Glycolic acid

Glycolic acid (hydroacetic acid or hydroxyacetic acid); chemical formula C2H4O3 (also written as HOCH2CO2H), 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.
A glycolate or glycollate is a salt or ester of glycolic acid.
History.
The name "glycolic acid" was coined in 1848 by French chemist Auguste Laurent (1807–1853). He proposed that the amino acid glycine—which was then called "glycocolle"—might be the amine of a hypothetical acid, which he called "glycolic acid" ("acide glycolique"). 
Glycolic acid was first prepared in 1851 by German chemist Adolph Strecker (1822–1871) and Russian chemist Nikolai Nikolaevich Sokolov (1826–1877). They produced it by treating hippuric acid with nitric acid and nitrogen dioxide to form an ester of benzoic acid and glycolic acid (C6H5C(=O)OCH2COOH), which they called "benzoglycolic acid" ("Benzoglykolsäure"; also benzoyl glycolic acid). They boiled the ester for days with dilute sulfuric acid, thereby obtaining benzoic acid and glycolic acid ("Glykolsäure").
Preparation.
Glycolic acid can be synthesized in various ways. The predominant approaches use a catalyzed reaction of formaldehyde with synthesis gas (carbonylation of formaldehyde), for its low cost.
It is also prepared by the reaction of chloroacetic acid with sodium hydroxide followed by re-acidification.
Other methods, not noticeably in use, include hydrogenation of oxalic acid, and hydrolysis of the cyanohydrin derived from formaldehyde. Some of today's glycolic acids are formic acid-free. Glycolic acid can be isolated from natural sources, such as sugarcane, sugar beets, pineapple, cantaloupe and unripe grapes.
Glycolic acid can also be prepared using an enzymatic biochemical process that may require less energy.
Properties.
Glycolic acid is slightly stronger than acetic acid due to the electron-withdrawing power of the terminal hydroxyl group. The carboxylate group can coordinate to metal ions forming coordination complexes. Of particular note are the complexes with Pb2+ and Cu2+ which are significantly stronger than complexes with other carboxylic acids. This indicates that the hydroxyl group is involved in complex formation, possibly with the loss of its proton.
Applications.
Glycolic acid is used in the textile industry as a dyeing and tanning agent, in food processing as a flavoring agent and as a preservative, and in the pharmaceutical industry as a skin care agent. It is also used in adhesives and plastics. Glycolic acid is often included in emulsion polymers, solvents and additives for ink and paint in order to improve flow properties and impart gloss. It is used in surface treatment products that increase the coefficient of friction on tile flooring. It is the active ingredient in the household cleaning liquid Pine-Sol.
Skin care.
Due to its capability to penetrate skin, glycolic acid finds applications in skin care products, most often as a chemical peel. 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 low as 2.5. 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 stratum corneum to be exfoliated, exposing live skin cells.
Organic synthesis.
Glycolic acid is 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). Commercially, important derivatives include the methyl (CAS# 96-35-5) and ethyl (CAS# 623-50-7) esters which are readily distillable (boiling points 147–149 °C 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 has good dissolving properties.
Agriculture.
Many plants make glycolic acid during photorespiration. Its role consumes significant amounts of energy. In 2017 researchers announced a process that employs a novel protein to reduce energy consumption/loss and prevent plants from releasing harmful ammonia. The process converts glycolate into glycerate without using the conventional BASS6 and PLGG1 route.
Safety.
Glycolic acid is a strong irritant depending on pH. Like ethylene glycol, it is metabolized to oxalic acid, which could make it dangerous if ingested.