Thioglycolic acid (TGA) is the

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

HSCH₂CO₂H. TGA is often called mercaptoacetic acid (MAA). It contains both a

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

(

mercaptan In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

) and

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

functional groups. It is a colorless liquid with a strongly

unpleasant odor An odor (American English) or odour (English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a smell or a scent caused by one or more volatilized ...

. TGA is miscible with polar organic solvents.''The Merck index'', 14th ed.; O’Neil, Maryadele J., Ed.; Merck & Co., Inc.: Whitehouse Station, NJ, 2006; p. 9342.Robert Rippel "Mercaptoacetic Acid and Derivatives" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2012, Wiley-VCH, Weinheim. .

Uses

TGA is used as a chemical

depilatory Hair removal is the deliberate removal of body hair or head hair. This process is also known as epilation or depilation. Hair is a common feature of the human body, exhibiting considerable variation in thickness and length across different pop ...

and is still used as such, especially in salt forms, including calcium thioglycolate and sodium thioglycolate. TGA is the precursor to ammonium thioglycolate, which is used for permanents. TGA and its derivatives break the

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

bonds in the cortex of hair. One reforms these broken bonds in giving hair a "perm". Alternatively and more commonly, the process leads to

depilation Hair removal is the deliberate removal of body hair or head hair. This process is also known as epilation or depilation. Hair is a common feature of the human body, exhibiting considerable variation in thickness and length across different po ...

, as is done commonly in

leather Leather is a strong, flexible and durable material obtained from the tanning (leather), tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffal ...

processing. It is also used as an acidity indicator, manufacturing of thioglycolates, and in bacteriology for preparation of thioglycolate media. Thioglycolysis reactions are used on

condensed tannin Condensed tannins (proanthocyanidins, polyflavonoid tannins, catechol-type tannins, pyrocatecollic type tannins, non-hydrolyzable tannins or flavolans) are polymers formed by the condensation of flavans. They do not contain sugar residues. They ...

s to study their structure. TGA has also been used to soften nails, either to reshape pincer nails into the correct position or to help the topical antifungal agent

terbinafine Terbinafine, sold under the brand name Lamisil among others, is an antifungal medication used to treat pityriasis versicolor, onychomycosis, fungal nail infections, and ringworm including jock itch and athlete's foot. It is either oral adminis ...

penetrate the nail.

Organotin Organotin chemistry is the scientific study of the synthesis and properties of organotin compounds or stannanes, which are organometallic compounds containing tin–carbon bonds. The first organotin compound was diethyltin diiodide (), discove ...

derivatives of thioglycolic acid isooctyl esters are widely used as stabilizers for PVC. These species have the formula R₂Sn(SCH₂CO₂C₈H₁₇)₂. Sodium thioglycolate is a component of

thioglycolate broth Thioglycolate broth is a multipurpose, enrichment, differential medium used primarily to determine the oxygen requirements of microorganisms. Sodium thioglycolate in the medium consumes oxygen and permits the growth of obligate anaerobes. This, co ...

, a special bacterial growth media. It is also used in so-called "fallout remover" or "wheel cleaner" to remove

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

residue from

wheels A wheel is a rotating component (typically circular in shape) that is intended to turn on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axl ...

Ferrous In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...

iron combines with thioglycolate to form red-violet ferric thioglycolate.

Production

Thioglycolic acid is prepared by reaction of sodium or potassium chloroacetate with alkali metal hydrosulfide in aqueous medium.Cosmetic, Toiletry, and Fragrance Association (CTFA). Thioglycolic Acid. 1987;(1987a). Submission of unpublished data by CTFA Code No. 3-25-2 It can be also prepared via the

Bunte salt In organosulfur chemistry, a Bunte salt is an archaic name for salts with the formula . They are also called ''S''-alkylthiosulfates or ''S''-arylthiosulfates. These compounds are typically derived from alkylation on the pendant sulfur of sodium t ...

obtained by reaction of

sodium thiosulfate Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is a reducing agent an ...

with

chloroacetic acid Chloroacetic acid, industrially known as monochloroacetic acid (MCA), is the organochlorine compound with the formula . This carboxylic acid is a useful building block in organic synthesis. It is a colorless solid. Related compounds are dichlo ...

