Question

What is the color of copper compound formed in Fehling’s test for aliphatic aldehydes?

• A. Green
• B. Blue
• C. Yellow
• D. Red brown

Answer 1

The Correct Option is D

Fehling's test involves a redox reaction when an aliphatic aldehyde reacts with Fehling's solution. This solution contains copper(II) sulfate, sodium potassium tartrate (Rochelle salt), and a strong base like sodium hydroxide. During the reaction, the blue copper(II) ions are reduced to copper(I) ions.

The general chemical equation for an aliphatic aldehyde in this test is:

RCHO + 2Cu²⁺ + 5OH^- → RCOO^- + Cu₂O + 3H₂O

In this equation, RCHO denotes the aliphatic aldehyde, and RCOO^- is its oxidized form, a carboxylate ion. The formation of copper(I) oxide (Cu₂O), which precipitates as a red-brown solid, is key to this reaction. The color change from blue to red-brown indicates the presence of an aliphatic aldehyde.

Observation	Interpretation
Red-brown precipitate	Positive for aliphatic aldehyde

Consequently, the color of the copper compound formed in Fehling’s test for aliphatic aldehydes is red-brown.