Oxidation, Reduction & $\alpha$-Hydrogen Reactions
Decode the most heavily tested reactions of Carbonyls. Master the Tollens vs. Fehling traps, the Aldol Condensation shortcut, and the Cannizzaro disproportionation.
Module Focus: The Alpha Carbon & Redox
Aldehydes and Ketones behave very differently under oxidation due to the presence (or absence) of a hydrogen atom directly attached to the carbonyl carbon. Furthermore, the strong electron-withdrawing nature of the carbonyl group makes the hydrogens on the adjacent carbon ($\alpha$-hydrogens) highly acidic. This acidity drives the legendary Aldol Condensation, while the lack of it drives the Cannizzaro reaction.
1. Oxidation & Distinction Tests
Aldehydes are easily oxidized to carboxylic acids by mild oxidizing agents because they have a $C-H$ bond on the carbonyl carbon. Ketones resist oxidation and require vigorous conditions (which cleave carbon-carbon bonds). We use mild oxidizers as qualitative tests to distinguish them.
Reagent: Ammoniacal Silver Nitrate $[Ag(NH_3)_2]^+$
Aldehydes reduce the $Ag^+$ ion to metallic Silver ($Ag$), which deposits as a shiny mirror on the test tube wall.
(Aliphatic AND Aromatic)
Reagent: Aqueous $CuSO_4$ mixed with Sodium Potassium Tartrate (Rochelle salt) in an alkaline medium.
Aldehydes reduce the blue $Cu^{2+}$ ion to a red-brown precipitate of Cuprous Oxide ($Cu_2O$).
(Benzaldehyde gives NO reaction)
Used to detect the presence of a Methyl Ketone group ($CH_3-CO-$) or a group that can oxidize to it ($CH_3-CH(OH)-$ like ethanol or 2-propanol).
Result: Bright Yellow Precipitate of Iodoform ($CHI_3$)
2. Reduction to Hydrocarbons (Alkanes)
The carbonyl group ($>C=O$) can be completely stripped of oxygen and reduced to a methylene group ($>CH_2$). The choice of reagent depends on the pH sensitivity of other functional groups in the molecule.
| Reaction Name | Reagents | Condition / Best Used For |
|---|---|---|
| Clemmensen Reduction | Zinc Amalgam ($Zn-Hg$) + Conc. $HCl$ | Highly Acidic (Use if molecule is base-sensitive) |
| Wolff-Kishner Reduction | Hydrazine ($NH_2NH_2$) then $KOH/\text{Glycol}, \Delta$ | Highly Basic (Use if molecule is acid-sensitive) |
3. Reactions due to $\alpha$-Hydrogen: Aldol Condensation
The $\alpha$-hydrogens of aldehydes and ketones are acidic because the resulting enolate carbanion is stabilized by resonance with the carbonyl group. Aldehydes and ketones having at least one $\alpha$-hydrogen undergo Aldol condensation in the presence of dilute alkali (like dilute $NaOH$ or $Ba(OH)_2$).
Line up the Carbonyl Oxygen of Molecule 1 with two $\alpha$-Hydrogens of Molecule 2. Remove $H_2O$ and double bond them!
When Aldol is carried out between two different aldehydes/ketones, and BOTH contain $\alpha$-hydrogens, it yields a messy mixture of four different products (2 self-aldol + 2 cross-aldol).
Smart Synthesis Rule: To get a good yield of a single cross-aldol product, use one molecule that has NO $\alpha$-hydrogens (like Benzaldehyde or Formaldehyde) and one that does!
4. Reactions lacking $\alpha$-Hydrogen: Cannizzaro Reaction
Aldehydes which do NOT have an $\alpha$-hydrogen atom undergo self-oxidation and reduction (Disproportionation) when treated with concentrated alkali (like 50% $NaOH$ or $KOH$).
Must be an aldehyde with ZERO $\alpha$-hydrogens.
Classic NEET Examples:
Benzaldehyde ($C_6H_5CHO$),
Chloral ($CCl_3CHO$)
One molecule is reduced to an Alcohol. The other molecule is oxidized to a Carboxylic Acid Salt.
$CH_3OH \ (\text{Methanol}) \ +$
$HCOOK \ (\text{Pot. Formate})$
Aromatic aldehydes and ketones (like Benzaldehyde or Acetophenone) undergo electrophilic substitution. The carbonyl group ($>C=O$) is a strong electron-withdrawing group ($-M$ effect). Therefore, it deactivates the ring and directs incoming electrophiles strictly to the meta position.
NEET Grand Test: Carbonyl Reactions
15 High-Yield Questions testing Tollens/Fehling traps, Aldol product predictions, and Cannizzaro conditions.
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