User:Cic12345/Alpha hydroxy acid

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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, lactic acid, mandelic acid and citric acid.

As part of the carboxylic acids family, some may assume AHAs are as weak as their non-alpha hydroxy carboxylic acid counterpart. Despite both being weak acids when compared to all acids, AHAs are stronger acids than the corresponding non-alpha hydroxy carboxylic acid. This increase in acidity is due to the -OH functional groups in the carboxylic and hydroxyl groups allow for hydrogen bonding. This increases the acidity of AHAs compared to most other carboxylic acids. For example, lactic acid is ten times more acidic compared to propionic acid. This increase is due to the hydroxyl group on lactic acid’s alpha carbon, which is not present in propionic acid.

α-Hydroxy acids have a variety of applications. They are used industrially as feed additives and as the basis for polymers. They are also commonly used in cosmetic products to chemically exfoliate and moisturize.

Feed additives
2-Hydroxy-4-(methylthio)butyric acid is produced commercially as a racemic mixture to substitute for methionine in animal feed. In nature, 2-Hydroxy-4-(methylthio)butyric acid is an intermediate in the biosynthesis of 3-dimethylsulfoniopropionate, precursor to natural dimethyl sulfide. Dimethyl sulfide can then be used in the production of borane dimethylsulfide, a commonly used stable borane reagent.

Lactide-based polymers
Lactide-based polymers are molecules made of repeating lactic acid units. Lactic acid molecules are joined together via the acid’s oxygens. The physical properties of the polymer depend on the positioning of the individual acid molecules.

Lactide-based polymers can be used in medical implants, drug delivery systems, and stitches after surgery. These polymers are used as they will slowly break down into individual, non-toxic molecules in the body.

Glycolic acid based polymers
Glycolic acid can also be used to form compact polymers, or poly(glycolic acid) (PGA). The dense nature of this polymer results in desirable physical properties, including: high crystallinity, thermal stability, and mechanical strength.

PGA is often used as a bioplastic alternative for traditional plastics. This polymer is fully biodegradable and completely compostable, making it an environmentally friendlier option than regular plastic.

Mandelic acid based polymers
Another AHA, mandelic acid is important in health care applications. When mandelic acid is treated with sulfuric acid, the condensation product is SAMMA. In labortary testing, SAMMA was found to have anti-viral properties against strains of human immunodeficiency virus (HIV) and the herpes simplex virus (HSV).

Synthesis and reactions
α-Hydroxy acids can be synthesized through a variety of pathways.

α-Hydroxy acids are useful building blocks in organic synthesis. For example, they are precursors in the preparation aldehydes via oxidative bond cleavage. Compounds of this class are used on the industrial-scale and include glycolic acid, lactic acid, citric acid, and mandelic acid. They are susceptible to acid-catalyzed decarbonylation to give, in addition to carbon monoxide, a ketone/aldehyde and water.

α-Hydroxy acids can form polyesters and membraneless protocellular structures.

α-Hydroxy acids are prepared by adding hydrogen cyanide to a ketone or aldehyde, followed by acidic hydrolysis of the resulting cyanohydrin product.



α-Hydroxy acids can also be prepared by the reaction between dilithiated carboxylic acids and oxygen after an aqueous workup:


 * RCHLiCO2Li + O2 → RCH(O2Li)CO2Li
 * RCH(O2Li)CO2Li +  H+ → RCH(OH)CO2H + 2Li+ + ...

Lastly, α-Keto aldehydes can undergo the Cannizaro reaction to give α-hydroxy acids:


 * RC(O)CHO + 2OH− → RCH(OH)CO2− + H2O

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 United States Food and Drug Administration (FDA) and Cosmetic Ingredient Review (CIR) Expert Panel both suggest that AHA’s are safe to use as long as they are sold at low concentrations, pH levels greater than 3.5, and include thorough safety instructions.

The FDA has warned consumers that care should be taken when using AHAs after an industry-sponsored study found that they can increase the likelihood of sunburns. This effect is reversible after stopping the use of AHAs. Other sources suggest that glycolic acid, in particular, may protect from sun damage.