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Glycolic acid

Glycolic acid

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Glycolic acid is the smallest α-hydroxy acid
Alpha hydroxy acid
α-Hydroxy acids, or alpha hydroxy acids , are a class of chemical compounds that consist of a carboxylic acid substituted with a hydroxyl group on the adjacent carbon. They may be either naturally occurring or synthetic. AHAs are well-known for their use in the cosmetics industry...

 (AHA). This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

. It is used in various skin-care products. Glycolic acid is found in some sugar-crops. Henry's law constant of glycolic acid is KH = 2.83 × 104 × exp[(33.5 × 103/R) × (1/T − 1/298)] .

Preparation


It is prepared by the reaction of chloroacetic acid
Chloroacetic acid
Chloroacetic acid, industrially known as monochloroacetic acid is the organochlorine compound with the formula ClCH2CO2H. This carboxylic acid is a useful building-block in organic synthesis.-Production:...

 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. Catalyzed reaction of formaldehyde with synthesis gas (carbonylation of formaldehyde) is a particularly economical preparative route. Other methods, not apparently in use, include hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

 of oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...

 with nascent hydrogen
Nascent hydrogen
Atomic hydrogen is the species denoted by H , contrasted with dihydrogen, the usual 'hydrogen' commonly involved in chemical reactions. It is claimed to exist transiently but long enough to effect chemical reactions...

 and the hydrolysis of the cyanohydrin derived from formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...

. Glycolic acid can be isolated from natural sources, such as sugarcane
Sugarcane
Sugarcane refers to any of six to 37 species of tall perennial grasses of the genus Saccharum . Native to the warm temperate to tropical regions of South Asia, they have stout, jointed, fibrous stalks that are rich in sugar, and measure two to six metres tall...

, sugar beets, pineapple
Pineapple
Pineapple is the common name for a tropical plant and its edible fruit, which is actually a multiple fruit consisting of coalesced berries. It was given the name pineapple due to its resemblance to a pine cone. The pineapple is by far the most economically important plant in the Bromeliaceae...

, canteloupe, and unripe grape
Grape
A grape is a non-climacteric fruit, specifically a berry, that grows on the perennial and deciduous woody vines of the genus Vitis. Grapes can be eaten raw or they can be used for making jam, juice, jelly, vinegar, wine, grape seed extracts, raisins, molasses and grape seed oil. Grapes are also...

s.

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. Each route can be evaluated for its relative costs and benefits.

Uses


Due to its excellent capability to penetrate skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

, glycolic acid finds applications in skin care products, most often as a chemical peel
Chemical peel
A chemical peel is a body treatment technique used to improve and smooth the texture of the facial skin using a chemical solution that causes the dead skin to slough off and eventually peel off. The regenerated skin is usually smoother and less wrinkled than the old skin. Thus the term chemical...

 performed by a dermatologist, plastic surgeon or licensed aesthetician in concentrations of 20 to 70% or at-home kits in lower concentrations between 10 and 20%. In addition to concentration, pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 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. Glycolic acid is used to improve the skin's appearance and texture. It may reduce wrinkles, acne scarring
Acne vulgaris
Acne vulgaris is a common human skin disease, characterized by areas of skin with seborrhea , comedones , papules , pustules , Nodules and possibly scarring...

, hyperpigmentation
Hyperpigmentation
In dermatology, hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin.-Causes:Hyperpigmentation may be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris...

 and improve many other skin conditions
Cutaneous conditions
There are many conditions of or affecting the human integumentary system—the organ system that comprises the entire surface of the body and includes skin, hair, nails, and related muscle and glands.- Diseases :...

, including actinic keratosis
Actinic keratosis
Actinic keratosis is a premalignant condition of thick, scaly, or crusty patches of skin. It is more common in fair-skinned people. It is associated with those who are frequently exposed to the sun, as it is usually accompanied by solar damage...

, hyperkeratosis
Hyperkeratosis
Hyperkeratosis is thickening of the stratum corneum, often associated with a qualitative abnormality of the keratin, and also usually accompanied by an increase also in the granular layer...

, and seborrheic keratosis
Seborrheic keratosis
A seborrheic keratosis is a noncancerous benign skin growth that originates in keratinocytes. Like liver spots, seborrheic keratoses are seen more often as people age. In fact they are sometimes humorously referred to as the "barnacles of old age". They appear in various colors, from light tan to...

. 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
Stratum corneum
The stratum corneum is the outermost layer of the epidermis, consisting of dead cells that lack nuclei and organelles. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress...

 to be exfoliated
Exfoliation (cosmetology)
Exfoliation involves the removal of the oldest dead skin cells on the skin's outermost surface, and has been used for many years to help maintain skin. Exfoliation is involved in the process of all facials, during microdermabrasion or chemical peels at medical spas...

, exposing live skin cells. Highly purified grades of glycolic acid are commercially available for personal care applications.

Glycolic acid is also a useful intermediate for organic synthesis, in a range of reactions including: oxidation-reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

, esterification and long chain polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

. It is used as a monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

 in the preparation of polyglycolic acid
Polyglycolide
Polyglycolide or Polyglycolic acid is a biodegradable, thermoplastic polymer and the simplest linear, aliphatic polyester. It can be prepared starting from glycolic acid by means of polycondensation or ring-opening polymerization. PGA has been known since 1954 as a tough fiber-forming polymer...

 and other biocompatible copolymer
Heteropolymer
A heteropolymer or copolymer is a polymer derived from two monomeric species, as opposed to a homopolymer where only one monomer is used...

s (e.g. PLGA
PLGA
PLGA or poly is a copolymer which is used in a host of Food and Drug Administration approved therapeutic devices, owing to its biodegradability and biocompatibility. PLGA is synthesized by means of random ring-opening co-polymerization of two different monomers, the cyclic dimers of glycolic...

). Among other uses this compound finds employment in the textile industry as a dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

ing and tanning
Tanning
Tanning is the making of leather from the skins of animals which does not easily decompose. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name . Coloring may occur during tanning...

 agent, in food processing as a flavor
Flavor
Flavor or flavour is the sensory impression of a food or other substance, and is determined mainly by the chemical senses of taste and smell. The "trigeminal senses", which detect chemical irritants in the mouth and throat as well as temperature and texture, are also very important to the overall...

ing 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 varnish
Varnish
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...

es, being desirable because it is nonvolatile and good dissolving properties.

Safety


Glycolic acid is a strong irritant. Like ethylene glycol
Ethylene glycol
Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...

, it is metabolized to oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...

, which could make it dangerous if ingested.