Thioglycolic acid (TGA) is the organic compound HSCH₂CO₂H. TGA is often called mercaptoacetic acid (MAA). It contains both a thiol (

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 gro ...

) and

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

functional groups. It is a colorless liquid with a strongly unpleasant odor. 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 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 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, as is done commonly in leather processing. It is also used as an acidity indicator, manufacturing of thioglycolates, and in bacteriology for preparation of thioglycolate media. In fact thioglycolysis reactions 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.

Organotin Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide (), discovered by ...

derivatives of thioglycolic acid isooctyl esters are widely used as stabilizers for PVC. These species have the formula R₂Sn(SCH₂CO₂C₈H₁₇)₂. Applying TGA can soften nails and then fix pincer nails in the correct position. 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, com ...

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

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...

residue from wheels. Ferrous iron combines with thioglycolate to form red-violet ferric thioglycolate.

Production

Thioglycolic acid is prepared by reaction of sodium or potassium chloracetate 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 RSSO3–Na+. They are also called S-alkylthiosulfates or S-arylthiosulfates. These compounds are typically derived from alkylation on the pendant sulfur of sodi ...

obtained by reaction of sodium thiosulfate with chloroacetic acid: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 pKa of 3.83 is about 10 times stronger an acid 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 component ...

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

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

, silver, and tin. Thioglycolic acid reacts with diethyl acetylmalonate to form acetylmercaptoacetic acid and diethyl malonate, the reducing agent in 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 David Rockwell Goddard (January 3, 1908 – July 9, 1985) was an American plant physiologist. Goddard was most known for his contributions to the large-scale production of penicillin, bacitracin and vitamin B2 in World War II. His later research ...

, in the early 1930s, identified TGA as a useful reagent for reducing the disulfide bonds in proteins, including keratin (hair protein), while studying why protease 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 depilatory.

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 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