Baeyer's chief achievements include the synthesis and description of the plant dye Indigo, the discovery of the phthalein dyes, and the investigation of polyacetylenes, oxonium salts, nitroso compounds (1869) and uric acid derivatives (1860 and onwards) (including the discovery of barbituric acid (1864), the parent compound of the barbiturates). He was the first to propose the correct formula for indole in 1869, after publishing the first synthesis three years earlier. His contributions to theoretical chemistry include the 'strain' (Spannung) theory of triple bonds and strain theory in small carbon rings.