Acetaldehyde
Properties
| State | Liquid at room temperature (volatile) |
| Color | Colorless |
| Solubility | Miscible with water |
| Melting Point | -123.4 °C |
| Boiling Point | 20.2 °C |
About Acetaldehyde
Acetaldehyde is the two-carbon aldehyde that sits one oxidation step above ethanol and one step below acetic acid, and most of its chemistry — biochemical and industrial — flows from that middle position. In the liver, alcohol dehydrogenase oxidizes ethanol to acetaldehyde, then aldehyde dehydrogenase oxidizes that to acetate. The hangover lives in the gap: when ALDH can't keep up, acetaldehyde accumulates, and the headache, nausea, and facial flushing that follow are the body running into a metabolite it doesn't tolerate well. About 40% of people of East Asian descent carry an ALDH2 variant that slows the second step dramatically, which is the genetic basis for the alcohol flush reaction and, paradoxically, a real protective effect against alcohol use disorder. Industrially, the molecule is the workhorse intermediate behind the Wacker process — ethylene plus oxygen over Pd(II)/Cu(II) gives acetaldehyde — which historically fed straight into acetic acid manufacture before methanol carbonylation took over that role. Molar mass works out to 44.05 g/mol, low enough that it boils at 20 °C and behaves more like a refrigerant than a typical liquid reagent.
Where you'll encounter it
If you smell ripe apples, a fresh pour of coffee, or warm bread, you're picking up acetaldehyde — it's a common volatile in plant metabolism and one of the dominant aroma compounds in fermenting fruit. In a teaching lab, it's the molecule that gives a positive Tollens silver-mirror test, since the aldehyde reduces Ag(I) to elemental silver while being oxidized to acetate. In the analytical chemistry literature, it shows up as the IARC Group 1 designation tied specifically to alcohol-derived exposure — the same biochemistry that produces hangovers also produces the carcinogenic dose, which is why alcohol-flush carriers face elevated upper-aerodigestive cancer risk if they drink heavily.
Common Uses
- Feedstock for acetic acid via stepwise oxidation
- Intermediate for pyridine derivatives and pentaerythritol
- Tollens-test reagent for silvering glassware
- Trace flavor compound in food and beverage processing
- Reference aldehyde for teaching nucleophilic addition reactions
Safety Information
Flammable with a flash point of -39 °C — ignites readily at room temperature. Vapors irritate the eyes and respiratory tract; chronic inhalation exposure raises cancer risk, with IARC classifying it as Group 1 in the context of alcohol-related exposure. Handle in a fume hood with appropriate gloves and eye protection; keep away from oxidizers and ignition sources.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.