Aniline, CAS 62-53-3 and EINECS 200-539-3, is an aromatic primary amine with the molecular formula C6H5NH2 and molar mass 93.13 g/mol. It is supplied as a clear to pale yellow oily liquid with a normal boiling point of 184.1°C, freezing point of -6.2°C, density of 1.0217 g/cm³ at 20°C, refractive index 1.5863 at 20°C, and vapor pressure of 0.49 mm Hg at 25°C. The conjugate acid of aniline has a pKa of 4.6; the free base is sparingly soluble in water at about 36 g/L at 25°C, and its octanol–water partition coefficient is approximately 0.90 log P. Flash point is 70°C closed cup, and autoignition temperature is approximately 615°C.

Commercial aniline is marketed in at least three industrial grades: MDI-grade for polyurethane feedstock, rubber-chemical-grade for diphenylamine and accelerator production, and dyestuff-grade for azo and indigo dye intermediates. The grade differentiation is based on trace impurity limits rather than on differences in the aniline molecule. A representative MDI-grade certificate of analysis lists aniline content ≥99.80% by gas chromatography, nitrobenzene ≤0.01%, water ≤0.10%, distillation range 183.0–186.0°C at 101.3 kPa, and APHA color ≤50. Rubber-chemical-grade material may accept slightly broader water and color limits, but downstream condensation and reduction systems often require nitrobenzene to remain below 0.02% to limit color-forming side products.

The dominant manufacturing route is catalytic hydrogenation of nitrobenzene over copper- or palladium-based fixed-bed or fluid-bed catalysts. The reaction operates at 200–300°C and 0.3–1.0 MPa; heat removal is critical because the reaction is highly exothermic and thermal excursions can increase nitrobenzene breakthrough. Crude aniline is dehydrated and fractionated. Plant-scale overhead condenser data show that the aniline–water separation step must be operated with sufficient residence time to prevent water carry-over into the product tank, because water in stored aniline accelerates corrosion in carbon steel transfer lines and reduces downstream phosgenation efficiency.

Does water content control phosgenation yield in MDI production more than nitrobenzene concentration?

In methylene diphenyl diisocyanate production, aniline is reacted with aqueous formaldehyde in the presence of hydrochloric acid to produce methylene dianiline. The industrial condensation is carried out at 60–100°C with an aniline-to-formaldehyde molar ratio of at least 2.0:1 to suppress higher oligomers. The 4,4'-MDA isomer is the desired species for rigid polyurethane and elastomer applications; typical high-selectivity MDA processes report the 4,4'-isomer content above 98.0% of the isomer mixture, although published data for exact plant-level isomer distributions are limited.

After condensation, MDA is phosgenated in monochlorobenzene or another inert solvent at 120–180°C. Water is the primary feed-quality variable because water above 0.10% hydrolyzes phosgene, increasing carbon dioxide and hydrogen chloride formation and raising scrubber heat duty. Nitrobenzene above 0.01% does not cause the same phosgene consumption, but it can contribute to colored diamines and to final MDI color instability. In practice, MDI-grade aniline specifications therefore prioritize low water content and low nitrobenzene simultaneously, with water limits typically enforced by Karl Fischer titration using ASTM E203 and nitrobenzene limits by capillary gas chromatography.

These limits are representative technical-data-sheet values for MDI-grade aniline; they are not universal legal limits. Individual producers may report lower water limits for phosgenation units that use refrigerated monochlorobenzene recovery, and some end users may require nitrobenzene below 0.005% for optical-grade polyurethane intermediates. Published data for exact yield impact at each impurity level remain limited because plant-specific phosgene excess and scrubber capacities influence the tolerable water ingress.

Rubber accelerator and antioxidant pathways, with process-specific impurity constraints

Rubber-chemical-grade aniline is converted into diphenylamine, 4-aminodiphenylamine, and mercaptobenzothiazole-type accelerators. Diphenylamine is produced by vapor-phase condensation of aniline over a solid acid catalyst in fixed-bed reactors with a typical catalyst bed temperature of 350–450°C. Excess aniline is recycled after condensation, and the reactor pressure drop is monitored continuously. Operating bulletins for fixed-bed units indicate that water above 0.15% in the feed promotes oligomeric deposits on the catalyst surface and shortens cycle length; this is one reason rubber-chemical-grade aniline retains a tight water specification despite being less sensitive than MDI-grade.

4-ADPA is an intermediate in the production of the tire antiozonant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, commonly identified as 6PPD. Modern routes may use nitrobenzene as the primary nitrogen source, with aniline entering in a condensation step followed by hydrogenation over a supported metal catalyst. The resulting 6PPD is evaluated in rubber compounds according to ASTM D1149 for ozone cracking and ASTM D1171 for outdoor exposure or dynamic ozone resistance; these tests are used to set addition levels in sidewall and tread compounds. Aniline-derived diarylamine antioxidants such as diphenylamine and 6PPD are lower in volatility and higher in molecular weight than aniline itself, which reduces migration kinetics in polymer matrices and extends protection under dynamic ozone exposure.

