Question

Which of the following compound(s) give negative Fehling's test?

• A. Benzaldehyde
• B. Aliphatic ketone
• C. Terminal $\alpha$-hydroxy ketone
• D. Aliphatic aldehyde

Answer 1

Answer: (A), (B)

Aliphatic ketone

Fehling’s test consists of a solution that is usually prepared fresh in laboratories. Initially, the solution exists in the form of two separate solutions which are labelled as Fehling’s A and Fehling’s B. Fehling’s A is a solution containing copper(II) sulphate, which is blue. Fehling’s B is a clear liquid consisting of potassium sodium tartrate (Rochelle salt) and a strong alkali, usually sodium hydroxide. During the test solutions A and B are prepared individually and stored.

The reaction is as follows:
Benzaldehyde, aliphatic ketones gives negative Fehling's test. Aliphatic aldehyde and $\alpha$-hydroxy ketone (terminal) give positive Fehling's test.