:Saeed M. Hameed N. Madan V. Mansoor S. Preparation and Mechanisms studies of Thioglycolic Acid. ''Pak. J. Sci. Ind. Res.'' 1992, 35: 131-132 :ClCH₂CO₂H + Na₂S₂O₃ → Na ₃S₂CH₂CO₂H+ NaCl :Na ₃S₂CH₂CO₂H+ H₂O → HSCH₂CO₂H + NaHSO₄

Reactions

Thioglycolic acid with a p''K''_a of 3.83 is an acid about 8.5 times stronger than

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

(p''K''_a 4.76): :HSCH₂CO₂H → HSCH₂CO₂⁻ + H⁺ The second ionization has a p''K''_a of 9.3: : HSCH₂CO₂⁻ → ⁻SCH₂CO₂⁻ + H⁺ Thioglycolic acid is a reducing agent, especially at higher pH. It oxidizes to the corresponding

(2- carboxymethyl)disulfanylcetic acid or dithiodiglycolic acid): :2 HSCH₂CO₂H + "O" → CH₂CO₂Hsub>2 + H₂O

With metal ions

Thioglycolic acid, usually as its dianion, forms complexes with metal ions. Such complexes have been used for the detection of

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

, and

tin Tin is a chemical element; it has symbol Sn () and atomic number 50. A silvery-colored metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, a bar of tin makes a sound, the ...

. Thioglycolic acid reacts with diethyl acetylmalonate to form acetylmercaptoacetic acid and diethyl malonate, the reducing agent in the conversion of Fe(III) to Fe(II).Lee CW, Phil M. The detection of iron traces on hands by ferrozine sparys: a report on the sensitivity and interference of the method and recommended procedure in forensic science investigation. ''J Forensic Sci.'' 1986, 31:920-930.

History

Scientist David R. Goddard, in the early 1930s, identified TGA as a useful reagent for reducing the

disulfide bond In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inor ...

s in

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s, including

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

(hair protein), while studying why

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

enzymes could not easily digest hair, nails, feathers, and such. He realized that while the disulfide bonds, which stabilize proteins by cross-linking, were broken, the structures containing these proteins could be reshaped easily, and that they would retain this shape after the disulfide bonds were allowed to re-form.National Academies Press:Biographical Memoirs:David Rockwell Goddard:by Ralph O. Erickson
/ref> TGA was developed in the 1940s for use as a chemical

Safety and detection

The LD₅₀ (oral, rat) is 261 mg/kg, LC₅₀ inhalation for rat is 21 mg/m³ for 4 h, and LD₅₀ dermal for rabbit is 848 mg/kg.Sigma-Aldrich MSDS. http://www.sigmaaldrich.com/safety-center.html?cm_sp=Search-_-MSDS-_-MSDS1 (accessed Nov 10, 2013). Product Number – T3758 Mercaptoacetic acid in hair waving and depilatory products containing other mercapto acids can be identified by using thin-layer chromatography and gas chromatography.Goetz N, Gataud P, Bore P. Determination of mercaptoacetic acid in hair waving and depilatory products. ''Analyst.'' 1979,104:1062-1069Goetz N, Gataud P, Bore P. Gas-chromatographic determination of mercaptoacetic acid in hair-waving and diplatory products. ''Cosmet Sci Technol Ser.'' 1985, 4:65-79. MAA also has been identified by using potentiometric titration with

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

solution.Vandeputte M, Dryon L, Van Den Winkel P, Mertens J, Massart DL. Determination of thioglycolic acid using a silver sulfide single crystal electrode. ''Analysis.'' 1975,3:500-504.

Uses

Production

Reactions

With metal ions

History

Safety and detection

See also

References

Further reading