Isopentane
[CAS 78-78-4]

Identification

• Classification: Pharmaceutical intermediate >> Heterocyclic compound intermediate >> Quinoline compound
• Name: Isopentane
• Synonyms: 2-Methylbutane
• Molecular Formula: C₅H₁₂
• Molecular Weight: 72.15
• CAS Registry Number: 78-78-4
• EC Number: 201-142-8
• SMILES: CCC(C)C

Properties

• Density: 0.6±0.1 g/cm³ Calc.^*, 0.62 g/mL (Expl.)
• Melting point: -160 °C (Expl.)
• Boiling point: 26.2±3.0 °C 760 mmHg (Calc.)^*, 30 °C (Expl.)
• Flash point: -56.7 °C (Calc.)^*, -57 °C (Expl.)
• Index of refraction: 1.369 (Calc.)^*, 1.354 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS02;GHS07;GHS08;GHS09 DangerGHS02
• Risk Statements: H224-H304-H336-H411
• Safety Statements: P210-P233-P240-P241-P242-P243-P261-P271-P273-P280-P301+P316-P303+P361+P353-P304+P340-P319-P331-P370+P378-P391-P403+P233-P403+P235-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Aspiration hazard	Asp. Tox.	1	H304
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411
Specific target organ toxicity - single exposure	STOT SE	3	H336
Flammable liquids	Flam. Liq.	1	H224
Flammable liquids	Flam. Liq.	2	H225
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335

• Transport Information: UN 1265

chemBlink Chemical Story

A hydrocarbon can participate in generating power without being burned. Isopentane, CAS 78-78-4, is studied as a working fluid in organic Rankine cycles, where supplied heat drives evaporation and expansion. The molecule circulates through a machine while an external source supplies the thermal energy. Distinguishing the working fluid from the heat source is the first step toward understanding this application.

Isopentane is 2-methylbutane, a branched alkane with the formula C₅H₁₂. It is a constitutional isomer of n-pentane and neopentane: all three have the same elemental composition but different carbon connectivity. Those differences affect physical properties, so the word pentane without further qualification can be inadequate in a process specification. Its identity is also distinct from cyclopentane, the saturated ring compound with two fewer hydrogen atoms.

The Rankine-cycle idea uses a working fluid to carry energy through repeated changes of state. A liquid is pressurized, heated, and vaporized; the vapor then expands through a work-producing device. Cooling condenses the fluid so that it can return to the liquid side of the cycle. Actual arrangements may add heat recovery or other features, but the basic objective is to obtain useful work from a heat flow.

An organic Rankine cycle uses an organic working fluid instead of relying on water as the circulating substance. This allows engineers to examine fluids whose thermodynamic properties suit particular heat sources and cooling conditions. The label organic refers to the chemical working fluid, not to a biological origin or an environmental certification. Choosing a hydrocarbon does not by itself establish that an installation is renewable or low impact.

For a geothermal example, heat can pass from the geothermal stream across a heat exchanger into a separate working-fluid circuit. Isopentane need not mix with the source water or steam, and it is not consumed to provide the heat. The distinction also applies to recovering industrial waste heat: a circulating fluid can help extract useful work from a thermal stream without being the fuel that originally created that stream.

A 2013 study by Garg and colleagues evaluated isopentane, R-245fa, and their mixtures as organic Rankine-cycle working fluids. This comparison illustrates why fluid choice is tied to a defined thermodynamic problem. A pure substance and a mixture need not evaporate and condense in the same manner. Conclusions depend on the selected heat source, temperature range, cycle assumptions, and performance criteria, rather than on one substance being universally superior.

Boiling behavior and chemical stability must also be distinguished. A fluid can repeatedly evaporate and condense while retaining its molecular identity, yet sufficiently severe thermal exposure may cause chemical change. Pasetti and colleagues investigated thermal stability in 2014 for several candidate fluids, including isopentane. Their experimental approach examined changes after thermal stress, showing why an operating envelope needs evidence beyond a normal boiling point.

Pressure is central to that distinction as well. A boiling point quoted at atmospheric pressure is not the temperature at which the fluid must boil everywhere in a closed machine. The pressure and temperature together determine phase conditions. Meanwhile, the useful output comes from the complete cycle, after accounting for internal power requirements and losses; observing vapor expansion alone is not a measurement of net electricity generation.

Isopentane is volatile and flammable, properties that belong in the design of a contained working-fluid system. Its energy application is best understood as coordinated heat transfer and fluid circulation. A small branched molecule becomes useful through a machine that repeatedly changes its physical state while preserving, as far as intended operation allows, its chemical identity.

References:
1. NIST Chemistry WebBook. Isopentane, CAS 78-78-4. https://webbook.nist.gov/cgi/cbook.cgi?ID=C78784&Mask=4
2. Garg P et al. Evaluation of isopentane, R-245fa and their mixtures as working fluids for organic Rankine cycles. Applied Thermal Engineering (2013). https://doi.org/10.1016/j.applthermaleng.2012.08.056
3. Pasetti M et al. Thermal stability of working fluids for organic Rankine cycles: An improved survey method and experimental results for cyclopentane, isopentane and n-butane. Applied Thermal Engineering (2014). https://doi.org/10.1016/j.applthermaleng.2014.08.017

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