Ammonium thioglycolate
Encyclopedia
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
:
, one should consider two chemical facts. First is the thiol-disulfide equilibrium:
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 thiol
ate 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.
, 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 cysteine
s 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.
, law student Elle Woods wins her first case by her knowledge of the chemistry of ammonium thioglycolate (which she pronounces "ammonium thyglocolate"), something she asserts "any Cosmo
girl would have known."
Being the salt of a weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...
and weak base, ammonium thioglycolic acid exists in solution as an equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
mixture of the salt itself as well as the free carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
thioglycolic acid
Thioglycolic acid
Thioglycolic acid is the organic compound HSCH2CO2H. It contains both a thiol and a carboxylic acid. It is a clear liquid with a strong unpleasant odor...
(HSCH2CO2H) and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
:
- HSCH2COO− + NH4+AmmoniumThe ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
HSCH2COOHThioglycolic acidThioglycolic acid is the organic compound HSCH2CO2H. It contains both a thiol and a carboxylic acid. It is a clear liquid with a strong unpleasant odor...
+ NH3AmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
Chemical concepts related to perms
When discussing the chemistry of permsPermanent wave
A permanent wave, commonly called a perm, involves the use of chemicals to break and reform the bonds of the hair. The hair is washed and wrapped on a perm rod and waving lotion is applied with a base. This solution creates a chemical reaction that softens the inner structure of the hair by...
, 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 thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
ate 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 hairHair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
, rendering it permeable. The thioglycolic acid
Thioglycolic acid
Thioglycolic acid is the organic compound HSCH2CO2H. It contains both a thiol and a carboxylic acid. It is a clear liquid with a strong unpleasant odor...
in the perm solution reduces
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
the disulfide cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...
bonds in the cortex
Cortex (anatomy)
In anatomy and zoology the cortex is the outermost layer of an organ. Organs with well-defined cortical layers include kidneys, adrenal glands, ovaries, the thymus, and portions of the brain, including the cerebral cortex, the most well-known of all cortices.The cerebellar cortex is the thin gray...
of the hair. In a sense, the thioglycolate removes crosslinks. After washing, the hair is treated with a mild solution of hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
, which oxidizes the cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
s back to cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...
. These new chemical bonds impart the structural rigidity necessary for a successful perm. The rigidification process is akin to the vulcanization
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
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 BlondeLegally Blonde
Legally Blonde is a 2001 American comedy film directed by Robert Luketic, written by Karen McCullah Lutz and Kirsten Smith, and produced by Marc E. Platt...
, law student Elle Woods wins her first case by her knowledge of the chemistry of ammonium thioglycolate (which she pronounces "ammonium thyglocolate"), something she asserts "any Cosmo
Cosmopolitan (magazine)
Cosmopolitan is an international magazine for women. It was first published in 1886 in the United States as a family magazine, was later transformed into a literary magazine and eventually became a women's magazine in the late 1960s...
girl would have known."