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Abstract
Practical forms of colour development are restricted to the oxidative condensation, discovered by Fischer, of p-aminodi-alkylanilines with couplers. Electrochemical and stereochemical considerations explain the activity of the developing agents and the reactions of their oxidation products. The various types of open-chain or cyclic active methylene couplers and of methine couplers are surveyed and the sites of coupling, elimination coupling, and the stereochemistry of coupling are discussed. Suggestions made for the mechanism of the condensation reaction are discussed and the most likely mechanism is elaborated. The absorption of the dyes in solution can be understood by an application of the resonance theory but additional considerations apply when the dyes are deposited in the image.