CID 177

CH₃CHO

Acetaldehyde

Ball-and-stick Stick Space-filling

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7 atoms, 6 bonds.

IUPAC name: acetaldehyde
Molar mass: 44.0530(58) g/mol
Atoms: 7
Polarity: polar
SMILES: CC=O
InChIKey: IKHGUXGNUITLKF-UHFFFAOYSA-N
PubChem CID: 177

Made of

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

In the definition of ChEBI (Chemical Entities of Biological Interest), acetaldehyde is the aldehyde formed when acetic acid’s carboxy group, –COOH, is reduced to –CHO. It is built from carbon, hydrogen and oxygen alone, its molar mass is 44.05 g/mol, and its IUPAC name is simply acetaldehyde.

At room temperature it is a colourless gas, fruity and pungent in smell, according to the US National Toxicology Program (NTP). The Hazardous Substances Data Bank (HSDB) gives its boiling point as 20.8 °C and describes it as a volatile liquid or gas. It mixes with water in all proportions.

Commercial production began in 1916, and US output peaked in 1969, NTP reports. In western Europe in 1983, most of it came from the catalytic oxidation of ethylene, the Wacker-Hoechst process in HSDB’s account. HSDB calls oxidising ethanol the oldest laboratory method of preparing it.

Structure and bonding

Six bonds hold together acetaldehyde’s two carbons, one oxygen and four hydrogens. Three hydrogens sit on the CH₃ carbon; the other carbon holds the fourth and a double bond to oxygen, drawn as a double stick. Chem1 notes that multiple bonds also join unlike atoms; this one is built like ethylene’s carbon–carbon bond.

No single shape word fits, because the two carbons take different geometries. The viewer draws PubChem’s computed model, in which the angles around the CHO carbon are 123.4° (O–C–C), 121.0° (O–C–H) and 115.6° (C–C–H). They sum to 360°, so that carbon and its three partners lie flat, lining up when seen edge-on. At the CH₃ carbon every angle falls between 109.1° and 110.1°, near the 109.5° Chem1 gives for four bonds around one atom.

Stick thins it to bare bonds; Space-filling swells it into one compact body, oxygen at one end. HSDB records a dipole moment of 2.689 debyes, so the molecule is polar: its centres of positive and negative charge do not coincide. The viewer draws no partial charges, so that shift of charge never shows.

Where it turns up

Most higher plants make acetaldehyde as a normal step in respiration, NTP reports, and traces turn up in apples, broccoli, coffee, grapefruit and grapes. HSDB links it to the tart taste of unripe fruit, and it has been reported in the tea plant, Camellia sinensis. Cheese, yoghurt and milk carry it as well. In wine it can form after contact with air, which is one reason alcoholic drinks contain it.

Residential fireplaces and woodstoves are its two largest emission sources, ahead of industry, according to the US Environmental Protection Agency (EPA). Vehicle exhaust and tobacco smoke carry it too, and ChEBI ranks it first by abundance among the carcinogens in tobacco smoke. Forest fires, volcanoes and animal wastes give it off naturally. NTP names drinking alcohol, and the body’s breakdown of it, as the general public’s main exposure; the EPA adds that many people take it in from ambient air.

Why it matters

Industry uses acetaldehyde mainly as a stepping stone to other chemicals. In 1982, 61 per cent of it went to making acetic acid, and in 1986, 50 per cent, on HSDB’s figures. NTP lists pyridine and pyridine bases, peracetic acid, pentaerythritol and chloral among its products, as well as metaldehyde, widely used against slugs and snails. HSDB described China as its largest consumer and expected new Chinese plants to make acetic acid by methanol carbonylation instead.

In the body, enzymes called alcohol dehydrogenases (ADH) turn ethanol into acetaldehyde, and aldehyde dehydrogenases (ALDH) turn that into acetic acid, NTP explains. HSDB adds a second route, the microsomal ethanol-oxidising system (MEOS), and places ALDH2, the main enzyme clearing acetaldehyde, in the mitochondria.

A variant of the ALDH2 gene, common among Asian people, makes an enzyme with almost no ability to detoxify it. HSDB puts roughly 30 per cent of East Asian people as short of ALDH2 activity. Disulfiram, a drug for controlling alcoholism, makes acetaldehyde build up. Ethanol is not its only parent in the body; choline yields it too.

Properties

Properties of acetaldehyde Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 177.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂H₄O
Molar mass	44.0530(58) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	2
Hydrogen atoms	4
Oxygen atoms	1
Bonds	6
Single bonds	5
Double bonds	1
PubChem molecular weight	44.05 g/mol	g/mol	PubChem

Safety

Pure acetaldehyde is an extremely flammable liquid and vapour, and EU rules add that it may cause cancer, is suspected of causing genetic defects and irritates eyes and airways. In contact with air it can generate explosive peroxides, the International Chemical Safety Card warns. NIOSH, the US National Institute for Occupational Safety and Health, lists it as a potential occupational carcinogen, citing nasal cancer in animals. The NIOSH card, which carries workplace exposure limits, and the PubChem Laboratory Chemical Safety Summary are the authoritative sheets.

Questions

What shape is an acetaldehyde molecule?

No single shape word covers it, because it has two central carbons that behave differently. The carbon joined to oxygen holds its three partners in one flat plane: in PubChem's computed model its three angles add up to 360°. The other carbon carries three hydrogens and spreads its four bonds at close to 109.5°, Chem1's angle for an atom with four bonds. That end of the molecule is not flat.

Is acetaldehyde a carcinogen?

Agencies call it a likely or possible one. The US National Toxicology Program (NTP) has listed it since 1991 as reasonably anticipated to cause cancer in people, on animal evidence. The US Environmental Protection Agency classes it as a probable human carcinogen, Group B2. IARC, the International Agency for Research on Cancer, places acetaldehyde itself in Group 2B, its tier for possible causes of cancer. In 2009, NTP reports, IARC found that acetaldehyde from drinking alcohol causes cancer in humans.

Does acetaldehyde cause hangovers?

It may contribute, though the link is not settled. The Hazardous Substances Data Bank (HSDB) reports evidence that acetaldehyde triggers nociception, the sensing of pain, by activating TRPA1, an ion channel on pain-sensing nerves. This may help explain hangover pain. Since it is the main product of ethanol's breakdown, ethanol's effects hint at acetaldehyde's own, on HSDB's account. Long-term drinking also inhibits the mitochondrial enzyme that clears it, while ethanol's conversion into acetaldehyde speeds up.

Is acetaldehyde used in food?

Yes. The US Food and Drug Administration considers it generally recognised as safe as a flavouring agent, the US National Toxicology Program reports. It is especially useful for orange, apple and butter flavours. Vinegar makers use it, and it serves as a preservative for fruit and fish. Food makers add it to baked goods, milk products, sweets, fruit juices, desserts and soft drinks.

Made of the same elements

C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.

All 51 compounds made of carbon, hydrogen and oxygen

CH₂O Formaldehyde 4 atoms, 3 bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 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.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.