Colour reaction of phenols with the gibbs reagent. The reaction mechanism and decomposition and stabilisation of the reagent

Abstract

The decomposition of the Gibbs reagents (2,6-dichloro- and 2,6-dibromoquinonechloroimines) in solution is shown to occur first under the formation of the quinoneimine (which reacts very rapidly with phenol and is the operative species in the colour reaction) and then products (one of which is the tetrahaloindophenol derivative) which do not react with phenol. The rate decomposition increases with increasing pH. The reagents, when purified by crystallisation fromn-heptane and kept in the dark at room temperature, are stable. Stock 0.01M solutions in ethanol or acetone are stable for at least 6 months if stored in a refrigerator. The optimum pH for the colour reaction with phenols is 8.5. Borate, phosphate or Britton-Robinson buffers are suitable, but not ammonia or glycocoll buffers.