Ammonium thioglycolate

Chemistry

Being the salt of a weak acid and weak base, ammonium thioglycolic acid exists in solution as an equilibrium mixture of the salt itself as well as thioglycolic acid and ammonia:

Thioglycolate, in turn, is able to cleave disulfide bonds, capping one side with a hydrogen and forming a new disulfide with the other side:

The chemistry of perms

A solution containing ammonium thioglycolate contains a lot of free ammonia, which swells hair, rendering it permeable. The thioglycolic acid in the perm solution reduces the disulfide cystine bonds in the cortex of the hair. In a sense, the thioglycolate removes crosslinks. After washing, the hair is treated with a mild solution of hydrogen peroxide, which oxidizes the cysteines back to cystine. These new chemical bonds impart the structural rigidity necessary for a successful perm. The rigidification process is akin to the vulcanization of rubber, where commonly polysulfide linkages are used to crosslink the polymer chains. However, not as many disulfide bonds are reformed as there were before the permanent. As a result, the hair is weaker than before the permanent was applied and repeated applications over the same spot may eventually cause strand breakage.

Since polar molecules are less volatile than nonpolar ones, the glycolate substituent makes the thiol non-volatile and hence less odorous. An added advantage is that the glycolate confers some solubility in water. One could almost certainly use HSCH₃ and ammonia to give a perm, but there would be serious olfactory consequences.