Kjeldahl’s method cannot be used for:This question has multiple correct optionsA. PhN{{O}_{2}}B. PhNHCOPhC. AzobenzeneD. Pyridine

Kjeldahl's method is commonly used for the determination of nitrogen content in organic compounds, particularly those containing nitrogen in the form of amino groups (-NH2). Let's evaluate each option:

A. PhNO2 (Nitrobenzene): This compound doesn't contain nitrogen in a form suitable for analysis by Kjeldahl's method. Nitrogen is present in the form of a nitro group (-NO2), not an amino group. So, A is correct.

B. PhNHCOPh (Benzamide): This compound contains a nitrogen atom in an amide group (-NHCO-), which can be converted to ammonium ions and analyzed by Kjeldahl's method. So, B is incorrect.

C. Azobenzene: Azobenzene contains two nitrogen atoms connected via a double bond (-N=N-). While it doesn't have an amine group, it can potentially be analyzed by Kjeldahl's method if the nitrogen atoms are converted into an analyzable form. So, C could be correct depending on the specific conditions.

D. Pyridine: Pyridine contains a nitrogen atom in a heterocyclic ring. It does not contain an amine group, and its nitrogen cannot be analyzed directly by Kjeldahl's method. So, D is correct.

So, the correct options are A and D.