N-Propanol CAS Number, Formula, Molecular Weight and Properties
The Chemical Abstracts Service registry number 71-23-8 identifies the linear primary alcohol n-propanol, which appears under synonyms including propan-1-ol, 1-propanol, and propyl alcohol in the EINECS inventory number 200-746-9 and in national chemical inventories. The molecular formula is C3H8O, the condensed formula is CH3CH2CH2OH, and the molar mass calculated from the IUPAC 2013 standard atomic weight table is 60.095 g mol-1; the monoisotopic mass is 60.0575 Da. At 20 °C the compound is a clear, colorless, hygroscopic liquid with a mild alcohol odor, a density of 0.8034 g cm-3, and a refractive index at the sodium D line of 1.3850. The normal boiling point at 101.325 kPa is 97.2 °C, and the freezing point is −126.5 °C, which supports liquid handling across a broad ambient window but requires venting or pressure-rated equipment above 60 °C to control vapor accumulation. The CAS identifier separates n-propanol from branched propan-2-ol, which carries CAS 67-63-0 and a normal boiling point of 82.5 °C; this distinction is mandatory for import classification, analytical certificates of analysis, and REACH registration reporting.
| Property | Value | Preferred method or reference |
|---|---|---|
| Molecular formula | C3H8O | structural formula |
| Molar mass | 60.095 g mol-1 | IUPAC 2013 atomic weights |
| Density at 20 °C | 0.8034 g cm-3 | ASTM D4052 / DIN 51757 |
| Normal boiling point | 97.2 °C | ebulliometric measurement at 101.325 kPa |
| Freezing point | −126.5 °C | differential scanning calorimetry |
| Refractive index nD20 | 1.3850 | ISO 5661 / ASTM D1218 |
| Dynamic viscosity at 20 °C | 2.26 mPa·s | ASTM D445 / ISO 3104 |
| Dielectric constant at 25 °C | 20.3 | cavity perturbation |
| log P octanol/water | 0.25 | OECD 107 shake-flask |
| Closed-cup flash point | 24 °C | ASTM D56 / ISO 3679 |
| Autoignition temperature | 371 °C | ASTM E659 |
| Flammability limits in air | 2.2–13.7 vol% | ASTM E681 |
| Aqueous solubility | miscible | qualitative visual method |
What Limits Closed-Cup Flash Point Test Reproducibility in n-Propanol?
In a Setaflash closed-cup apparatus conforming to ISO 3679 or in a Tag closed-cup apparatus conforming to ASTM D56, the flash point of n-propanol is reported as 24 °C, but values of 23 °C appear in older literature when the interlaboratory repeatability window is applied. The vapor pressure at 20 °C is 1.99 kPa and increases to approximately 2.8 kPa at 25 °C, so a temperature rise of 5 °C shifts the equilibrium headspace concentration by approximately 40% relative to the 20 °C value. The lower and upper flammability limits in dry air are 2.2 vol% and 13.7 vol% respectively when measured according to ASTM E681; therefore, a closed vessel at 24 °C can form an ignitable headspace mixture if the vapor concentration lies within that range. The autoignition temperature is 371 °C according to ASTM E659, which is high enough that steam tracing at 121 °C does not approach autoignition but can boil the liquid because the normal boiling point is 97.2 °C. Under 29 CFR 1910.106, the liquid is classified as a Class IC flammable liquid when the flash point is at or above 22.8 °C and below 37.8 °C, although the borderline reported values of 23 °C or 24 °C require conservative bonding, grounding, and ventilation practices. The GHS/CLP classification is Flam. Liq. 2 with H225, and transport is controlled under UN 1274, Class 3, with closed equipment and local exhaust ventilation specified for packaging and storage.
Because the terminal hydroxyl group in n-propanol participates in hydrogen bonding with a strength intermediate between ethanol and butanol, the compound is fully miscible with water and with a range of polar solvents including acetone, ethyl acetate, and chloroform; the experimental octanol-water partition coefficient is 0.25, indicating a moderate preference for aqueous phases over lipid phases. Hansen solubility parameters reported for n-propanol are approximately 15.8 MPa0.5 dispersion, 6.8 MPa0.5 polar, and 17.4 MPa0.5 hydrogen bonding, which supports its function as a co-solvent in aqueous coatings, as a mobile-phase modifier in reversed-phase chromatography, and as a reaction medium for aromatic substitution and Grignard coupling. In binary distillation with water, the system forms a minimum-boiling azeotrope at approximately 87.8 °C and 71.7 wt% n-propanol at 101.3 kPa; this behavior prevents anhydrous n-propanol recovery by simple atmospheric distillation above the azeotropic composition and forces the use of azeotropic distillation with an entrainer, extractive distillation with glycols, molecular sieve adsorption, or pervaporation. The dynamic viscosity of 2.26 mPa·s at 20 °C and density of 0.8034 g cm-3 produce a kinematic viscosity of 2.81 mm2 s-1; in positive-displacement pumping systems, this kinematic viscosity requires correction of slip flow and volumetric efficiency compared with methanol, which has a kinematic viscosity near 0.75 mm2 s-1. The vapor-liquid equilibrium properties also affect condenser sizing, because the condensation load at the azeotropic overhead is dominated by the water-enriched vapor rather than by pure n-propanol vapor.
