Aniline
Properties
| State | Liquid at room temperature |
| Color | Colorless to pale yellow (darkens on exposure to air) |
| Solubility | Slightly soluble in water (3.6 g/100 mL at 25 °C) |
| Melting Point | -6.3 °C |
| Boiling Point | 184.1 °C |
About Aniline
Aniline is the prototypical aromatic amine and a textbook case of how attaching a nitrogen lone pair to an aromatic ring changes the chemistry on both sides. The basicity drops sharply: aniline's pKb of 9.4 is roughly five orders of magnitude weaker than methylamine's pKb of 3.4, because the nitrogen lone pair delocalizes into the ring rather than sitting available for protonation. The same delocalization pushes electron density into the ortho and para positions of the ring, making aniline strongly activated toward electrophilic aromatic substitution — Friedel–Crafts and similar reactions hit the para position with high regioselectivity. The molecule's place in chemical history is hard to overstate. In 1856, an 18-year-old William Henry Perkin tried to synthesize quinine by oxidizing aniline with potassium dichromate; he got a black sludge instead, but on extracting it with ethanol pulled out a brilliant purple solution that turned out to be mauveine — the first synthetic dye. The discovery launched the German dye industry that grew into BASF, Bayer, and Hoechst, and effectively created modern industrial organic chemistry. Today's production of around 6 million tonnes a year flows almost entirely into methylene diphenyl diisocyanate (MDI), the building block for polyurethane foams that show up in mattresses, refrigerator insulation, and shoe soles.
Where you'll encounter it
If you've worked with rubber, dyes, or polyurethanes, the chemistry traces back to aniline somewhere upstream. In a research lab, aniline is the starting material for the Sandmeyer reaction (where the diazonium chemistry built off aniline gives chlorides, bromides, cyanides, and other aryl substitutions cleanly), the Doebner reaction for quinoline synthesis, and a long list of pharmaceutical intermediates including paracetamol/acetaminophen. The compound is also infamous for skin absorption — bare-handed exposure can produce methemoglobinemia within hours, where the absorbed aniline gets metabolized to nitrosobenzene and oxidizes hemoglobin's iron, leaving the patient with cyanotic-looking skin and impaired oxygen transport.
Common Uses
- MDI feedstock for polyurethane foams and insulation
- Aryl-amine substrate for Sandmeyer and diazonium chemistry
- Pharmaceutical intermediate (paracetamol, sulfa drugs, dyes)
- Rubber-vulcanization accelerator and antioxidant precursor
- Reference compound for aromatic-amine spectroscopy and reactivity
Safety Information
Readily absorbed through intact skin — the fat-solubility plus the dermal permeability mean bare-handed exposure produces systemic toxicity within hours. Once absorbed, aniline metabolism generates nitrosobenzene and phenylhydroxylamine, both of which oxidize hemoglobin's Fe(II) to Fe(III) and produce methemoglobinemia, a potentially fatal failure of oxygen transport. Suspected human carcinogen by chronic exposure (IARC Group 2B). Always handle in a fume hood with butyl- or nitrile-rubber gloves; check gloves for permeation degradation since aniline penetrates many common rubber materials.
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.