Glycolic acid

Ball-and-stick Stick Space-filling

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9 atoms, 8 bonds.

IUPAC name: 2-hydroxyacetic acid
Molar mass: 76.0510(78) g/mol
Atoms: 9
SMILES: C(C(=O)O)O
InChIKey: AEMRFAOFKBGASW-UHFFFAOYSA-N
PubChem CID: 757

Made of

Structural formula of glycolic acid. Each unlabelled corner is a carbon atom, and the two hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

An extra –OH on the carbon beside the acid group is all that separates glycolic acid from acetic acid. ChEBI (Chemical Entities of Biological Interest) describes it as acetic acid with its methyl group hydroxylated, and classes it as an alcohol and an acid with one –COOH group. The IUPAC name is 2-hydroxyacetic acid. Two carbon, four hydrogen and three oxygen atoms give a molar mass of 76.05 g/mol.

HSDB, the Hazardous Substances Data Bank, gives its pK_a as 3.83 at 25 °C and also as 3.6, both lower than acetic acid’s. What the figure measures is set out under phosphoric acid. The pure acid forms colourless, odourless prisms that are hygroscopic, pulling water from the air, and dissolve very readily in water. Its crystals take two forms: alpha, melting at 78 to 80 °C, and a metastable beta form, less stable than alpha, melting at 63 °C.

Structure and bonding

Glycolic acid holds two carbons, three oxygens and four hydrogens. One carbon carries the carboxyl group, –COOH, the functional group Chem1 calls typical of organic acids: a double bond to one oxygen, a single bond to an O–H. The other carbon, a CH₂, holds the second O–H. In Ball-and-stick, the computed angles at the carboxyl carbon are 124.0°, 125.8° and 110.2°, totalling 360°: that carbon and its three neighbours lie flat, a trigonal planar centre. At the CH₂ carbon they run from 108.3° to 110.3°, near the tetrahedral angle.

Turned edge-on, both carbons and all three oxygens line up in one plane, the two CH₂ hydrogens on either side. Stick shows the C=O as two parallel rods. Space-filling swells everything into one compact body with red oxygen at both ends.

The viewer draws the neutral acid. In water the carboxyl hydrogen can leave, and ChEBI names glycolic acid the conjugate acid of the glycolate ion left behind. The alcohol O–H keeps its hydrogen: on Chem1’s account, an alcohol’s –COH gives one up even less readily than water.

Where it turns up

Sugar cane juice contains glycolic acid, HSDB records, and ChEBI gives it the role of a metabolite, a product of living chemistry. In rats it is a main urinary breakdown product of dichloroacetic acid, beside glyoxylic and oxalic acid, and it has a part in antifreeze poisoning. In cosmetics, the US Food and Drug Administration (FDA) names glycolic and lactic acid as the commonest alpha hydroxy acids, or AHAs.

Haz-Map, a database of occupational hazards, lists it for cleaning metals, water wells and dairy equipment, deliming hides, processing furs and dyeing leather. It is also used in adhesives, copper pickling, an acid clean of the metal, flux for printed wiring boards and the acidifying of oil wells. It breaks petroleum emulsions and acts as a chelating agent for iron, binding the metal’s ions, according to HSDB. Sold as a light straw-coloured solution, it smells of burnt sugar.

Why it matters

In skin products glycolic acid acts as an AHA, and such products cause exfoliation, the shedding of the skin’s surface layer, according to the FDA. How far it goes depends on the AHA, its concentration and pH, and the other ingredients. In studies the FDA reports, a cosmetic-type cream base carried an AHA deeper into the skin than a plain solution. Serious reactions seem to come most often from the products that exfoliate hardest, such as skin peelers. The agency received 114 reports of skin reactions to AHA products between 1992 and February 2004.

In the United States, Du Pont has made it commercially from formaldehyde, carbon monoxide and water, according to HSDB. It can also come from monochloroacetic acid, or from oxalic acid reduced electrolytically, and it has served as an intermediate in making ethylene glycol. For 2023 the US volume reported to the Environmental Protection Agency (EPA) was from 25 million to under 40 million pounds. It is also used in biopolymers made for absorbable sutures and drug-delivery systems, HSDB reports.

Properties

Properties of glycolic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 757.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂H₄O₃
Molar mass	76.0510(78) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	2
Hydrogen atoms	4
Oxygen atoms	3
Bonds	8
Single bonds	7
Double bonds	1
PubChem molecular weight	76.05 g/mol	g/mol	PubChem

Safety

Pure glycolic acid is corrosive, with the signal word Danger in notifications to the European Chemicals Agency (ECHA). It causes severe skin burns and eye damage, the statement in 99.7% of the company reports behind those notifications. According to the International Chemical Safety Card, it also irritates the airways, is corrosive if swallowed and may affect the kidneys. The PubChem Laboratory Chemical Safety Summary gathers its hazard classifications and the exposure criteria set for it.

Questions

What shape is a glycolic acid molecule?

No single shape word fits it: the molecule is a two-carbon chain with two centres rather than one central atom. The carboxyl carbon and its three neighbours lie flat, a trigonal planar centre, while the bond angles at the CH₂ carbon, 108.3° to 110.3° in the model, sit close to the tetrahedral angle. PubChem's computed model joins nine atoms with eight bonds and keeps both carbons and all three oxygens in one plane.

Why is glycolic acid used in skin care?

Glycolic acid is an alpha hydroxy acid (AHA), and the US Food and Drug Administration (FDA) names it with lactic acid as the commonest AHAs in cosmetic products. AHA products make the outer skin shed, a process called exfoliation, and are marketed to smooth fine lines and surface wrinkles, even out texture and tone, and clear pores. Products meant to treat acne or lighten skin count as drugs under US law.

Does glycolic acid make skin more sensitive to the sun?

Yes, for a while. According to the US Food and Drug Administration (FDA), AHAs on the skin raised volunteers' sensitivity to reddening by ultraviolet light by 18 per cent, an effect that faded soon after they stopped using the cream. How AHAs cause it was not identified, and in mice glycolic acid did not alter how light-induced skin cancers developed. In January 2005 the FDA issued labelling guidance so AHA cosmetics warn buyers of this raised sun sensitivity.

Is glycolic acid a strong acid?

No. Like most organic acids it is a weak acid: in water only a small fraction of it gives up a proton. Its pK_a is 3.83 at 25 °C on the Hazardous Substances Data Bank's figure, lower than acetic acid's, so it is the stronger of the two. Of its two O–H hydrogens, only the one in the carboxyl group comes away in water, leaving the glycolate ion behind.

Made of the same elements

CH₃COOH Acetic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.