Alpha hydroxy acid

Short description: Class of chemical compounds

α-, β- and γ-hydroxy acids

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 internal hydrogen bonding.[1][2][3]

α-Hydroxy acids have a variety of applications. They are used industrially as feed additives and as the basis for polymers.[4][5][6][7] They are also commonly used in cosmetic products to chemically exfoliate and moisturize.[8]

## Contents

* 1 Industrial applications
* 1.1 Feed additives
* 1.2 Lactic acid- and glycolic acid-based polymers
* 1.3 Mandelic acid based polymers
* 2 Synthesis and reactions
* 2.1 Hydrolysis routes
* 2.2 Specialized routes
* 3 Occurrence
* 4 Safety
* 5 See also
* 6 Further reading
* 7 References
* 8 External links

## Industrial applications

### Feed additives

2-Hydroxy-4-(methylthio)butyric acid is produced commercially as a racemic mixture to substitute for methionine in animal feed.[9]

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

Lactic acid and its cyclic ester (lactide) are precursors to polylactic acid (PLA).[4] PLA can be used as biodegradable medical implants, drug delivery systems, and sutures.[5]

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.[6] 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.[6]

### 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.[7] In laboratory testing, SAMMA was found to have anti-viral properties against strains of human immunodeficiency virus (HIV) and the herpes simplex virus (HSV).[7]

## 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.[10][11] Compounds of this class are used on the industrial-scale and include glycolic acid, lactic acid, citric acid, and mandelic acid.[12][13] They are susceptible to acid-catalyzed decarbonylation to give, in addition to carbon monoxide, a ketone/aldehyde and water.[14]

### 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.[12]

α-Hydroxy acids are prepared by adding hydrogen cyanide to a ketone or aldehyde, followed by acidic hydrolysis of the resulting cyanohydrin product.[15]

RCHO + HCN → RCH(OH)CN
RCH(OH)CN + 2 H
2O → RCH(OH)CO
2H + NH

### Specialized routes

α-Hydroxy acids also arise by the reaction between dilithiated carboxylic acids and oxygen after an aqueous workup:[16]

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

Lastly, α-keto aldehydes can undergo the Cannizaro reaction to give α-hydroxy acids:[17]

RC(O)CHO + 2 OH− → RCH(OH)CO2− \+ H2O

## Occurrence

2-Hydroxy-4-(methylthio)butyric acid is an intermediate in the biosynthesis of 3-dimethylsulfoniopropionate, precursor to natural dimethyl sulfide.[18]

## 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.[8] 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.[8]

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.[8] 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.[8]

## See also

* Beta hydroxy acid
* Hydroxybutyric acid
* Omega hydroxy acid

## Further reading

* "Glycolic acid peeling in the treatment of acne". Journal of the European Academy of Dermatology and Venereology 12 (2): 119–22. March 1999. doi:10.1111/j.1468-3083.1999.tb01000.x. PMID 10343939.
* "Alpha Hydroxy Acids for Skin Care". Cosmetic Dermatology, Supplement: 1–6. October 1994.
* "Chemical peeling--glycolic acid versus trichloroacetic acid in melasma". Indian Journal of Dermatology, Venereology and Leprology 67 (2): 82–4. 2001. PMID 17664715.
* "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. June 1998. PMID 9640557.

