Question

When acetaldehyde is heated with Fehling’s solution, it gives a precipitate of:
a)$C{u_2}O$
b)$Cu + C{u_2}O + CuO$
c)$Cu$
d)$CuO$

Answer 1

Hint: Fehling’s solution is a chemical reagent which is used to distinguish between water- soluble aldehyde and ketone functional group. It is also used as a test for reducing and non-reducing sugars. The precipitate formed is red in colour and is only given when Fehling’s test is done for aldehyde.

Complete step by step answer:
Fehling’s solution is a widely used reagent to distinguish between aldehydes and ketones. Fehling’s solution is prepared by mining Fehling A (Deep blue aqueous solution of copper (II) sulphate) and Fehling B (a colourless solution of aqueous Potassium Sodium tartrate, also known as Rochelle salt) and then made strongly alkaline with potassium hydroxide. The deep blue active ingredient in Fehling’s solution is the bis complex of $Cu^{2+}$. The net reaction between an aldehyde and the copper (II) ions in Fehling’s solution may be written as:
\[RCHO{\text{ }} + {\text{ }}2C{u^{2 + }} + SO{H^ - } \to {\text{ }}RCO{O^ - } + {\text{ }}C{u_2}O{\text{ }} + 3{H_2}O\]
Thus, the red precipitate formed when Fehling’s solution reacts with aldehyde is\[C{u_2}O\].
Thus, option A is the correct answer.

Note:
Fehling’s solution does not give a red precipitate with ketone except for alpha-hydroxy ketone which are oxidised by it. The major difference between aldehyde and ketone is the H present in aldehyde. It makes aldehydes very easy to oxidise or in another way, aldehydes are strong reducing agents. Because ketones do not have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents like potassium permanganate solution oxidise ketones, by breaking carbon-carbon bonds.