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Q8.18(ii):
Give plausible explanation for each of the following: (ii) There are two \(-NH_{2}\) groups in semicarbazide. However, only one is involved in the formation of semicarbazones.

Solution :

1. Structural Analysis of Semicarbazide

Semicarbazide is represented by the chemical formula $H_2N-NH-CO-NH_2$. It contains three nitrogen atoms, but the two nitrogen atoms typically referenced in the context of nucleophilic attack are the amino group ($-NH_2$) attached to the hydrazine moiety and the terminal amino group attached to the carbonyl ($-CO-NH_2$) group.

H₂N — NH — C(=O) — NH₂ Site of Nucleophilic Attack

2. Electronic Environment of the Nitrogen Atoms

The reactivity of a nitrogen atom as a nucleophile depends on the availability of its lone pair of electrons for donation to the electrophilic carbonyl carbon ($C=O$).

  • The Hydrazinic Nitrogen ($-NH_2$): The nitrogen adjacent to the $-NH-$ group possesses a lone pair that is localized and sterically accessible. This lone pair is available for attack on the carbonyl carbon of the aldehyde or ketone [Per the mechanism of Nucleophilic Addition-Elimination].
  • The Amide Nitrogen ($-NH_2$ attached to $C=O$): The lone pair on the terminal amino group is involved in resonance stabilization with the carbonyl group ($C=O$).

3. Resonance Stabilization Argument

The resonance structure of the $-CONH_2$ fragment significantly reduces the nucleophilicity of the terminal nitrogen:

$$H_2\ddot{N}-C(=O)-NH_2 \longleftrightarrow H_2N^+=C(O^-)-NH_2$$

Because the lone pair of this nitrogen is delocalized into the carbonyl group, its electron density is partially shifted toward the oxygen atom [By Resonance Theory]. Consequently, this nitrogen is much less nucleophilic and cannot effectively attack the carbonyl carbon of the substrate.

4. Comparative Reactivity Table

Nitrogen Group Electronic Status Nucleophilicity
Hydrazinic $-NH_2$ Localized lone pair High (Nucleophilic)
Amide $-NH_2$ Delocalized (Resonance) Low (Non-nucleophilic)

5. Conclusion

The formation of semicarbazone proceeds via the nucleophilic attack of the more basic/nucleophilic nitrogen on the carbonyl carbon. Since the terminal $-NH_2$ is part of an amide functional group, its lone pair is conjugated with the $C=O$ group, rendering it electronically deficient for nucleophilic participation. Thus, only the hydrazine-type nitrogen is involved in the reaction.

Final Solution: The terminal $-NH_2$ group in semicarbazide is not involved in the formation of semicarbazones because its lone pair of electrons is involved in resonance with the adjacent carbonyl group ($C=O$), thereby reducing its electron density and nucleophilicity. Conversely, the other $-NH_2$ group remains nucleophilic and reactive.


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