A practical Brønsted acid promoted benzylic C-H functionalization of 2-alkylazaarenes and nucleophilic addition to nitroso compounds was developed under mild conditions. Switched by Brønsted acids, this method can afford azaarene-2-aldimines, azaarene-2-carbaldehyde, or azaarene-2-oximes selectively. No metal, base, oxidant, or other additives were required.