Thermal Degradation and Storage Exotherm Boundaries
The normal boiling point of 97.2 °C limits unpressurized storage to temperatures below this value, while the lower flammability limit of 2.2 vol% requires that storage tanks be inerted or vented with flame arresters; nitrogen blanketing to an oxygen concentration below 50 ppm is often specified in pharmaceutical and fine-chemical bulk storage because primary alcohols can slowly autoxidize to propionaldehyde and propionic acid through a free-radical pathway. The temperature rise from oxidation is not classified as a self-reactive hazard, but water-contaminated n-propanol in carbon steel storage may show measurable corrosion at temperatures above 40 °C because the acid formed by oxidative degradation lowers the local pH. In pilot-plant and production-scale equipment, wetted parts of 316L stainless steel, fluoropolymer gaskets such as PTFE, and borosilicate glass are generally used for transfer lines and reactor internals; however, compatibility with nitrile, neoprene, and natural rubber gaskets is not guaranteed because alcohols can swell and extract plasticizers from elastomers. Published data for long-term elastomer compatibility in warm n-propanol-water mixtures is limited, and ASTM G31 immersion coupon tests or manufacturer-specific chemical resistance tabulations should be used before specifying a gasket on a 5000 L storage tank. Ventilation design for process areas should maintain the airborne concentration below 10% of the lower explosive limit, which in volumetric terms is 0.22 vol%; this requirement is one of the reasons that flammable-liquids storage cabinets conforming to 29 CFR 1910.106 and EN 14470-1 are applied to stock bottles and intermediate containers.
For workplace exposure assessment, the OSHA permissible exposure limit for n-propanol is 200 ppm as an 8-hour TWA with an approximate mass concentration of 500 mg m-3 under 29 CFR 1910.1000 Table Z-1; the ACGIH threshold limit value is 100 ppm with a 15-minute STEL of 150 ppm, and the NIOSH recommended exposure limit is 200 ppm TWA with a 250 ppm STEL. The GHS/CLP classification includes Eye Dam. 1 with H318 because direct splash contact produces serious eye irritation and corneal injury; it also includes STOT SE 3 with H336 because high vapor concentrations can cause central nervous system depression, drowsiness, and dizziness. The compound has a detectable alcohol odor, but olfaction fatigue is a known limitation for prolonged exposure and should not be relied upon as a warning property. Air monitoring should use charcoal tube sampling followed by gas chromatography with flame ionization detection according to NIOSH 1405, with sample collection at the operator breathing zone and comparison against the regulatory limits.
| Authority or standard | Limit or classification | Value |
|---|---|---|
| 29 CFR 1910.1000 Table Z-1 | PEL 8-hour TWA | 200 ppm (500 mg m-3) |
| ACGIH TLV | 8-hour TWA | 100 ppm |
| ACGIH STEL | 15-minute STEL | 150 ppm |
| NIOSH REL | TWA / STEL | 200 ppm / 250 ppm |
| CLP Flam. Liq. 2 | classification | H225 |
| CLP Eye Dam. 1 | classification | H318 |
| CLP STOT SE 3 | classification | H336 |
When Excess Molar Volume Governs n-Propanol-Water Batch Correction
When water and n-propanol are mixed for cleaning, chromatographic eluent preparation, or reaction solvent dilution, the final volume is not the arithmetic sum of the two component volumes because the binary system exhibits negative excess molar volume through much of the composition range at ambient temperature; the hydrogen-bonding network contracts upon mixing, and literature reports show measurable negative excess volumes at intermediate mole fractions. Published data for this specific configuration is limited in production-scale archives, but density correction is required for preparing exact molar solutions. Batch records based on volumetric additions therefore require conversion to mass fractions and measured density using ASTM D4052 or ISO 15212-1 before release; a target concentration of 0.500 mol kg-1 cannot be prepared reliably by adding 37.3 mL of n-propanol to 1.000 L of water without density correction. In production-scale mixing vessels, load cells calibrated to ISO 7500-1 or Coriolis mass flow meters calibrated to ISO 10790 provide mass-based dosing, while the final volume is confirmed by a calibrated sight gauge or radar level transmitter. For reactions in which water is generated or consumed, the mass balance should use the molar mass of 60.095 g mol-1, not the rounded 60.1 g mol-1 value commonly printed on analytical certificates, because a 1000 kg batch at 0.5 mol kg-1 requires 30.0475 kg of n-propanol, and the difference between exact and rounded molecular mass becomes analytically significant at batch sizes above several hundred kilograms.
