glycolic acid

Glycolic acid
IUPAC name	2-Hydroxyethanoic acid
Other names	glycolic acid, hydroxyacetic acid
Identifiers
CAS number	79-14-1 Y
RTECS number	MC5250000
SMILES	O=C(O)CO
ChemSpider ID	737
Properties
Molecular formula	C2H4O3
Molar mass	76.05 g/mol
Appearance	white, powdery solid
Density	1.27 g/cm3
Melting point	75 °C
Boiling point	decomposes
Solubility in water	70% solution
Solubility in other solvents	alcohols, acetone, acetic acid and ethyl acetate[1]
Acidity (pKa)	3.83
Hazards
R-phrases	R22-R34
S-phrases	S26-S36/37/39-S45
NFPA 704	1 3 0
Flash point	129 °C[2]
Related compounds
Related α-hydroxy acids	lactic acid
Related compounds	acetic acid, glycerol
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Glycolic acid (or hydroxyacetic acid) is the smallest α-hydroxy acid (AHA). This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used in various skin-care products. Glycolic acid is found in some sugar-crops.

Preparation

It is prepared by the reaction of chloroacetic acid with sodium hydroxide followed by re-acidification. The net reaction can be summarized as follows:

ClCH₂CO₂H + NaOH → HOCH₂CO₂H + NaCl

In this way, a few million kilograms are produced annually. Other methods, not apparently in use, include hydrogenation of oxalic acid and the hydrolysis of the cyanohydrin derived from formaldehyde.^[3] Glycolic acid can be isolated from natural sources, such as sugarcane, sugar beets, pineapple, canteloupe, and unripe grapes.^{[citation needed]}.

Glycolic acid can also be prepared using an enzymatic biochemical process which produces fewer impurities compared to traditional chemical synthesis, requires less energy in production and produces less co-product. This process is used by the specialty chemical company CrossChem.^[4]

Uses

Due to its excellent capability to penetrate skin, glycolic acid finds applications in skin care products, most often as a chemical peel performed by a dermatologist in concentrations of 20 to 70% or at-home kits in lower concentrations between 10 and 20%. In addition to concentration, pH also plays a large part in determining the potency of glycolic acid in solution. Physician-strength peels can have a pH as low as 0.6 (strong enough to completely keratolyze the epidermis), while acidities for home peels can be as high as 2.5. ^[5] Glycolic acid is used to improve the skin's appearance and texture. It may reduce wrinkles, acne scarring, hyperpigmentation and improve many other skin conditions, including actinic keratosis, hyperkeratosis, and seborrheic keratosis. Once applied, glycolic acid reacts with the upper layer of the epidermis, weakening the binding properties of the lipids that hold the dead skin cells together. This allows the outer skin to "dissolve" revealing the underlying skin.

Glycolic acid is also a useful intermediate for organic synthesis, in a range of reactions including: oxidation-reduction, esterification and long chain polymerization. It is used as a monomer in the preparation of polyglycolic acid and other biocompatible copolymers (e.g. PLGA). Among other uses this compound finds employment in the textile industry as a dyeing and tanning agent, in food processing as a flavoring agent and as a preservative. Glycolic acid is often included into emulsion polymers, solvents and additives for ink and paint in order to improve flow properties and impart gloss.

From the commercial perspective, important derivatives include the methyl (CAS#[96-35-5) and ethyl (CAS# 623-50-7) esters which are readily distillable (b.p. 147-9 and 158-159 °C, respectively), unlike the parent acid. The butyl ester (b.p. 178-186 °C) is a component of some varnishes, being desirable because it is nonvolatile and good dissolving properties.^[3]

Safety

Glycolic acid is a strong irritant.^[6] Like ethylene glycol, it is metabolized to oxalic acid, which makes it dangerous if ingested.

References

1. ^ "DuPont Glycolic Acid Technical Information". http://www.dupont.com/glycolicacid/applications/index.html. Retrieved 2006-07-06. 
2. ^ "Glycolic Acid MSDS". University of Akron. http://ull.chemistry.uakron.edu/erd/chemicals/7/6540.html. Retrieved 2006-09-18. 
3. ^ ^{a b} Karlheinz Miltenberger "Hydroxycarboxylic Acids, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
4. ^ GlyAcid EBP Crystalline at CrossChem.net
5. ^ Glycolic Acid Essential Actives, KAVI.
6. ^ "Glycolic Acid MSDS". ICSC:NENG1537 International Chemical Safety Cards (WHO/IPCS/ILO). http://www.cdc.gov/niosh/ipcsneng/neng1537.html. Retrieved 2006-06-08.