Diazotization gives diazonium derivative, which is a precursor to some diazo dyes. Alizarine Yellow R is a yellow colored azo dye made by the diazo coupling reaction. In the Doyle-Kirmse reaction certain diazo compounds react with allyl sulfides to the homoallyl sulfide. Organic pigments such as diazo or phthalocyanine compounds feature conjugated systems of double bonds. Huisgen investigated a series of cycloadditions between the 1,3-dipolar diazo compounds and various dipolarophilic alkenes. The serum of patient is mixed with diazo reagent. Vesicular film is sensitised with a diazo dye, which after exposure is developed by heat. Barium manganate has also been used to oxidize hydrazones to diazo compounds. It links diazo dyes to give a fast black. In the Buchner ring expansion diazo compounds react with aromatic rings with ring-expansion. A commercially relevant diazo compound is ethyl diazoacetate (NCHCOOEt). Several methods exist for the preparation of diazo compounds. Azines have been used as precursors to hydrazones and diazo compounds. The hydrazone is oxidized by iodine into a diazo intermediate. Of considerable practical value in the dye industry are the diazo coupling reactions.