reactions of aldehydes and ketones, focusing on their carbonyl group.

Chemical Background

• 2,4-Dinitrophenylhydrazine (2,4-DNP):

  • Based on hydrazine.
  • One hydrogen in hydrazine is replaced with a benzene ring with two nitro groups.
  • Nitro groups are located at positions 2 and 4 (hence the name).

• Structure:

  • Hydrazine group on carbon 1 of the benzene ring.
  • Used in a solution with methanol and sulfuric acid, known as Brady's reagent.
  • This solution has a pale orange color.

Testing for Aldehydes and Ketones

• Reaction with Carbonyl Group:

  • 2,4-DNP reacts with carbonyl group in aldehydes and ketones.
  • Forms a yellow or orange precipitate.
  • Indicates presence of an aldehyde or ketone.
  • Forms 2,4-dinitrophenylhydrazone derivative.

• Identification Process:

  • Determine the melting point of the precipitate to identify the specific aldehyde or ketone.
  • Steps:
    1. React carbonyl compound with 2,4-DNP to produce a yellow-orange solid.
    2. Separate the solid by filtration.
    3. Dissolve solid in ethanol in a warm water bath.
    4. Cool solution to recrystallize pure crystals.
    5. Dry crystals and determine melting point.

Additional Identification Methods

• Database Reference:

  • Compare melting point with known data to identify the aldehyde or ketone.
  • Note: Some derivatives may have similar melting points.

• Boiling Point Consideration:

  • Combine melting point data with boiling point for accurate identification.

Next Steps

• The following video will cover distinguishing between aldehydes and ketones.