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When semicarbazide reacts with a ketone ...

When semicarbazide reacts with a ketone (or aldehyde) to form semicarbazone. Only one nitrogen atom of semicarbazide acts as a nucleophile and attack the carbonyl carbon of the ketone. The product fo the reaction consequently is `R_(2)C=N- NH-CONH_(2)` rather than `R_(2)C=NCONH-NH_2`. what factor account for the fact that two nitrogen atoms of semicarbazide are relatively non nucleophilic ?

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To address the question regarding the reactivity of semicarbazide with ketones or aldehydes, we need to analyze the structure of semicarbazide and the factors that influence the nucleophilicity of its nitrogen atoms. Here’s a step-by-step solution: ### Step 1: Understanding the Structure of Semicarbazide Semicarbazide has the following structure: - It contains three nitrogen atoms: one in the amine group (-NH2), one in the hydrazine-like part (-NH-), and one in the carbonyl part (-C(=O)NH2). - The general structure can be represented as H2N-NH-C(=O)-NH2. ### Step 2: Identifying the Nucleophilic Nitrogen When semicarbazide reacts with a ketone or aldehyde, the carbonyl carbon (C=O) is electrophilic and can be attacked by a nucleophile. In semicarbazide, the nitrogen atom that acts as a nucleophile is the one connected to the carbonyl carbon (the -NH- group). ### Step 3: Analyzing the Non-Nucleophilic Nitrogens The other two nitrogen atoms (the -NH2 and the -NH- connected to the carbonyl) are relatively non-nucleophilic due to the following reasons: - **Delocalization of Lone Pairs**: The lone pairs on the nitrogen atoms in semicarbazide can participate in resonance with the adjacent carbonyl group. This delocalization reduces the availability of these lone pairs for nucleophilic attack. - **Electronegativity**: Nitrogen is electronegative, and when it is adjacent to an electronegative atom (like oxygen in the carbonyl), it tends to hold onto its lone pair more tightly, making it less available for donation. ### Step 4: Conclusion on Nucleophilicity Due to the reasons mentioned above, only the nitrogen atom in the -NH- group is available to attack the carbonyl carbon of the ketone or aldehyde. The other two nitrogen atoms are involved in resonance and are therefore less nucleophilic. ### Final Answer The factor that accounts for the two nitrogen atoms of semicarbazide being relatively non-nucleophilic is the delocalization of their lone pairs with the adjacent carbonyl group, which makes them less available for nucleophilic attack. ---
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