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.[4][5] TGA is miscible with polar organic solvents.[6][7]
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.[7] Thioglycolysis reactions are used on
condensed tannins to study their structure.[8][9][10][11]
Organotin derivatives of thioglycolic acid isooctyl esters are widely used as stabilizers for
PVC. These species have the formula R2Sn(SCH2CO2C8H17)2.[7]
Sodium thioglycolate is a component of
thioglycolate broth, a special bacterial growth media. It is also used in so-called "fallout remover"[14] or "wheel cleaner" to remove
iron oxide residue from
wheels.[15]Ferrous iron combines with thioglycolate to form red-violet[16] ferric thioglycolate.[17][18]
Production
Thioglycolic acid is prepared by reaction of sodium or potassium chloroacetate with alkali metal hydrosulfide in aqueous medium.[19] It can be also prepared via the
Bunte salt obtained by reaction of
sodium thiosulfate with
chloroacetic acid:[7][20]
ClCH2CO2H + Na2S2O3 → Na[O3S2CH2CO2H] + NaCl
Na[O3S2CH2CO2H] + H2O → HSCH2CO2H + NaHSO4
Reactions
Thioglycolic acid with a
pKa of 3.83 [7] is an acid about 10 times stronger 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 the conversion of Fe(III) to Fe(II).[21]
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.[22] TGA was developed in the 1940s for use as a chemical
depilatory.
Safety and detection
The
LD50 (oral, rat) is 261 mg/kg,[7]LC50 inhalation for rat is 21 mg/m3 for 4 h, and LD50 dermal for rabbit is 848 mg/kg.[23] Mercaptoacetic acid in hair waving and depilatory products containing other mercapto acids can be identified by using thin-layer chromatography and gas chromatography.[24][25] MAA also has been identified by using potentiometric titration with
silver nitrate solution.[26]
^Lyons E (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.
ISSN0002-7863.
^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.
^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.
Further reading
Okada K, Okada E. Novel treatment using thioglycolic acid for pincer nails. J. Dermatol.2012, volume 39, pp. 996-999.