O634: Identification of Aldehydes – Benedict Test

Introduction

Aliphatic aldehydes and reducing sugars will form a yellow-to-red precipitate with Benedict's reagent, causing the blue solution to appear green-to-red.  This reaction can be used as a test for such aldehydes.

Equations

RCHO + 2Cu(citrate)22+ → RCOOH + Cu2O(s) + H2O

To Conduct Demonstration:  

  1. Measure 5mL 0.1% glucose solution into a 200mm test tube.
  2. Add 5mL Benedict's reagent to the tube.
  3. Place the test tube into a beaker of boiling water for 5 minutes.
  4. Measure 5mL Benedict's reagent and 5mL water into a second test tube and place in the boiling water (as a control).
  5. Place both test tubes in a lighted display rack for comparison.

 Safety and Disposal 

Wear gloves and avoid contact with skin by solutions.  Handle hot water bath with care.

Reference:

D.J. Pasto and C.R. Johnson, Laboratory Text for Organic Chemistry, p 409,1979.

Acknowledgment:

Margaret Asirvatham, Spring 1992