Thioglycolic acid

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
CAS Number	68-11-1 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:30065 ;
ChEMBL	ChEMBL116455 ;
ChemSpider	1101 ;
ECHA InfoCard	100.000.616
KEGG	C02086 ;
PubChem CID	1133;
UNII	7857H94KHM ;
CompTox Dashboard (EPA)	DTXSID8026141 ;
	InChI InChI=1S/C2H4O2S/c3-2(4)1-5/h5H,1H2,(H,3,4) Key: CWERGRDVMFNCDR-UHFFFAOYSA-N ; InChI=1/C2H4O2S/c3-2(4)1-5/h5H,1H2,(H,3,4) Key: CWERGRDVMFNCDR-UHFFFAOYAP;
	SMILES C(C(=O)O)S;
Properties
Chemical formula	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
Solubility in water	miscible[3]
Vapor pressure	10 mmHg (17.8°C)[3]
Magnetic susceptibility (χ)	-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).
	verify (what is ?)
	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 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]

References

Merck Index, 11th Edition, 9265
CDC - NIOSH Pocket Guide to Chemical Hazards
NIOSH Pocket Guide to Chemical Hazards. "#0610". National Institute for Occupational Safety and Health (NIOSH).
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. doi:10.1002/14356007.a16_265
"What is a fallout remover - DetailingWiki, the free wiki for detailers". DetailingWiki, the free wiki for detailers. 2016-03-17. Retrieved 2016-06-14.
"SONAX Xtreme Wheel Cleaner" Safety data sheet
Jr, John J. McKetta (1981-01-01). Encyclopedia of Chemical Processing and Design: Volume 12 - Corrosion to Cottonseed. CRC Press. p. 103. ISBN 9780824724627.
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.
"Limit Test of Iron - Web Formulas". www.web-formulas.com. Retrieved 2016-06-14.
Cosmetic, Toiletry, and Fragrance Association (CTFA). Thioglycolic Acid. 1987;(1987a). Submission of unpublished data by CTFA Code No. 3-25-2
Saeed M. Hameed N. Madan V. Mansoor S. Preparation and Mechanisms studies of Thioglycolic Acid. Pak. J. Sci. Ind. Res. 1992, 35: 131-132
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.
National Academies Press:Biographical Memoirs:David Rockwell Goddard:by Ralph O. Erickson
Sigma-Aldrich MSDS. http://www.sigmaaldrich.com/safety-center.html?cm_sp=Search-_-MSDS-_-MSDS1 (accessed Nov 10, 2013). Product Number – T3758
Goetz N, Gataud P, Bore P. Determination of mercaptoacetic acid in hair waving and depilatory products. Analyst. 1979,104:1062-1069
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.
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.