Mercaptoacetic acid
[CAS 68-11-1]

Identification

• Classification: Inorganic chemical industry >> Inorganic acid
• Name: Mercaptoacetic acid
• Synonyms: Thioglycollic acid
• Molecular Formula: C₂H₄O₂S
• Molecular Weight: 92.11
• CAS Registry Number: 68-11-1
• EC Number: 200-677-4
• SMILES: C(C(=O)O)S

Properties

• Density: 1.3±0.1 g/cm³ Calc.^*, 1.326 g/mL (Expl.)
• Melting point: -16 °C (Expl.)
• Boiling point: 225.5 °C 760 mmHg (Calc.)^*, 269.4 °C (Expl.)
• Flash point: 99.8±22.6 °C (Calc.)^*, 130 °C (Expl.)
• Solubility: water: Miscible (Expl.)
• Index of refraction: 1.503 (Calc.)^*, 1.505 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS05;GHS06 Danger
• Risk Statements: H301-H311-H314-H331
• Safety Statements: P260-P261-P262-P264-P270-P271-P280-P301+P316-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P354+P338-P316-P321-P330-P361+P364-P363-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1B	H314
Acute toxicity	Acute Tox.	3	H311
Acute toxicity	Acute Tox.	3	H301
Acute toxicity	Acute Tox.	3	H331
Serious eye damage	Eye Dam.	1	H318
Acute toxicity	Acute Tox.	1	H330
Skin sensitization	Skin Sens.	1B	H317
Acute toxicity	Acute Tox.	2	H330
Acute toxicity	Acute Tox.	2	H310
Acute toxicity	Acute Tox.	2	H300
Skin sensitization	Skin Sens.	1	H317
Acute toxicity	Acute Tox.	4	H332
Skin corrosion	Skin Corr.	1A	H314
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412
Specific target organ toxicity - single exposure	STOT SE	3	H335

• Transport Information: UN 1940; UN 2436

chemBlink Chemical Story

Mercaptoacetic acid, CAS 68-11-1, is better known as thioglycolic acid. It has the formula HSCH₂COOH and a molecular weight of 92.12. With only two carbon atoms, the molecule contains two chemically important functional groups: a thiol (-SH) and a carboxylic acid (-COOH). The thiol gives it reducing and metal-binding properties, while the carboxyl group gives the molecule its acidic character. One of the most familiar applications of this chemistry is surprisingly far removed from an analytical laboratory: thioglycolates helped make the modern permanent wave possible.

Human hair is made largely of keratin, a fibrous protein rich in structural interactions that help determine the shape and mechanical properties of each hair fiber. Among the strongest of these connections are disulfide bonds formed between cysteine residues. Two sulfur atoms become covalently joined to form a cystine linkage:

Keratin-S-S-Keratin

These sulfur bridges act as molecular cross-links within the keratin structure. Hydrogen bonds and other weaker interactions can be disturbed temporarily by water and heat, which is why wet hair can be shaped and then lose much of that shape later. Disulfide bonds are covalent and much more persistent. Producing a longer-lasting change therefore requires chemistry capable of altering some of these sulfur bridges.

This became the basis of the modern permanent-wave process. In the first chemical stage, a reducing formulation containing a thioglycolate is applied to the hair. Ammonium thioglycolate became especially important because it provides thioglycolate in an alkaline formulation suitable for penetrating and softening the hair fiber. Thioglycolate reacts with cystine disulfide bonds and converts part of them into thiol groups.

The process can be represented schematically as:

Keratin-S-S-Keratin + reducing agent → Keratin-SH + HS-Keratin

The actual chemistry in hair is more complicated than this single equation, but the essential result is straightforward: some of the covalent cross-links that helped hold the keratin structure in its original arrangement have been opened.

At the same time, the hair is physically placed into a new shape, traditionally by wrapping it around curling rods. Once enough disulfide bonds have been reduced, the keratin chains have greater freedom to rearrange relative to one another. The curl is therefore not created simply by wrapping wet hair around a cylinder. Mechanical shaping and chemical reduction work together.

