Alpha hydroxy acid

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α-, β- and γ-hydroxy acids

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.

Although these compounds are related to the ordinary carboxylic acids and are therefore weak acids, 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 pKa of lactic acid is 3.86, while that of the unsubstituted propionic acid is 4.87; a full pKa unit difference means that lactic acid is ten times stronger than propionic acid.[1][2]

Industrial applications[edit]

Feed additives[edit]

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

Lactide-based polymers[edit]

Synthesis and reactions[edit]

α-Hydroxy acids are classically prepared by addition of hydrogen cyanide to a ketone or aldehyde, followed by acidic hydrolysis of the resulting cyanohydrin product.[5]

Dilithiated carboxylic acids react with oxygen to give α-hydroxy acids after an aqueous workup:[6]

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

α-Keto aldehydes undergo the Cannizaro reaction to give α-hydroxy acids:[7]

RC(O)CHO + 2 OH → 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 via oxidative cleavage.[8][9] Compounds of this class are used on the industrial-scale and include glycolic acid, lactic acid, citric acid, and mandelic acid.[10][11] They are susceptible to acid-catalyzed decarbonylation to give, in addition to carbon monoxide, a ketone/aldehyde and water.[12]

α-Hydroxy acids can form polyesters[13] and membraneless protocellular structures.[14][15][13][16]


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.[17]

The United States Food and Drug Administration 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.[18] Other sources suggest that glycolic acid, in particular, may have a photoprotective effect.[19]

See also[edit]

Further reading[edit]

  • 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. PMID 10343939. S2CID 9721678.
  • "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. PMID 17664715.
  • 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. PMID 9640557.


  1. ^ Dawson RM, et al. (1959). Data for Biochemical Research. Oxford: Clarendon Press.
  2. ^ Handbook of Chemistry and Physics, CRC Press, 58th edition, page D147 (1977)
  3. ^ Lemme, A.; Hoehler, D.; Brennan, JJ; Mannion, PF (2002). "Relative effectiveness of methionine hydroxy analog compared to DL-methionine in broiler chickens". Poultry Science. 81 (6): 838–845. doi:10.1093/ps/81.6.838. PMID 12079051.
  4. ^ Curson, Andrew R. J.; Liu, Ji; Bermejo Martínez, Ana; Green, Robert T.; Chan, Yohan; Carrión, Ornella; Williams, Beth T.; Zhang, Sheng-Hui; Yang, Gui-Peng; Bulman Page, Philip C.; Zhang, Xiao-Hua; Todd, Jonathan D. (2017). "Dimethylsulfoniopropionate biosynthesis in marine bacteria and identification of the key gene in this process" (PDF). Nature Microbiology. 2 (5). doi:10.1038/nmicrobiol.2017.9. PMID 28191900. S2CID 21460292.
  5. ^ Vollhardt KP, Schore NE (2018-01-29). Organic chemistry : structure and function (8th ed.). New York. ISBN 9781319079451. OCLC 1007924903.
  6. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 813, ISBN 978-0-471-72091-1
  7. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 1864, ISBN 978-0-471-72091-1
  8. ^ Ôeda H (1934). "Oxidation of some α-hydroxy-acids with lead tetraacetate". Bulletin of the Chemical Society of Japan. 9 (1): 8–14. doi:10.1246/bcsj.9.8.
  9. ^ Nwaukwa S, Keehn P (1982). "Oxidative cleavage of α-diols, α-diones, α-hydroxy-ketones and α-hydroxy- and α-keto acids with calcium hypochlorite [Ca(OCl)2]". Tetrahedron Letters. 23 (31): 3135–3138. doi:10.1016/S0040-4039(00)88578-0.
  10. ^ Miltenberger K (2000). "Hydroxycarboxylic Acids, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a13_507. ISBN 978-3527306732.
  11. ^ Ritzer E, Sundermann R (2000). "Hydroxycarboxylic Acids, Aromatic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a13_519. ISBN 978-3527306732.
  12. ^ Chandler NR (1993). Principles of organic synthesis. Coxon, J. M. (James Morriss), 1941- (3rd. ed.). London: Blackie Academic & Professional. ISBN 978-0751401264. OCLC 27813843.
  13. ^ a b Chandru K, Guttenberg N, Giri C, Hongo Y, Butch C, Mamajanov I, Cleaves HJ (2018-05-31). "Simple prebiotic synthesis of high diversity dynamic combinatorial polyester libraries". Communications Chemistry. 1 (1). doi:10.1038/s42004-018-0031-1. ISSN 2399-3669.
  14. ^ Jia TZ, Chandru K, Hongo Y, Afrin R, Usui T, Myojo K, Cleaves HJ (August 2019). "Membraneless polyester microdroplets as primordial compartments at the origins of life". Proceedings of the National Academy of Sciences of the United States of America. 116 (32): 15830–15835. doi:10.1073/pnas.1902336116. PMC 6690027. PMID 31332006.
  15. ^ Parker ET, Cleaves HJ, Bada JL, Fernández FM (September 2016). "Quantitation of α-hydroxy acids in complex prebiotic mixtures via liquid chromatography/tandem mass spectrometry". Rapid Communications in Mass Spectrometry. 30 (18): 2043–51. Bibcode:2016RCMS...30.2043P. doi:10.1002/rcm.7684. PMID 27467333.
  16. ^ Chandru K, Mamajanov I, Cleaves HJ, Jia TZ (January 2020). "Polyesters as a Model System for Building Primitive Biologies from Non-Biological Prebiotic Chemistry". Life. 10 (1): 6. doi:10.3390/life10010006. PMC 7175156. PMID 31963928.
  17. ^ "AHA LÀ GÌ? CÔNG DỤNG CHÍNH CỦA AHA TRONG MỸ PHẨM". Y Khoa Blog (in Vietnamese). 2021-09-08. Retrieved 2021-10-04.
  18. ^ *Kurtzweil P (March–April 1998). "Alpha Hydroxy Acids for Skin Care". FDA Consumer. 32 (2): 30–5. PMID 9532954. Archived from the original on February 7, 2006. Retrieved February 5, 2006.
  19. ^ Perricone NV, DiNardo JC (May 1996). "Photoprotective and antiinflammatory effects of topical glycolic acid". Dermatologic Surgery. 22 (5): 435–7. doi:10.1111/j.1524-4725.1996.tb00343.x. PMID 8634805. S2CID 37313380.

External links[edit]