Thioglycolic acid

"Thioglycolate" redirects here. For the microbiology broth, see thioglycollate broth.
Thioglycolic acid[1]
Names
IUPAC name
2-Sulfanylacetic acid
Other names
Acetyl mercaptan
Mercaptoacetate
Mercaptoacetic acid
Thioglycolic acid
Thiovanic acid[2]
Identifiers
68-11-1 YesY
ChEBI CHEBI:30065 YesY
ChEMBL ChEMBL116455 YesY
ChemSpider 1101 YesY
Jmol interactive 3D Image
KEGG C02086 YesY
PubChem 1133
UNII 7857H94KHM YesY
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]
Hazards
Flash point > 110 °C; 230 °F; 383 K [3]
Explosive limits 5.9%-?[3]
US health exposure limits (NIOSH):
none[3]
TWA 1 ppm (4 mg/m3) [skin][3]
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

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]

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 stabilzers 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.

Production

Thioglycolic acid is prepared by reaction of sodium or potassium chloracetate with alkali metal hydrosulfide in aqueous medium.[6] It can be also prepared via the Bunte salt obtained by reaction of sodium thiosulphate with chloroacetic acid:[5][7]

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

Reactions

It is about 100 times stronger acid than acetic acid with a pKa of 3.83:

HSCH2CO2H → HSCH2CO2− + H+

The second ionization has a pKa of 9.3:

HSCH2CO2− → −SCH2CO2− + H+

It 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

TGA, usually as its dianion, forms complexes with metal ions. Such complexes have been used for the detection of iron, molybdenum, silver, and tin. TGA reacts with diethyl acetylmalonate to form acetylmercatoacetic acid and diethyl malonate, the reducing agent in conversion of Fe(III) to Fe(II).[8]

History

The 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.[9] 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.[10] Mercaptoacetic acid in hair waving and depilatory products containing other mercapto acids can be identified by using thin-layer chromatography and gas chromatography.[11][12] MAA also has been identified by using potentiometric titration with silver nitrate solution.[13]

See also

References

  1. ↑ Merck Index, 11th Edition, 9265
  2. ↑ CDC - NIOSH Pocket Guide to Chemical Hazards
  3. 1 2 3 4 5 6 7 8 9 "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. 1 2 3 4 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. ↑ Cosmetic, Toiletry, and Fragrance Association (CTFA). Thioglycolic Acid. 1987;(1987a). Submission of unpublished data by CTFA Code No. 3-25-2
  7. ↑ Saeed M. Hameed N. Madan V. Mansoor S. Preparation and Mechanisms studies of Thioglycolic Acid. Pak. J. Sci. Ind. Res. 1992, 35: 131-132
  8. ↑ 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.
  9. ↑ National Academies Press:Biographical Memoirs:David Rockwell Goddard:by Ralph O. Erickson
  10. ↑ Sigma-Aldrich MSDS. http://www.sigmaaldrich.com/safety-center.html?cm_sp=Search-_-MSDS-_-MSDS1 (accessed Nov 10, 2013). Product Number – T3758
  11. ↑ Goetz N, Gataud P, Bore P. Determination of mercaptoacetic acid in hair waving and depilatory products. Analyst. 1979,104:1062-1069
  12. ↑ 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.
  13. ↑ 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

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