Ammonium thioglycolate

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Ammonium thioglycolate
Image:Ammonium thioglycolate.png
Systematic name	Ammonium 2-mercapto- acetate
Other names	Perm salt
Molecular formula	NH4+HSCH2COO−
Molar mass	109.15 g/mol
CAS number	[5421-46-5]
Density	? g/cm³
Solubility (water)	Soluble
Melting point	? °C
Boiling point	? °C
Disclaimer and references

Ammonium thioglycolate, also known as perm salt, is the chemical compound with the formula HSCH₂CO₂NH₄.

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 the free carboxylic acid thioglycolic acid (HSCH₂CO₂H) and ammonia:

HSCH₂COO⁻ + NH₄⁺ ⇌ HSCH₂COOH + NH₃

Contents 1 Chemical concepts related to perms 2 The actual chemistry of perms 3 Popular culture 4 External links

Chemical concepts related to perms

When discussing the chemistry of perms, one should consider two chemical facts. First is the thiol-disulfide equilibrium:

RSH + R'SSR' ⇌ R'SH + RSSR'

where R and R' are organic substituents such as methyl (-CH₃), ethyl (-C₂H₅), or -CH₂COO⁻.

The thiol-disulfide exchange reaction is accelerated by bases such as ammonia, because the base generates some thiolate anion (RS^-), which attacks the disulfide. Thus the ammonia plays multiple roles (and more, see below) in this application.

The second chemical fact is that polar molecules are less volatile than nonpolar ones. So the glycolate substituent makes the thiol non-volatile and hence non-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.

The actual 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.

Popular culture

In the 2001 film Legally Blonde, law student Elle Woods wins her first case by her knowledge of the chemistry of ammonium thioglycolate (which she pronounces "ammonium thyglocolate"), something she claims "any Cosmo girl would have known."

Wikipedia information about Ammonium thioglycolate This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ammonium thioglycolate". It may not have been reviewed by professional editors (see full disclaimer). Help contribute to Americola and edit this article.