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

Prior to Lecture  (prep time ~ 20 min.)

  1. Prepare Benedict's reagent by dissolving 86.5g hydrated sodium citrate and 50g anhydrous sodium carbonate in 350mL distilled water.  Add 8.65g CuSO4 (copper sulfate) dissolved in 50mL water and dilute to 500mL.

  2. Prepare 0.1% glucose solution in dropper bottle

                            0.2g glucose/200mL

  3.  Assemble the following equipment:   
  •  two 10mL graduated cylinders 
  • hot plate
  • 1000mL beaker with water
  • two 200mm test tubes
  • gloves
  • goggles 

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.