a process for converting an indene , a polyindene , a dihydronaphthalene or a polydihydronaphthalene to 4 - nitro - o - phthalic acid which involves nitrating said indene , polyindene , dihydronaphthalene or polydihydronaphthalene and then oxidizing said nitrated indene , polyindene , dihydronaphthalene or polydihydronaphthalene with nitric acid .