If the process stopped at this point, however, the new arrangement would not be securely fixed. A second chemical step is needed. After the reducing solution is removed, an oxidizing neutralizer, commonly based on hydrogen peroxide or another oxidizing system, is applied. Oxidation converts thiol groups back toward disulfide bonds:

2 Keratin-SH + oxidant → Keratin-S-S-Keratin

But the protein chains are now positioned differently because the hair has been held in its new shape. The newly formed sulfur bridges therefore help stabilize a different arrangement from the original one. In simplified terms, a permanent wave works by opening molecular bridges, reshaping the material, and then rebuilding bridges in new positions.

This chemistry explains the word "permanent," although the result is not literally permanent forever. The chemically modified portion of a hair fiber retains its altered structure much longer than a curl produced only with water and heat. New hair growing from the follicle has not undergone the treatment and therefore retains its natural characteristics. Chemical and mechanical damage, washing, weathering, and further treatments can also affect the appearance over time.

The same disulfide chemistry can be used in the opposite direction cosmetically. Chemical hair straightening systems based on thioglycolates reduce keratin disulfide bonds while the hair is held straighter rather than curled. Subsequent oxidation helps establish new disulfide connections in that configuration. Curling and straightening can therefore use closely related chemistry; the difference lies partly in the physical arrangement imposed while sulfur cross-links are being reorganized.

The history of this chemistry is closely associated with twentieth-century changes in hair treatment. Earlier permanent-wave methods relied heavily on heat and cumbersome equipment. The development of chemical "cold wave" processes in the 1930s and 1940s made it possible to reshape hair without the elaborate electrically heated machines previously used. Thioglycolate chemistry became central to this transformation and remains associated with permanent-wave formulations.

Mercaptoacetic acid also has important uses outside cosmetics. The combination of thiol and carboxyl groups makes it capable of interacting with metal ions, and thioglycolic acid and its salts are used in analytical, industrial, and synthetic chemistry. Thioglycolate media have long been important in microbiology as reducing environments, and thioglycolic acid is also used as a reagent or intermediate in chemical manufacture.

The compound's two functional groups explain much of this versatility. The carboxyl group can form salts such as ammonium thioglycolate, while the thiol participates in oxidation-reduction chemistry and can bind certain metals. A very small molecule can therefore behave as an acid, a reducing agent, a sulfur-containing ligand, and a building block for other compounds.

Its reducing power also demands respect. Thioglycolic acid is corrosive and can cause serious injury on contact, while cosmetic formulations must carefully control concentration, pH, exposure time, and subsequent neutralization. Hair itself can also be damaged if too many structural bonds are disrupted or treatment conditions are excessive. The useful chemistry and the potential for damage arise from the same fundamental ability to alter protein structure.

This is what makes mercaptoacetic acid particularly interesting. A permanent wave may appear to be simply a cosmetic change visible in a mirror, but at molecular scale it is an exercise in protein chemistry. Covalent bonds in keratin are deliberately reduced, the macroscopic object is mechanically reshaped, and oxidation is then used to establish stabilizing bonds again.

The transformation moves back and forth between two scales. At the molecular scale, sulfur atoms exchange partners. At the visible scale, a strand of hair changes shape.

Few everyday chemical processes demonstrate structure-property relationships so clearly. Change the cross-links inside a material, and the shape of the material can change with them. Thioglycolic acid made that principle practical enough to move from protein chemistry into millions of hair salons.

References

1. PubChem. Thioglycolic Acid, CAS 68-11-1. Identity, physicochemical properties, uses, and safety information.

2. Robbins, C. R. Chemical and Physical Behavior of Human Hair. Springer. Keratin structure, cystine chemistry, permanent waving, and chemical hair treatment.

3. Wolfram, L. J. Human hair: a unique physicochemical composite. Journal of the American Academy of Dermatology. Hair structure and chemical behavior.

4. Scientific and cosmetic-chemistry literature describing thioglycolate reduction of keratin disulfide bonds and oxidative neutralization during permanent waving.