Color reactions, general discussion

Oxidation. Hydrogen peroxide is a strong oxidant. Most of its uses and those of its derivatives depend on this property. Hydrogen peroxide oxidizes a wide variety of organic and inorganic compounds, ranging from iodide ions to the various color bodies of unknown stmcture in ceUulosic fibers. The rate of these reactions may be quite slow or so fast that the reaction occurs on a reactive shock wave. The mechanisms of these reactions are varied and dependent on the reductive substrate, the reaction environment, and catalysis. Specific reactions are discussed in a number of general and other references (4,5,32—35). 

The general aspects of the aldehyde-acid reaction were discussed in Chapter 2. Thus it is readily understood that catechins, for example, can react with aromatic aldehydes in the presence of strong acids to yield colored triphenylmethane dyes [26]. 

Butter Yellow.—The di-methyl derivative of amino azo benzene which we have referred to in our discussion of the general method for the formation of amino azo compounds by the Griess reaction (p. 569), is also a dye known as butter yellow It is insoluble in water, but soluble in oils and, therefore, is used to color butter. 

It was recognized centuries ago that many substances could be divided into the two general categories. Acids have a sour taste (as in lemon juice), dissolve many metals, and turn litmus paper red. Bases have a bitter taste (as in soaps), feel slippery, and turn litmus paper blue. The chemical reaction between an acid and a base is called neutralization. The products of neutralization reactions are neither acids nor bases. Litmus paper is an example of an acid-base indicator, a substance that changes color when changing from an acid to a base. Definitions of acids and bases are discussed in Skill 4.2c... 

Although not generally described as such, certain reactions of oxidation hair dyes are examples of in situ polymerizations in hair. These consist of the oxidation of electron-rich aromatic amine and phenol monomers that condense with each other and perhaps even attach to amino acid residues of hair. The net result, at least with products containing p-phenylenediamine (PPD) and resorcinol, is the formation of polyindophenol-type polymeric pigments [81-83] that render color to the hair. (See the discussion on oxidation hair dyes in Chapter 6 and its references for additional details.)... 

---