Pentane

C₅H₁₂ organic

Molar Mass

72.149 g/mol

Properties

State	Liquid (colorless, highly volatile, faint gasoline odor)
Color	Colorless
Solubility	Insoluble in water (0.04 g/L at 25 °C); miscible with ethanol, ether, and most organic solvents
Melting Point	-130 °C
Boiling Point	36 °C

About Pentane

Pentane is the five-carbon straight-chain alkane, formula C5H12, molar mass 72.149 g/mol. It's a colorless, extremely volatile liquid with a faint gasoline odor and the lightest n-alkane that's actually liquid at room temperature — butane (C4H10) is a gas at 20 °C, pentane boils at 36 °C. The C5H12 stoichiometry has three structural isomers, and they make a perfect teaching set for the relationship between branching and intermolecular forces: n-pentane (linear) boils at 36 °C, isopentane (2-methylbutane) at 28 °C, and neopentane (2,2-dimethylpropane) at 10 °C. Same molecular formula, same London dispersion forces in principle, but as branching pulls the molecule into a more spherical shape, the contact area between adjacent molecules drops, the dispersion attractions weaken, and the boiling point falls. In an organic lab, n-pentane is the workhorse non-polar solvent for low-temperature recrystallizations, for column chromatography of very non-polar substrates (steroids, fatty acid esters), and for liquid-liquid extractions where you want to keep DCM or diethyl ether out of the system. Its low boiling point makes rotovap removal trivial — a 30 °C bath strips it cleanly. Industrially, pentane is the modern blowing agent for expanded polystyrene (EPS) foam: pentane-impregnated polystyrene beads soften under steam and the dissolved pentane vaporizes to inflate them, replacing the CFCs and HCFCs that the Montreal Protocol phased out. It's also a major component of natural gasoline (the C5+ fraction stripped from natural gas) and the lighter fluid in disposable Bic-style lighters.

Where you'll encounter it

If you've ever lit a Bic lighter, blown insulation into an attic with EPS beads, or set up a normal-phase silica column with hexane-but-actually-pentane as the eluent, you've used pentane. EPS foam manufacturers store pentane-impregnated polystyrene beads (about 6 wt percent pentane) in vented warehouses with explosion-proof lighting because the beads slowly outgas during storage. In the petroleum industry, pentane is the standard solvent for SARA analysis (saturates, aromatics, resins, asphaltenes) and for asphaltene precipitation in heavy crude — adding 40 volumes of n-pentane to 1 volume of crude oil drops out the asphaltene fraction quantitatively. Synthetic chemists prefer pentane over hexane for very non-polar workups when they need to remove the solvent quickly under reduced pressure, and it shows up in NMR spectroscopy as the standard solvent for low-temperature variable-temperature studies down to about -40 °C.

Common Uses

Non-polar solvent for low-temperature recrystallization, column chromatography, and liquid-liquid extraction
Blowing agent for expanded polystyrene (EPS) foam, replacing CFC blowing agents post-Montreal Protocol
Asphaltene precipitation solvent in petroleum SARA analysis (40:1 pentane:crude ratio)
Component of natural gasoline and fuel for refillable cigarette lighters
Teaching substrate for structural isomerism, demonstrating how branching reduces boiling point

Safety Information

GHS: H225 (highly flammable liquid and vapor — flash point -49 °C), H304 (may be fatal if swallowed and enters airways — aspiration hazard causing chemical pneumonitis), H336 (may cause drowsiness or dizziness), H411 (toxic to aquatic life with long-lasting effects). Vapors are heavier than air and can travel along the floor to ignition sources — this is how pentane spills cause flash fires far from the spill point. OSHA PEL is 1000 ppm TWA, ACGIH TLV is 1000 ppm TWA (revised down to 600 ppm for some applications). Use only in a well-ventilated fume hood with no spark sources, ground containers when transferring, and store in flammable cabinets away from oxidizers.

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.

Constituent Elements

C — Carbon H — Hydrogen

Frequently Asked Questions

What is the molar mass of pentane?

The molar mass of pentane (C5H12) is 72.149 g/mol from 5 carbon (60.055) + 12 hydrogen (12.096). Density at 20 °C is 0.626 g/mL, so 1 mole of liquid pentane occupies about 115 mL — useful when scaling up extractions. The three structural isomers (n-pentane, isopentane, neopentane) all share this molar mass but have very different boiling points and densities.

How many structural isomers does pentane have?

Three: n-pentane (straight chain, bp 36 °C, density 0.626 g/mL), isopentane (2-methylbutane, one branch, bp 28 °C), and neopentane (2,2-dimethylpropane, two methyl branches, bp 10 °C — actually a gas at room temperature in summer). The progression in boiling points illustrates the surface-area dependence of London dispersion forces: linear molecules pack closely along their length and have strong dispersion contact, branched molecules are more spherical and contact less, so they boil lower despite having identical molar mass.

Why is pentane used in foam production?

Two reasons: the 36 °C boiling point and the inertness. Polystyrene beads are pre-impregnated with about 6 wt percent pentane during suspension polymerization. When the beads are heated with low-pressure steam (about 100 °C), the polystyrene softens above its 100 °C glass transition and the dissolved pentane flashes to vapor, expanding each bead 30-50 fold and locking it in foam form. Pentane replaced CFCs (CFC-11, CFC-12) after the Montreal Protocol because it doesn't deplete stratospheric ozone, though its flammability requires careful plant engineering and explosion-proof equipment.