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

In Fehling's test, when an aliphatic aldehyde is treated with Fehling's solution, a redox reaction occurs. Fehling's solution is composed of copper(II) sulfate, sodium potassium tartrate (Rochelle salt), and a strong base such as sodium hydroxide. The copper(II) ions in the solution (which give it a blue color) are reduced to copper(I) ions during the reaction.

The overall chemical reaction for an aliphatic aldehyde can be summarized as follows:

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

Here, RCHO represents the aliphatic aldehyde, and RCOO^- is its oxidized form (a carboxylate ion). The critical aspect of this reaction is the formation of copper(I) oxide (Cu₂O), which precipitates out as a red-brown solid. This color change from blue to red-brown confirms the presence of an aliphatic aldehyde.

Observation	Interpretation
Red-brown precipitate	Positive result for aliphatic aldehyde

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

Answer 2

In Fehling’s test for aliphatic aldehydes, a red-brown precipitate of copper(I) oxide (Cu₂O) is formed.

• Fehling’s solution contains copper(II) ions (Cu²⁺), which are blue in color.
• When an aldehyde is present, it reduces the copper(II) ions to copper(I) ions (Cu⁺), forming a red-brown precipitate of Cu₂O.
• This red-brown color indicates the presence of an aldehyde.

Thus, the color of the copper compound formed is red-brown.