Isobutane
[CAS 75-28-5]

Identification

• Classification: Inorganic chemical industry >> Industrial gases such as hydrogen, nitrogen and oxygen
• Name: Isobutane
• Synonyms: 2-Methylpropane; A 31; Purifrigor Iso 3; R 600a; Trimethylmethane; iso-Butane
• Molecular Formula: C₄H₁₀
• Molecular Weight: 58.12
• CAS Registry Number: 75-28-5
• EC Number: 200-857-2
• SMILES: CC(C)C

Properties

• Density: 0.6±0.1 g/cm³ Calc.^*
• Melting point: -159.4 °C (Expl.)
• Boiling point: -10.5±3.0 °C 760 mmHg (Calc.)^*, -11.7 °C (Expl.)
• Flash point: -71.5±10.3 °C (Calc.)^*, -83 °C (Expl.)
• Index of refraction: 1.352 (Calc.)^*

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

Safety Data

• Hazard Symbols: GHS02;GHS04 DangerGHS02
• Risk Statements: H220
• Safety Statements: P203-P210-P222-P280-P377-P381-P403

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Flammable gases	Flam. Gas	1	H220
Gases under pressure (liquid)	Press. Gas (Liq.)		H280
Gases under pressure (compressed)	Press. Gas (Comp.)		H280
Carcinogenicity	Carc.	1A	H350
Germ cell mutagenicity	Muta.	1B	H340
Specific target organ toxicity - single exposure	STOT SE	3	H336
Flammable gases	Flam. Gas	2	H220
Flammable gases	Flam. Gas	2	H221
Specific target organ toxicity - single exposure	STOT SE	1	H370

chemBlink Chemical Story

Inside a refrigerator, a circulating molecule can help move heat without being consumed as a fuel. Isobutane, CAS 75-28-5, performs that role in systems designed for the refrigerant R-600a. Its usefulness comes from controlled changes of pressure and physical state. The same hydrocarbon identity that makes combustion possible does not mean combustion is the mechanism used to cool food.

Isobutane is 2-methylpropane, a branched four-carbon alkane with the formula C₄H₁₀. It has the same molecular formula as n-butane but a different carbon skeleton. The refrigerant designations preserve that distinction: R-600a refers to isobutane, while R-600 refers to n-butane. Matching elemental composition is therefore insufficient when selecting or modeling a working fluid.

At ordinary room temperature and atmospheric pressure, isobutane is a gas. Under suitable pressure it can be maintained as a liquid, and the conditions for evaporation and condensation change with pressure. A refrigeration circuit uses that relationship to arrange heat absorption at one temperature and heat rejection at another. The fluid need not undergo a permanent chemical transformation during normal operation.

In the evaporator, refrigerant takes up heat as it evaporates under the selected low-pressure conditions. A compressor supplies work to raise the pressure of the vapor. In the condenser, the fluid releases heat and returns toward the liquid state, before a pressure-reducing device prepares it for another pass through the evaporator. This description identifies the functions of the cycle without prescribing operating settings for a particular appliance.

The energy balance explains a familiar observation: the warm side of a refrigerator releases more heat than the cabinet alone loses. The machine must reject the heat removed from the cooled space along with energy supplied to operate the cycle. Cooling performance therefore cannot be understood simply by asking how cold the refrigerant can become. Compressor behavior, heat exchangers, controls, and the surrounding temperature also matter.

Experimental research has examined isobutane in domestic refrigeration systems for decades. A 2002 paper by Lee and Su reported such studies using R-600a. This establishes a research basis for discussing the application, but results for one apparatus should not become a guarantee for all refrigerators. A working fluid is only one part of a system whose geometry, components, and operating conditions determine its measured performance.

A different kind of evidence comes from a 2024 Oak Ridge National Laboratory conference paper. The researchers used a detailed simulation platform to compare refrigerator configurations employing R-600a, including single- and dual-evaporator arrangements. This was a modeling study, distinct from a report that every configuration had been experimentally built. It illustrates how system design remains a research question even after a refrigerant has been chosen.

Isobutane contains neither chlorine nor fluorine, a structural distinction from halogenated refrigerants. That fact alone is not a complete environmental comparison. Electricity use, leakage, manufacturing, and end-of-life handling also affect an appliance's overall impact. Isobutane is flammable, so successful use depends on equipment designed around its properties; its familiar chemical name does not establish interchangeability with a different refrigerant in an existing machine.

The molecule's refrigeration story is consequently about engineering a repeatable physical cycle. Molecular identity supplies the thermodynamic possibilities, while the machine determines how those possibilities are used. Keeping chemistry, phase behavior, and system performance distinct makes it easier to explain both the value of R-600a and the limits of claims made for it.

References:
1. NIST Chemistry WebBook. Isobutane, CAS 75-28-5. https://webbook.nist.gov/cgi/cbook.cgi?ID=C75285&Mask=4
2. Lee YS, Su CC. Experimental studies of isobutane (R600a) as refrigerant in domestic refrigeration system. Applied Thermal Engineering (2002). https://doi.org/10.1016/S1359-4311(01)00106-5
3. Yang CM et al. Performance Evaluation of Various Configurations for Domestic Refrigerators with R600a. ORNL, conference paper (2024). https://www.ornl.gov/publications/performance-evaluation-of-various-configurations-for-domestic-refrigerators-with-r600a/

Read more stories