Question

Why cannot secondary tertiary amine undergo carbylamine reaction?

Answer 1

The product formed in carbylamine reaction is an isocynide i.e R-NC.

Which is formed after removal of two hydrogen from amine, which is only possible only in primary amines.

In 2° and 3° amines, to form an isocyanide , N-R bond is to be broken, which is very strong bond therefore they will not form isocyanides, hence it doesn't undego carbyl amine reaction. (Study about it's mechanism, it will help you for sure)

Since Carbylamine reaction is possible only in primary-amines, it is used as a test to check whether the given amine us primary or not.

[BTW, it's undergoes in the sentence]