Ascorbic acid

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20 atoms, 20 bonds.

IUPAC name: (2R)-2-[(1S)-1,2-dihydroxyethyl]-3,4-dihydroxy-2H-furan-5-one
Molar mass: 176.124(20) g/mol
Atoms: 20
SMILES: C(C(C1C(=C(C(=O)O1)O)O)O)O
InChIKey: CIWBSHSKHKDKBQ-JLAZNSOCSA-N
PubChem CID: 54670067

Made of

Structural formula of ascorbic acid. Each unlabelled corner is a carbon atom, and the four hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Scurvy, with its bleeding gums and wounds that refuse to heal, is what a diet short of ascorbic acid produces. The compound is better known as vitamin C, and the form PubChem’s record describes is L-ascorbic acid, C₆H₈O₆, with a molar mass of 176.12 g/mol. Its IUPAC name, (2R)-2-[(1S)-1,2-dihydroxyethyl]-3,4-dihydroxy-2H-furan-5-one, spells out a five-membered ring with a two-carbon tail.

The pure solid is a crystalline powder, white or a very pale yellow, almost odourless and with a pleasant, sharp acid taste. Kept dry, it is stable in air. Dissolved in water, it is oxidised by air rapidly. HSDB, the Hazardous Substances Data Bank, calls it a strong reducing agent, a substance that hands its electrons to another. That reducing power, together with its value as a nutrient and its low toxicity, is the main reason for its many uses in food and medicine, according to HSDB.

Structure and bonding

The model holds 20 atoms, six carbon, eight hydrogen and six oxygen, joined by 20 bonds. With Labels on, four C and one O close a five-membered ring. The ring carbon beside that oxygen carries a C=O double bond, and the pair forms a lactone, a cyclic ester, in CAMEO Chemicals’ terms. Next along, a C=C double bond joins two ring carbons that each hold an O–H group, the enediol group that oxidation strips of hydrogen.

A two-carbon side chain leaves the ring with two more O–H groups. In Ball-and-stick, the ring carbon at that junction and the first chain carbon each carry a single hydrogen. They are stereocentres, carbons whose four different neighbours allow two mirror-image arrangements, marked 2R and 1S in the IUPAC name. Stick drops the spheres, and the doubled C=O and C=C sticks stand out side by side.

The drawing is the neutral acid, every O–H intact and no charges shown. In water it gives up a hydrogen ion; HSDB lists a first pK_a, a measure of how readily that ion leaves, of 4.17. In the body the vitamin is described as ascorbate, the anion. The oxidised partner, dehydroascorbic acid, is not drawn.

Where it turns up

OpenStax lists citrus fruits, red berries, peppers, tomatoes, broccoli and dark green leafy vegetables among the foods that supply vitamin C. HSDB adds potatoes, a significant dietary source in many countries, and they show how easily it goes. Over three months of storage their reduced ascorbic acid fell by half, from 25 to 13 mg per 100 g. Washing and cooking then took it down to 6 mg.

Plants also carry ascorbic acid oxidase, an enzyme that can cause large losses when tissue is crushed without the enzyme first being inactivated. The European Chemicals Agency (ECHA) registers it under the food-additive name E-300. JECFA, the expert committee on food additives run jointly by the UN’s food and health agencies, classes it as an antioxidant. The US Food and Drug Administration (FDA) also lists it as a flavouring, a dough strengthener and a preservative. PubChem’s consumer product data name 252 personal care and pet care products that contain it.

Why it matters

Vitamin C donates electrons to eight human enzymes, HSDB reports. Three of them hydroxylate collagen, adding O–H groups to the protein; two build a fat-carrying molecule, carnitine; three make hormones and amino acids. In the collagen enzymes, prolyl and lysyl hydroxylase, ascorbate is believed to reactivate each one by reducing its metal site, iron in the first and copper in the second. Scurvy, DrugBank notes, mainly damages collagenous structures, with lesions in bone and blood vessels.

It also reduces most reactive oxygen species in the body, oxygen-containing molecules and fragments that damage cells. HSDB reports that glutathione and enzymes running on NADH and NADPH, two electron-carrying coenzymes, then regenerate its oxidised forms.

OpenStax says water-soluble vitamins are not stored in the body, yet HSDB puts healthy body stores at about 1.5 g. Its plasma half-life, 16 days on one HSDB figure, ranges from 8 to 40 days and falls to hours when the body holds a surplus. Excess leaves unchanged in the urine, and HSDB names a second route as well: part is metabolised to ascorbate-2-sulfate and oxalic acid.

Properties

Properties of ascorbic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 54670067.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₆H₈O₆
Molar mass	176.124(20) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	6
Hydrogen atoms	8
Oxygen atoms	6
Bonds	20
Single bonds	18
Double bonds	2
PubChem molecular weight	176.12 g/mol	g/mol	PubChem

Safety

In food and supplements ascorbic acid is essentially harmless. On PubChem’s record, 97.3% of reports find it meets no GHS (Globally Harmonized System) hazard criteria. For the bulk powder, the PubChem Laboratory Chemical Safety Summary notes irritation of skin, eyes and airways, dust that can explode in air, and violent reactions with oxidants. Very large amounts swallowed can upset the gut, and that summary is the reference for anyone handling the pure substance.

Questions

Is ascorbic acid the same as vitamin C?

Yes. DrugBank, quoted on PubChem's record, describes ascorbic acid as vitamin C, a water-soluble vitamin, and the US Food and Drug Administration (FDA) classes it as Vitamin C. The substance meant is L-ascorbic acid, the form whose solution turns polarised light to the right by +20.5 to +21.5 degrees at 25 °C. Its oxidised partner, dehydroascorbic acid, forms reversibly from it, and DrugBank counts both as physiologically active in body fluids.

Why is vitamin C called an acid if it has no carboxyl group?

The ring's C=O belongs to an ester, a lactone, not to a COOH group. Even so, the Hazardous Substances Data Bank (HSDB) gives a first pK_a, a measure of how readily a hydrogen ion leaves, of 4.17. HSDB measured a pH of about 2 to 3 for its solutions, and CAMEO Chemicals describes the taste as pleasant, sharp and acidic.

What does the ascorbic acid molecule look like in 3D?

The model shows 20 atoms joined by 20 bonds. A five-membered ring of four carbons and one oxygen forms its core, with a two-carbon tail branching from one ring carbon. Besides the ring oxygen, five more oxygens sit on the frame: one in the ring's C=O and four in O–H groups. The ring also holds one C=C. Two carbons, one in the ring and one in the tail, are the stereocentres the IUPAC name labels 2R and 1S.

Why does vitamin C break down in cooked or stored food?

Dry ascorbic acid is stable in air, but once dissolved it is oxidised by air quickly. HSDB reports that alkalis, iron and copper speed the loss, and CAMEO Chemicals adds that its rate depends on pH and on how much oxygen is present. Light slowly darkens the solid as well. In plant foods, the enzyme ascorbic acid oxidase adds to the loss unless it is inactivated.

Made of the same elements

C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic 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.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 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.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.