α-Hydroxy acids are
stronger acids compared to their non-alpha hydroxy counterparts, a property enhanced by internal
hydrogen bonding.[2][3][4] AHAs serve a dual purpose; industrially, they are utilized as additives in animal feed and as precursors for polymer synthesis.[5][6][7][8] In cosmetics, they are commonly used for their ability to chemically exfoliate the skin and
moisturize.[9]
Uses
The synthesis and utilization of
polymers based on
lactic acid, including
polylactic acid (PLA) and its cyclic ester
lactide, are used in the creation of biodegradable materials such as
medical implants,
drug delivery systems, and
sutures.[6] Similarly,
glycolic acid serves as a foundation for the development of poly(glycolic acid), spelled
polyglycolide (PGA), a polymer distinguished by its high crystallinity, thermal stability, and mechanical strength, despite its synthetic origins.[5] Both PLA and PGA are fully biodegradable.[7]
α-Hydroxy acids, such as glycolic acid, lactic acid, citric acid, and mandelic acid, serve as precursors in
organic synthesis, playing a role in the industrial-scale preparation of various compounds.[12][13] These acids are used when synthesizing
aldehydes through
oxidative cleavage.[14][15] α-Hydroxy acids are particularly prone to acid-catalyzed
decarbonylation, yielding
carbon monoxide, a
ketone or aldehyde, and water as by-products.[16]
One common synthesis route involves the
hydrolysis of α-halocarboxylic acids, readily available precursors, to produce 2-hydroxycarboxylic acids. For instance, the production of glycolic acid typically follows this method, utilizing a base-induced reaction, followed by acid workup. Similarly, unsaturated acids and
fumarate and
maleate esters undergo hydration to yield
malic acid derivatives from esters, and
3-hydroxypropionic acid from
acrylic acid.[12]
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.[9] 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.[9]
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.[9] 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.[9]
Atzori L, Brundu MA, Orru A, Biggio P (March 1999). "Glycolic acid peeling in the treatment of acne". Journal of the European Academy of Dermatology and Venereology. 12 (2): 119–22.
doi:
10.1111/j.1468-3083.1999.tb01000.x.
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"Alpha Hydroxy Acids for Skin Care". Cosmetic Dermatology, Supplement: 1–6. October 1994.
Kalla G, Garg A, Kachhawa D (2001). "Chemical peeling--glycolic acid versus trichloroacetic acid in melasma". Indian Journal of Dermatology, Venereology and Leprology. 67 (2): 82–4.
PMID17664715.
Kempers S, Katz HI, Wildnauer R, Green B (June 1998). "An evaluation of the effect of an alpha hydroxy acid-blend skin cream in the cosmetic improvement of symptoms of moderate to severe xerosis, epidermolytic hyperkeratosis, and ichthyosis". Cutis. 61 (6): 347–50.
PMID9640557.
^Dawson RM, et al. (1959). Data for Biochemical Research. Oxford: Clarendon Press.
^Handbook of Chemistry and Physics, CRC Press, 58th edition, page D147 (1977)
^The strength of the hydrogen bonding is refelected also in the
Proton nuclear magnetic resonance-spectrum of these compounds: instead of giving rise to a contribution to the broad signal of rappidly exchanged protons (between COOH, OH, NH, etc) in 2-phenyl-2-hydroxyacetic acid (
mandelic acid) the proton on the alpha carbon and the proton trapped in the internal hydrogen bridge show a nice pair of doublets instead a singlet (H on alpha-C) and the formentioned broad signal of exchangable protons. So on the NMR-time scale the exchange equilibrium for the alpha-hydroxy group is frozen.