Which of the following nitrogen containing compound does not give Lassaigne's test?

Step 1: Recall the Principle of Lassaigne's Test

In Lassaigne's test, the organic compound is fused with sodium metal. Covalently bonded elements are converted into their sodium salts:

• N → $\ce{NaCN}$ (detectable as Prussian blue with $\ce{FeSO4}$)
• S → $\ce{Na2S}$ (detectable as purple with sodium nitroprusside)
• Halogens → $\ce{NaX}$

For the Prussian blue test for nitrogen, $\ce{NaCN}$ must be formed.

Step 2: Which Compounds Can Form NaCN?

For N to be detected, it must be present in a form that can be converted to $\ce{NaCN}$ during sodium fusion.

• Urea $\ce{(NH2)2CO}$ ✅ → Na fusion → $\ce{NaCN}$ → Prussian blue
• Glycine $\ce{H2NCH2COOH}$ ✅ → Na fusion → $\ce{NaCN}$ → Prussian blue
• Hydrazine $\ce{N2H4}$ ✅ → Na fusion → $\ce{NaCN}$ → Prussian blue
• Phenylhydrazine $\ce{C6H5NHNH2}$ — This is a reducing agent. When fused with sodium, the N-N bond is weak and N may not be effectively converted to $\ce{NaCN}$. More importantly, the $\ce{N-N}$ hydrazine linkage in phenylhydrazine does not readily give the Prussian blue test.

Actually, the key point is: compounds with N-N bonds (hydrazo compounds, azo compounds) may NOT give the Prussian blue test because N is not converted to $\ce{CN^-}$ efficiently.

Step 3: Conclusion

Phenylhydrazine has N-N bonds — the nitrogen does not convert efficiently to $\ce{NaCN}$ during sodium fusion. Therefore it does NOT give a positive Lassaigne's test for nitrogen.

Answer: (a) Phenyl hydrazine

Key Points to Remember:

• Compounds with N-N bonds (phenylhydrazine, hydrazones, azo compounds) may not give Prussian blue in Lassaigne's test
• Azo compounds ($\ce{-N=N-}$) also do NOT give Kjeldahl's test for nitrogen
• Lassaigne's test works for: amines, amides, amino acids, nitriles, nitro compounds
• Exception: diazonium salts and azo compounds require special methods