Question

What happens when Acetaldehyde is treated with Fehling’s Solution?

Answer 1

1. When Acetaldehyde is treated with Fehling's Solution, a red precipitate of Cuprous ions is formed.
2. Fehling's solution is a complex of Tartaric acid of Cupric ions which is a weak oxidizing agent.
3. It does not oxidize aromatic aldehydes because of the presence of the carbonyl group which acts as an electron-withdrawing group and the absence of alpha hydrogen.
4. In this reaction, Acetaldehyde reduces Cupric ions(blue color) into Cuprous ions(red color) but itself does not get oxidized to form a carboxylic acid. It gets oxidized to acetate ion

$$\begin{gathered} {\mathrm{CH}}_3\mathrm{CHO}\hspace{0.17em}\left(\mathrm{aq}\right)+2{\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)+5{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{CH}}_3{\mathrm{COO}}^{-}\left(\mathrm{aq}\right)+\hspace{0.17em}{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Acetaldehyde}\mathrm{Red}\mathrm{ppt}. \end{gathered}$$