Hydroxycarboxylic acids Structural Formulae V

α-Hydroxy acids, or alpha hydroxy acids (AHAs), are a class of chemical compounds that consist of a carboxylic acid with a hydroxyl group substituent on the adjacent (alpha) carbon. Prominent examples are

glycolic acid Glycolic acid (or hydroxyacetic acid; chemical formula HOCH2CO2H) is a colorless, odorless and hygroscopic crystalline solid, highly soluble in water. It is used in various skin-care products. Glycolic acid is widespread in nature. A glycolate (s ...

lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...

mandelic acid Mandelic acid is an aromatic alpha hydroxy acid with the molecular formula C6H5CH(OH)CO2H. It is a white crystalline solid that is soluble in water and polar organic solvents. It is a useful precursor to various drugs. The molecule is chiral. T ...

and

citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in ...

. Although these compounds are related to the ordinary carboxylic acids and are therefore

weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...

s, their chemical structure allows for the formation of an internal hydrogen bond between the hydrogen at the hydroxyl group and one of the oxygen atoms of the carboxylic group. The net effect is an increase in acidity. For example, the pK_a of lactic acid is 3.86, while that of the unsubstituted propionic acid is 4.87; a full pK_a unit difference means that lactic acid is ten times stronger than propionic acid.

Industrial applications

Feed additives

2-Hydroxy-4-(methylthio)butyric acid is produced commercially as a racemic mixture to substitute for methionine in animal feed. In nature, the same compound is an intermediate in the biosynthesis of 3-dimethylsulfoniopropionate, precursor to natural dimethyl sulfide.

Lactide-based polymers

Synthesis and reactions

α-Hydroxy acids are classically prepared by addition of hydrogen cyanide to a ketone or

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

, followed by acidic hydrolysis of the resulting

cyanohydrin In organic chemistry, a cyanohydrin or hydroxynitrile is a functional group found in organic compounds in which a cyano and a hydroxy group are attached to the same carbon atom. The general formula is , where R is H, alkyl, or aryl. Cyanohydr ...

product. Dilithiated carboxylic acids react with

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

to give α-hydroxy acids after an aqueous workup: :RCHLiCO₂Li + O₂ → RCH(O₂Li)CO₂Li :RCH(O₂Li)CO₂Li + 2H⁺ → RCH(OH)CO₂H + 2Li⁺ + ... α-Keto aldehydes undergo the

Cannizaro reaction The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid. ...

to give α-hydroxy acids: :RC(O)CHO + 2OH⁻ → RCH(OH)CO₂⁻ + H₂O α-Hydroxy acids are useful building blocks in organic synthesis. For example, α-hydroxy acids are precursors in the preparation

aldehydes In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

via

oxidative Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

cleavage. Compounds of this class are used on the industrial-scale and include

, and

. They are susceptible to acid-catalyzed

decarbonylation Decarbonylation is a type of organic reaction that involves loss of CO. It is often an undesirable reaction since it represents a degradation. In the chemistry of metal carbonyls, decarbonylation describes a substitution process, whereby a CO lig ...

to give, in addition to carbon monoxide, a ketone/aldehyde and water. α-Hydroxy acids can form polyesters and membraneless

protocell A protocell (or protobiont) is a self-organized, endogenously ordered, spherical collection of lipids proposed as a stepping-stone toward the origin of life. A central question in evolution is how simple protocells first arose and how they could ...

ular structures.

Safety

AHAs are generally safe when used on the skin as a cosmetic agent using the recommended dosage. The most common side-effects are mild skin irritations, redness and flaking. The severity usually depends on the pH and the concentration of the acid used. Chemical peels tend to have more severe side-effects including blistering, burning and skin discoloration, although they are usually mild and go away a day or two after treatment. The United States

Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respon ...

has also warned consumers that care should be taken when using AHAs after an industry-sponsored study found that they can increase

photosensitivity Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicit ...

to the sun.* Other sources suggest that glycolic acid, in particular, may have a

photoprotective Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative st ...

effect.

References

External links

U.S. Food and Drug Administration: Alpha Hydroxy Acids in Cosmetics

The Ordinary
Application of AHA technology in cosmetic production {{DEFAULTSORT:Alpha Hydroxy Acid Cosmetics chemicals