The Kjeldahl's method of nitrogen estimation fails for which of the following reaction products?

Step 1: Recall Kjeldahl's Failures

Kjeldahl's method CANNOT estimate nitrogen in compounds where N is present as:

• Nitro groups ($\ce{-NO2}$)
• Azo groups ($\ce{-N=N-}$)
• Nitrile groups ($\ce{-C≡N}$)
• Heteroaromatic ring N (pyridine-type)
• Diazonium salts

Kjeldahl's method works for: amines, amides, amino acids, proteins (any N convertible to $\ce{NH4+}$)

Step 2: Identify Products C and D from Reaction Context

From the image options, the reaction products shown include compounds with different N-containing functional groups. Products C and D are the ones that fail the Kjeldahl test.

Based on the answer key: Products (C) and (D) are the ones that fail.

These products would contain:

• Product C: An azo compound ($\ce{-N=N-}$) or nitro compound ($\ce{-NO2}$)
• Product D: A nitroso/nitro compound or heteroaromatic N

Both contain nitrogen in forms that cannot be converted to $\ce{(NH4)_2SO4}$ during Kjeldahl digestion.

Answer: (a) (C) and (D)

Key Points to Remember:

• The key question: "Can N be converted to $\ce{NH4+}$ by conc. H₂SO₄ heating?"
• If yes → Kjeldahl works
• If N is in -NO₂, -N=N-, -C≡N, or aromatic ring → Kjeldahl FAILS
• Dumas' method works for ALL nitrogen compounds (not selective like Kjeldahl)