Reactions with Ammonia and its Derivatives
Aldehydes and Ketones react with Ammonia ($NH_3$) and its derivatives ($NH_2-Z$) to form compounds containing a carbon-nitrogen double bond ($>C=N-Z$) with the elimination of a water molecule. This is a characteristic Nucleophilic Addition-Elimination reaction.
1. General Reaction
The general formula for ammonia derivatives is $H_2N-Z$, where $Z$ can be various groups like Alkyl, Hydroxyl, Amino, Phenyl, etc.
Mechanism Summary:
- Addition: The nucleophilic nitrogen attacks the electrophilic carbonyl carbon. A proton transfers from nitrogen to oxygen to form a tetrahedral intermediate (carbinolamine).
- Elimination: The intermediate loses a molecule of water (acid-catalyzed dehydration) to form the $C=N$ double bond.
2. The Critical Role of pH
pH Control (Optimum pH $\approx$ 3.5)
The reaction requires a weak acid catalyst to protonate the carbonyl oxygen (making it more electrophilic) and to facilitate the elimination of water ($OH$ is a poor leaving group, $H_2O$ is good).
- If pH is too low (Strongly Acidic): The amine ($NH_2-Z$) gets protonated to form an ammonium salt ($NH_3^+-Z$). Since it loses its lone pair, it is no longer nucleophilic and cannot attack the carbonyl.
- If pH is too high (Basic): There is no protonation of the carbonyl oxygen or the -OH group, making the reaction extremely slow.
3. Specific Derivatives and Products
The table below summarizes the reagents and their corresponding products.
| Reagent Name | Formula ($NH_2-Z$) | Product Name | Product Structure |
|---|---|---|---|
| Hydroxylamine | $NH_2-OH$ | Oxime | $>C=N-OH$ |
| Hydrazine | $NH_2-NH_2$ | Hydrazone | $>C=N-NH_2$ |
| Phenylhydrazine | $NH_2-NHPh$ | Phenylhydrazone | $>C=N-NHPh$ |
| 2,4-DNP | $NH_2-NH-C_6H_3(NO_2)_2$ | 2,4-DNP Hydrazone | Orange/Red PPT |
| Semicarbazide | $NH_2-NH-CO-NH_2$ | Semicarbazone | $>C=N-NH-CO-NH_2$ |
| Primary Amine | $R-NH_2$ | Schiff's Base (Imine) | $>C=N-R$ |
4. Key Exam Points
1. Semicarbazide Reaction Site
Semicarbazide has two $-NH_2$ groups. However, only one reacts.
The $NH_2$ attached directly to the carbonyl group ($CO$) is involved in resonance (amide-like), making its lone pair unavailable for nucleophilic attack.
Reaction occurs at the terminal $-NH_2$ group attached to the other nitrogen.
Correct Active Site: $\mathbf{H_2N}-NH-CO-NH_2$
2. Brady's Reagent (2,4-DNP)
2,4-Dinitrophenylhydrazine (2,4-DNP) is used to identify aldehydes and ketones. A positive test results in a Yellow, Orange, or Red precipitate of the 2,4-DNP hydrazone. This does not happen with carboxylic acids or esters.
5. Stereochemistry of Products
The formation of Oximes and Hydrazones creates a $C=N$ double bond. If the groups attached to the carbon are different ($R \neq R'$), Geometrical Isomerism (Syn/Anti) is possible.
Derivatives Quiz
Test your concepts on Carbonyl derivatives. 10 MCQs with explanations.
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