Fehling's Test

Overall Reaction:

$$\text{R--CHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \xrightarrow{\Delta} \text{R--COO}^- + \text{Cu}_2\text{O}\downarrow \text{ (Brick-Red Precipitate)} + 3\text{H}_2\text{O}$$

Concept: Diagnostic oxidation distinguishing between types of carbonyl setups using an alkaline copper sulfate tartrate solution mixture. The weak reagent can only target highly reactive aliphatic forms, yielding a distinct colored solid settlement.

Notation:

• $$\text{Cu}_2\text{O}(s)$$ = Cuprous oxide forming a bright red precipitate;
• $$\text{Positive for aliphatic aldehydes}$$ = Formaceous or extended straight chains react completely;
• $$\text{Negative for aromatic aldehydes and ketones}$$ = Benzaldehyde variations fail due to resonance stabilization of the carbonyl group.