## References

1. ↑ Data for Biochemical Research. Oxford: Clarendon Press. 1959.
2. ↑ Handbook of Chemistry and Physics, CRC Press, 58th edition, page D147 (1977)
3. ↑ 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.
4. ↑ 4.0 4.1 Casalini, Tommaso; Rossi, Filippo; Castrovinci, Andrea; Perale, Giuseppe (2019). "A Perspective on Polylactic Acid-Based Polymers Use for Nanoparticles Synthesis and Applications". Frontiers in Bioengineering and Biotechnology 7: 259. doi:10.3389/fbioe.2019.00259. ISSN 2296-4185. PMID 31681741.
5. ↑ 5.0 5.1 Storti, G.; Lattuada, M. (2017-01-01). Perale, Giuseppe; Hilborn, Jöns. eds. "8 - Synthesis of bioresorbable polymers for medical applications" (in en). Bioresorbable Polymers for Biomedical Applications (Woodhead Publishing): 153–179. doi:10.1016/b978-0-08-100262-9.00008-2. ISBN 978-0-08-100262-9. https://www.sciencedirect.com/science/article/pii/B9780081002629000082. Retrieved 2023-04-01.
6. ↑ 6.0 6.1 6.2 Samantaray, Paresh Kumar; Little, Alastair; Haddleton, David M.; McNally, Tony; Tan, Bowen; Sun, Zhaoyang; Huang, Weijie; Ji, Yang et al. (2020). "Poly(glycolic acid) (PGA): a versatile building block expanding high performance and sustainable bioplastic applications" (in en). Green Chemistry 22 (13): 4055–4081. doi:10.1039/D0GC01394C. ISSN 1463-9262. http://xlink.rsc.org/?DOI=D0GC01394C.
7. ↑ 7.0 7.1 7.2 Herold, B. C.; Scordi-Bello, I.; Cheshenko, N.; Marcellino, D.; Dzuzelewski, M.; Francois, F.; Morin, R.; Casullo, V. Mas et al. (2002-11-15). "Mandelic Acid Condensation Polymer: Novel Candidate Microbicide for Prevention of Human Immunodeficiency Virus and Herpes Simplex Virus Entry" (in en). Journal of Virology 76 (22): 11236–11244. doi:10.1128/JVI.76.22.11236-11244.2002. ISSN 0022-538X. PMID 12388683.
8. ↑ 8.0 8.1 8.2 8.3 8.4 Nutrition, Center for Food Safety and Applied (2022-11-22). "Alpha Hydroxy Acids" (in en). FDA. https://www.fda.gov/cosmetics/cosmetic-ingredients/alpha-hydroxy-acids.
9. ↑ 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.
10. ↑ "Oxidation of some α-hydroxy-acids with lead tetraacetate". Bulletin of the Chemical Society of Japan 9 (1): 8–14. 1934. doi:10.1246/bcsj.9.8.
11. ↑ "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. 1982. doi:10.1016/S0040-4039(00)88578-0.
12. ↑ 12.0 12.1 "Hydroxycarboxylic Acids, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry. 2000. doi:10.1002/14356007.a13_507. ISBN 978-3527306732.
13. ↑ "Hydroxycarboxylic Acids, Aromatic". Ullmann's Encyclopedia of Industrial Chemistry. 2000. doi:10.1002/14356007.a13_519. ISBN 978-3527306732.
14. ↑ Principles of organic synthesis.. Coxon, J. M. (James Morriss), 1941- (3rd. ed.). London: Blackie Academic & Professional. 1993. ISBN 978-0751401264. OCLC 27813843.
15. ↑ Vollhardt, K Peter C; Schore, Neil Eric (2018-01-29). Organic chemistry:structure and function (8th ed.). New York. ISBN 9781319079451. OCLC 1007924903.
16. ↑ 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, https://books.google.com/books?id=JDR-nZpojeEC&printsec=frontcover
17. ↑ 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, https://books.google.com/books?id=JDR-nZpojeEC&printsec=frontcover
18. ↑ Curson, Andrew R. J.; Liu, Ji; Bermejo Martínez, Ana; Green, Robert T.; Chan, Yohan; Carrión, Ornella; Williams, Beth T.; Zhang, Sheng-Hui et al. (2017). "Dimethylsulfoniopropionate biosynthesis in marine bacteria and identification of the key gene in this process". Nature Microbiology 2 (5): 17009. doi:10.1038/nmicrobiol.2017.9. PMID 28191900. https://ueaeprints.uea.ac.uk/id/eprint/62649/1/Todd_Figure_1.pdf.

## External links

* U.S. Food and Drug Administration: Alpha Hydroxy Acids in Cosmetics

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