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.