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 In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...

with a

hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

substituent on the adjacent (alpha) carbon. Prominent examples are glycolic acid,

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 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

s 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 In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

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 Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a li ...

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 In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

to substitute for

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ...

animal feed Animal feed is food given to domestic animals, especially livestock, in the course of animal husbandry. There are two basic types: fodder and forage. Used alone, the word ''feed'' more often refers to fodder. Animal feed is an important input to ...

. In nature, the same compound is an intermediate in the biosynthesis of 3-dimethylsulfoniopropionate, precursor to natural

dimethyl sulfide Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula (CH3)2S. Dimethyl sulfide is a flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from co ...

Lactide-based polymers

Synthesis and reactions

α-Hydroxy acids are classically prepared by addition of

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on a ...

to a

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

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 gro ...

, 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. Cyanohyd ...

product. Dilithiated

s 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 ...

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 to give α-hydroxy acids: :RC(O)CHO + 2OH⁻ → RCH(OH)CO₂⁻ + H₂O α-Hydroxy acids are useful building blocks in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

. For example, α-hydroxy acids are

precursors Precursor or Precursors may refer to: *Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of unr ...

in the preparation aldehydes 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 ...

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

, and mandelic acid. 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

polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natura ...

s and membraneless protocellular 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 federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

has also warned consumers that care should be taken when using AHAs after an industry-sponsored study found that they can increase photosensitivity to the sun.* Other sources suggest that glycolic acid, in particular, may have a photoprotective 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