Alpha hydroxy acid
# Alpha hydroxy acid

Alpha hydroxy acids, or α-hydroxy acids (also known as 2-hydroxy acids), 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.

Alpha hydroxy acids are stronger acids than the corresponding non-alpha hydroxy carboxylic acid. Their increased acidity is attributed to hydrogen bonding.

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.

## Industrial applications

### Feed additives

2-Hydroxy-4-(methylthio)butyric acid is produced commercially as a racemic mixture to substitute for methionine in animal feed.

### Lactic acid- and glycolic acid-based polymers

Lactic acid and its cyclic ester (lactide) are precursors to polylactic acid (PLA). PLA can be used as biodegradable medical implants, drug delivery systems, and sutures.

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. Although PGA is sometimes touted as a bioplastic, the monomer is not obtained from natural glycolic acid, but is synthetic. Like PLA, PGA is fully biodegradable, making it an environmentally friendlier option than regular plastic.

### Mandelic acid based polymers

Another alpha hydroxy acid, mandelic acid is important in health care applications. When mandelic acid is treated with sulfuric acid, the condensation product is SAMMA. In laboratory 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 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.

### Hydrolysis routes

α-Halocarboxylic acids, which are often easily obtained, hydrolyze to give 2-hydroxycarboxylic acids. Glycolic acid is produces in this manner. The reaction usually is conducted with base, followed by an acid workup. The net reaction is:

    $RCH(Cl)CO_2H$ + $H_2O$ → $RCH(OH)CO_2H$ + HCl

Unsaturated acids and esters also hydrate. In this way, fumarate and maleate esters give malic acid derivatives. Acrylic acid gives 3-hydroxypropionic acid.

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

    RCHO + HCN → RCH(OH)CN
    RCH(OH)CN + 2 $H_2O$ → $RCH(OH)CO_2H$ + $NH_3$

### Specialized routes

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

    $RCHLiCO_2Li$ + $O_2$ → $RCH(O_2Li)CO_2Li$
    $RCH(O_2Li)CO_2Li$ + $H^+$ → $RCH(OH)CO_2H$ + 2 $Li^+$ + ...

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

    RC(O)CHO + 2 $OH^−$ → $RCH(OH)CO_2^−$ + $H_2O$

## Occurrence

2-Hydroxy-4-(methylthio)butyric acid is an intermediate in the biosynthesis of 3-dimethylsulfoniopropionate, precursor to natural dimethyl sulfide.

## Safety

Alpha hydroxy acids 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 expert panels both suggest that alpha hydroxy acids 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 alpha hydroxy acids after an industry-sponsored study found that they can increase the likelihood of sunburns. This effect is reversible after stopping the use of alpha hydroxy acids. Other sources suggest that glycolic acid, in particular, may protect from sun damage.

