Fehling's Test for Carbonyls
Welcome to the specialized laboratory unit on Fehling's Test! Abhishek Sengar Sir demonstrates how Fehling's solution (a mixture of copper sulfate and alkaline Rochelle salt) selectively oxidizes aliphatic aldehydes to carboxylates, forming a classic brick-red precipitate of cuprous oxide, while remaining inert to ketones and aromatic aldehydes.
Video Lecture Broadcast
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Detailed Notes: Fehling's Test & Carbonyl Analysis
What is Fehling's Solution?
Fehling's Solution is an alkaline oxidizing reagent used specifically to detect aliphatic aldehydes and reducing sugars (such as Glucose and Fructose). It is prepared by mixing equal volumes of two distinct commercial stock solutions:
Fehling's Solution A
An aqueous solution of Copper(II) Sulfate Pentahydrate ($CuSO_4 \cdot 5H_2O$), which displays a clear, light blue color due to solvated $[Cu(H_2O)_6]^{2+}$ ions.
Fehling's Solution B
A clear, colorless alkaline solution containing Sodium Potassium Tartrate ($\text{KNaC}_4\text{H}_4\text{O}_6 \cdot 4\text{H}_2\text{O}$) (classically known as Rochelle Salt) dissolved in Sodium Hydroxide ($NaOH$).
Mechanism & Reaction Equations
When an aliphatic aldehyde ($R-CHO$) is heated with the deep-blue Fehling's mixture, the aldehyde is oxidized to a carboxylate anion ($R-COO^-$), while copper(II) is reduced to copper(I) oxide:
Aliphatic Aldehyde + Tartrato-Copper(II) $\implies$ Carboxylate + Cuprous Oxide Precipitate
Observed Transition Stages:
During heating, the color of the solution transitions from **deep blue** to a **muddy green/yellow-green intermediate** (as a mixture of blue $Cu^{2+}$ and red $Cu^+$ forms) and finally yields a dense, solid **brick-red precipitate of Cuprous Oxide ($Cu_2O$)** that settles at the bottom of the tube.
The Aromatic Aldehyde Exception (Highly Tested)
As Abhishek Sir strongly emphasizes, Fehling's test cannot be used to oxidize aromatic aldehydes (such as Benzaldehyde, $C_6H_5CHO$). This makes it a crucial reagent to distinguish between aliphatic and aromatic aldehydes.
Why Aromatic Aldehydes show a NEGATIVE test:
In aromatic aldehydes, the carbonyl carbon is directly attached to the benzene ring. The electron-donating resonance ($+M$) effect of the benzene ring delocalizes the positive charge on the carbonyl carbon, making it far **less electrophilic** and highly resistant to oxidation by weaker oxidizing agents like Fehling's solution. Tollens' reagent, being a slightly stronger oxidizer, is required to oxidize Benzaldehyde.
Summary of Fehling's Test Outcomes
| Compound Class | Representative Example | Visual Result | Reaction Chemistry |
|---|---|---|---|
| Aliphatic Aldehyde | Acetaldehyde / Propionaldehyde | Brick-Red Precipitate | Oxidized to acetate; $Cu^{2+}$ reduced to $Cu_2O\downarrow$. |
| Reducing Sugar | Glucose / Fructose | Brick-Red Precipitate | Hemiacetal ring opens to form free aldehyde; yields positive test. |
| Aromatic Aldehyde | Benzaldehyde | Remains Deep Blue | Carbonyl carbon deactivated by benzene $+M$ resonance. |
| Ketones | Acetone / Butanone | Remains Deep Blue | No oxidizable hydrogen directly bound to carbonyl carbon. |
| Alcohols | Ethanol | Remains Deep Blue | No reaction. Alcohols do not reduce Cu(II) complexes. |
Fehling's Lab Visualizer
Mix Fehling's A & B, add your organic sample, and heat the test tube to watch coordinates shift and precipitates form live!
Amine specimen loaded.
Select Step 1 to mix Fehling's A & B solutions together!
Rochelle Salt Complex Builder
Investigate the coordination complex chemistry parameters that keep copper(II) ions dissolved in basic medium.
Lecture Supplementary Quiz
Validate your understanding of aldehydes, ketones, and Fehling's coordination redox with immediate results.
Need help with Fehling's oxidation?
If you have doubts regarding aldehyde distinctions, Rochelle salt chelation, or organic qualitative tests, email Abhishek Sir directly!
Email abhishek.sengar@chemca.in →
Very clear and straight to the point.
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