In dye manufacture, aniline is diazotized with sodium nitrite in hydrochloric acid at 0–5°C and coupled to aromatic phenols or amines to form azo dyes. The diazonium salt is thermally unstable; acid concentration and temperature are therefore controlled within narrow limits. Dyestuff-grade aniline should have APHA color ≤50 and nitrobenzene ≤0.01%, because colored oxidation impurities can shift final dye shade and survive diazo coupling. Aniline is also used in the production of aniline black, a polyquinonoid oxidation pigment, and in the older aniline–chloroacetic acid route to synthetic indigo. In batch azo dye units, aniline color above the accepted APHA threshold is a common cause of shade drift and rework, as reported in plant quality-control records.

Aniline is stored at 15–30°C in carbon steel or stainless steel vessels with nitrogen blanketing. Below -6.2°C, the product solidifies; reheating should use low-pressure steam or warm water rather than high-temperature electrical tracing, because wall temperatures above 70°C can increase oxidation and color development. The flash point is 70°C closed cup, and the autoignition temperature is approximately 615°C, so area classification and grounding are required at bulk storage installations. Aniline is incompatible with strong oxidizers, nitric acid, and nitrous acid; contact with copper, brass, or copper-containing alloys can form colored copper-amine complexes and accelerate resin formation. The ACGIH TLV-TWA is 2 ppm (7.6 mg/m³) with skin notation, and the OSHA PEL is 5 ppm (19 mg/m³) as an 8-hour time-weighted average. Closed-loop transfer, vapor recovery, and local exhaust ventilation are commonly specified for drum and tank handling stations.

Aniline is absorbed through skin and can produce methemoglobinemia via N-hydroxylation to phenylhydroxylamine, which oxidizes ferrous hemoglobin to ferric methemoglobin. Industrial hygiene monitoring for aniline and related aromatic amines uses silica gel sampling and gas chromatography; the relevant OSHA method is 5002. Spill control uses inert absorbents, and aqueous rinse water must be captured because aniline is toxic to aquatic life and requires pH-controlled oxidation or biological treatment before discharge. REACH registration and Safety Data Sheet exposure scenarios require specific risk-management measures for liquid transfer, sampling, and maintenance, including chemically resistant gloves and eye protection.

Aniline differs from nitrobenzene in oxidation state, reactivity, and phase behavior. Nitrobenzene has a normal boiling point of 210.9°C and a freezing point of 5.7°C, whereas aniline boils at 184.1°C and freezes at -6.2°C. The amino group of aniline is activating and ortho/para-directing, enabling diazotization and formaldehyde condensation; nitrobenzene is deactivated and meta-directing, so it cannot replace aniline in azo dye diazotization or MDA production. In supply chains, nitrobenzene is the upstream precursor to aniline rather than a competing product for the same amine-specific reactions.

Compared with N-methylaniline, aniline contains a primary amino group with two reactive hydrogen atoms. N-methylaniline has a normal boiling point of 196.3°C and is used mainly as a solvent and alkylation intermediate; it cannot undergo the same bifunctional polycondensation with formaldehyde to MDA. Compared with toluidine isomers, aniline carries no ring methyl substituent, which reduces steric hindrance in MDA condensation and produces a narrower distillation range. The comparative physical data are shown in the following table.

Physical constants are compiled from standard reference data and are not product specifications. Aniline is selected over N-methylaniline when the process requires diazonium salt formation or bifunctional condensation; N-methylaniline is selected when a monofunctional secondary amine is required to avoid crosslinking.

When phenyl isocyanate and sulfonamide intermediates require primary amine functionality

Aniline is selected over nitrobenzene and over secondary amines when the downstream reaction depends on the primary –NH2 group. In phenyl isocyanate production, aniline is reacted with phosgene in an inert solvent under controlled temperature; the operation typically requires aniline water content below 0.05% and nitrobenzene below 0.01%, because water consumes phosgene and nitrobenzene contributes aromatic impurities that are difficult to separate from the isocyanate stream. Isocyanate content is monitored by ASTM D2572, and purity is checked by gas chromatography. Production-scale batch records show that water ingress from incomplete drying of transfer lines is a recurring cause of off-spec isocyanate yield and increased phosgene usage.

In pharmaceutical intermediate synthesis, aniline is acylated to acetanilide or converted to phenylurea and sulfonamide precursors. The primary amine permits stoichiometric acylation under controlled pH, whereas N-methylaniline would generate tertiary amide structures with different biological and toxicological profiles. Aniline used in these applications may require additional purification beyond technical grade, such as vacuum distillation to reduce color and high-boiling residues. The handling boundary is the same as for technical material: exposure must be controlled to below the OSHA PEL of 5 ppm (19 mg/m³), and nitrite-bearing streams must be segregated to avoid diazonium formation. Published data for exact pharmaceutical purification yields are product-specific and limited.