Chemistry Of Permanent Hair Coloring

Chemistry of permanent hair coloring[edit]

Permanent hair coloring requires three components: (1) 1,4-diaminobenzene (historically) or 2,5-diaminotoluene (currently), (2) a coupling agent, and (3) an oxidant. The process is typically performed under basic conditions. The mechanism of oxidation dyes involves three steps:1) Oxidation of 1,4-diaminobenzene derivative to the quinone state. 2) Reaction of this diimine with a coupler compound (more detail below). 3) Oxidation of the resulting compound to give the final dye.

The preparation (dye precursors) is in the leuco (colorless) form. Oxidizing agents are usually hydrogen peroxide, and the alkaline environment is usually provided by ammonia. The combination of hydrogen peroxide and ammonia causes the natural hair to be lightened, providing a "blank canvas" for the dye. Ammonia opens the hair shaft pores so that the dye can actually diffuse inside the fiber. These dye intermediates and coupler compounds can undergo oxidation and coupling reaction as shown in the scheme below to form high molecular weight products, which are trapped in the hair matrix and cannot be readily removed through washing.

Various combinations of primary intermediates and coupler compounds provide a spectrum of shades of hair colors. The primary intermediates are aromatic para compounds, such as 1,4-diaminobenzene or 4-aminophenol. The coupler compounds (couplers) are meta-substituted derivatives of aniline. They come in three major classes based on the color that they produce when they react with the primary intermediate.

Couplers are chemical compounds that define the color of the hair dye. Shown here are three red couplers (A, B, C), two yellow-green couplers (D, E) and a blue coupler (F).

  • Blue couplers include 1,3-diaminobenzene and its derivatives.

  • Red couplers include phenols and naphthols, such as 3-aminophenol (CAS#591-27-5), 5-amino-2-methylphenol (CAS#2835-95-2) and 1-naphthol (CAS#90-15-3). The combination of 2,5-diaminotoluene with the coupler 3-aminophenol gives a magenta-brown dye, while the combination of 2,5-diaminotoluene with the coupler 1-naphthol gives a purple dye.

  • Yellow-green couplers include resorcinol, 4-chlororesorcinol, and benzodioxoles. These compounds produce broad-band absorption when they react to form dyes, allowing for more natural-looking hair colors. The combination of 2,5-diaminotoluene with the coupler resorcinol gives a greenish brown dye.

The first step shows the oxidation of p-phenylenediamine to the quinonediimine (C6H4(NH)2):

Quinodiimine.png

This species exists in equilibrium with the monoprotonated form (C6H4(NH)(NH2)+) (not shown). The second step involves the attack of this quinonediimine on the coupler. Inorganic chemistry, this reaction is called electrophilic aromatic substitution:

Quinodiimine+Coupler.png

In the third and final step, the product from the quinonediimine-coupler reaction oxidizes to the final hair dye.

QuinodiimineCouplerOx.png

It was once believed that the dye forms in the above reaction bonds to hair permanently.[5] It was later shown that the main reason that this reaction imparts a permanent color on hair by producing larger dye molecules, which is locked inside the hair.