The Kjeldahl method of Nitrogen estimation fails for which of the following reaction products?

The Kjeldahl method involves converting organic nitrogen into ammonium sulfate by heating with concentrated sulfuric acid. However, this method fails for compounds containing nitrogen in:

1. Nitro groups (-NO2)
2. Azo groups (-N=N-)
3. Nitrogen present in a ring structure (like Pyridine)

This is because the nitrogen in these functional groups cannot be quantitatively converted to ammonium sulfate under standard Kjeldahl digestion conditions.

Note: Amines (-NH2), amides, and nitriles (-CN) generally give a positive Kjeldahl test because they can be successfully reduced/hydrolyzed to ammonia.