N-nitrosamine
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R1 and R2 could be hydrogen atoms, alkyl groups, aryl groups, or any combination thereof.
eg:
An N-nitrosamine is stable only if neither R1 nor R2 is a hydrogen atom. Thus, 1 is unstable, and 2 and 3 are stable. 1 exists mostly as the tautomer (4), which is called a diazotic acid.
see also nitroso group