The group in which nitrogen contains double bond and present in hetrocyclic ring cannot be estimated by Kjeldahl’s method.
The structure of Aniline is as follows,
The nitrogen present in this structure does not contain double bond so it can be estimated by Kjeldahl’s method.The structure of Azobenzene possesses, NN−=− group.
The nitrogen present in this structure contains double bond so it cannot be estimated by Kjeldahl’s method.
The structure of Nitrobenzene is as follows,
The nitrogen present in this structure contains double bond so it cannot be estimated by Kjeldahl’s method.
The structure of Pyridine is as follows,
In this structure nitrogen is present in hetrocyclic ring so it cannot be estimated by Kjeldahl’s method.Thus, the correct option is (1).