Thioglycolic acid

Thioglycolic acid (TGA) is the organic compound HSCH2CO2H. TGA is often called mercaptoacetic acid (MAA). It contains both a thiol (mercaptan) and carboxylic acid functional groups. It is a colorless liquid with a strongly unpleasant odor. TGA is miscible with polar organic solvents.[4][5]

Thioglycolic acid[1]
Names
Preferred IUPAC name
Sulfanylacetic acid
Other names
2-Sulfanylacetic acid
2-Mercaptoacetic acid
Acetyl mercaptan
Mercaptoacetate
Mercaptoacetic acid
Thioglycolic acid
Thiovanic acid[2]
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.000.616
KEGG
UNII
Properties
C2H4O2S
Molar mass 92.11 g·mol−1
Appearance colorless, clear liquid[3]
Odor strong, disagreeable[3]
Density 1.32 g/cm3
Melting point −16 °C (3 °F; 257 K)
Boiling point 96 °C (205 °F; 369 K) at 5 mmHg
miscible[3]
Vapor pressure 10 mmHg (17.8°C)[3]
-50.0·10−6 cm3/mol
Hazards
Flash point > 110 °C; 230 °F; 383 K [3]
Explosive limits 5.9%-?[3]
NIOSH (US health exposure limits):
PEL (Permissible)
 none[3]
REL (Recommended)
 TWA 1 ppm (4 mg/m3) [skin][3]
IDLH (Immediate danger)
 N.D.[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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 that 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.[5] In fact thioglycolysis reactions used on condensed tannins to study their structure.

Organotin derivatives of thioglycolic acid isooctyl esters are widely used as stabilizers for PVC. These species have the formula R2Sn(SCH2CO2C8H17)2.[5]

Applying TGA can soften nails and then fix pincer nails in the correct position.

Sodium thioglycolate is a component of a special bacterial growth media : thioglycolate broth. It is also used in so-called "fallout remover"[6] or "wheel cleaner" to remove iron oxide residue from rims.[7] Ferrous iron combines with thioglycolate to form red-violet[8] ferric thioglycolate.[9][10]

Production

Thioglycolic acid is prepared by reaction of sodium or potassium chloracetate with alkali metal hydrosulfide in aqueous medium.[11] It can be also prepared via the Bunte salt obtained by reaction of sodium thiosulfate with chloroacetic acid:[5][12]

ClCH2CO2H + Na2S2O3 → Na[O3S2CH2CO2H] + NaCl
Na[O3S2CH2CO2H] + H2O → HSCH2CO2H + NaHSO4

Reactions

Thioglycolic acid with a pKa of 3.83 [5] is about 10 times stronger an acid than acetic acid (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)disulfanyl]acetic acid or dithiodiglycolic acid):

2 HSCH2CO2H + "O" → [SCH2CO2H]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, 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).[13]

History

Scientist David R. Goddard, 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.[14] TGA was developed in the 1940s for use as a chemical depilatory.

Safety and detection

The LD50 (oral, rat) is 261 mg/kg,[5] LC50 inhalation for rat is 21 mg/m3 for 4 h, and LD50 dermal for rabbit is 848 mg/kg.[15] Mercaptoacetic acid in hair waving and depilatory products containing other mercapto acids can be identified by using thin-layer chromatography and gas chromatography.[16][17] MAA also has been identified by using potentiometric titration with silver nitrate solution.[18]

See also

References
  1. Merck Index, 11th Edition, 9265
  2. CDC - NIOSH Pocket Guide to Chemical Hazards
  3. NIOSH Pocket Guide to Chemical Hazards. "#0610". National Institute for Occupational Safety and Health (NIOSH).
  4. The Merck index, 14th ed.; O’Neil, Maryadele J., Ed.; Merck & Co., Inc.: Whitehouse Station, NJ, 2006; p 9342.
  5. Robert Rippel "Mercaptoacetic Acid and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH, Weinheim. doi:10.1002/14356007.a16_265
  6. "What is a fallout remover - DetailingWiki, the free wiki for detailers". DetailingWiki, the free wiki for detailers. 2016-03-17. Retrieved 2016-06-14.
  7. "SONAX Xtreme Wheel Cleaner" Safety data sheet
  8. Jr, John J. McKetta (1981-01-01). Encyclopedia of Chemical Processing and Design: Volume 12 - Corrosion to Cottonseed. CRC Press. p. 103. ISBN 9780824724627.
  9. Lyons, Edward (1927-08-01). "Thioglycolic Acid as a Color Test for Iron". Journal of the American Chemical Society. 49 (8): 1916–1920. doi:10.1021/ja01407a010. ISSN 0002-7863.
  10. "Limit Test of Iron - Web Formulas". www.web-formulas.com. Retrieved 2016-06-14.
  11. Cosmetic, Toiletry, and Fragrance Association (CTFA). Thioglycolic Acid. 1987;(1987a). Submission of unpublished data by CTFA Code No. 3-25-2
  12. Saeed M. Hameed N. Madan V. Mansoor S. Preparation and Mechanisms studies of Thioglycolic Acid. Pak. J. Sci. Ind. Res. 1992, 35: 131-132
  13. 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.
  14. National Academies Press:Biographical Memoirs:David Rockwell Goddard:by Ralph O. Erickson
  15. Sigma-Aldrich MSDS. http://www.sigmaaldrich.com/safety-center.html?cm_sp=Search-_-MSDS-_-MSDS1 (accessed Nov 10, 2013). Product Number – T3758
  16. Goetz N, Gataud P, Bore P. Determination of mercaptoacetic acid in hair waving and depilatory products. Analyst. 1979,104:1062-1069
  17. Goetz 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.
  18. 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.

Further reading
  • Okada K, Okada E. Novel treatment using thioglycolic acid for pincer nails. J. Dermatol. 2012, volume 39, pp. 996-999.
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