Nitrobenzene
Properties
| State | Liquid (pale yellow oily liquid with almond-like odor) |
| Color | Pale yellow |
| Solubility | Slightly soluble in water (1.9 g/L at 20°C); miscible with most organic solvents |
| Melting Point | 5.7°C |
| Boiling Point | 211°C |
About Nitrobenzene
Nitrobenzene is a pale yellow oily liquid (formula C6H5NO2, molar mass 123.110 g/mol) with a sharp, bitter-almond odor — Victorian perfumers called it 'oil of mirbane' and used it in soaps and shoe polish until the toxicology caught up in the 1930s. It's produced industrially at over 5 million tonnes per year by mixed-acid nitration of benzene: a 1:1 mixture of concentrated HNO3 and H2SO4 generates the nitronium ion NO2+, which attacks benzene in the textbook electrophilic aromatic substitution mechanism that every sophomore organic student learns. The nitro group is the canonical meta-director and ring deactivator — its powerful electron-withdrawal through both inductive (-I) and resonance (-M) effects destabilizes carbocation intermediates at ortho and para positions, leaving meta as the kinetically preferred site for any second electrophilic attack. Over 95% of all nitrobenzene produced gets catalytically hydrogenated (H2 over Pd/C, Pt/C, or sponge nickel) to aniline, and aniline is the entry point into a massive synthesis tree: methylene diphenyl diisocyanate (MDI) for polyurethane insulation foam and mattresses, rubber accelerators (MBT, CBS) for tire vulcanization, dyestuffs (azo and triphenylmethane families), and pharmaceutical scaffolds. Nitrobenzene also serves as a high-boiling polar aprotic solvent for Friedel-Crafts acylations where AlCl3 needs to stay in solution.
Where you'll encounter it
If you've ever pulled the cap off an old jar of nitrobenzene in a stockroom and gotten a hit of marzipan-like sweetness, that's the bitter-almond odor that gave the compound its 19th-century commercial appeal — and the reason 'oil of mirbane' showed up in soap recipes until the methemoglobinemia cases started accumulating. In a teaching organic lab, the bench-scale nitration of benzene (or more commonly methyl benzoate or bromobenzene, since pure benzene is now restricted) is the reaction students run to see directing-group effects firsthand: do the nitration, recrystallize, take a melting point, and confirm meta-substitution from the symmetry pattern in proton NMR. Industrially, every kilogram of polyurethane foam in your couch cushions or refrigerator insulation traces back through MDI, through aniline, through nitrobenzene. World aniline production runs around 5.5 million tonnes per year and is essentially all derived this way.
Common Uses
- Sole industrial precursor for aniline via catalytic hydrogenation over Pd/C, Pt/C, or sponge nickel
- Feedstock for MDI polyurethane foam, rubber accelerators, and azo dye intermediates downstream of aniline
- High-boiling polar aprotic solvent for Friedel-Crafts acylations requiring dissolved AlCl3
- Teaching example for electrophilic aromatic substitution and meta-directing effects in undergraduate organic
- Mild oxidizing agent in the Skraup quinoline synthesis from aniline and glycerol
- Component in shoe polish and metal polish formulations (largely phased out in consumer products)
- Feedstock for explosives precursors including 1,3,5-trinitrobenzene via further nitration
Safety Information
GHS classifications: H301 + H311 + H331 (toxic if swallowed, in contact with skin, or inhaled), H351 (suspected carcinogen), H360F (may damage fertility), H372 (organ damage with prolonged exposure - blood). OSHA PEL is 1 ppm (5 mg/m3) as 8-hour TWA with skin notation; ACGIH TLV is 1 ppm. The signature toxicity is methemoglobinemia: nitrobenzene is reduced in vivo to nitrosobenzene and phenylhydroxylamine, which oxidize hemoglobin Fe²⁺ to Fe³⁺, blocking O2 transport. Symptoms — cyanosis (blue lips and nail beds), headache, fatigue, dizziness — appear hours after exposure and worsen over 24-48 hours. Skin absorption is significant; gloves must be butyl rubber or Viton (nitrile fails). Always work in a fume hood, with eye protection and lab coat. Methylene blue (1-2 mg/kg IV) is the antidote for severe methemoglobinemia. IARC Group 2B (possibly carcinogenic to humans).
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.