What is isobutane? What is the use of isobutane?

Isobutane (i-butane) is an isomer of butane with the chemical formula C4H10. That means it has the same number of atoms (therefore same weight), but has a different chemical arrangement.

It is also commonly used as an aerosol propellant and as an LPG gas, as are butane and propane. It is mainly used in refineries as a gasoline petrol additive.

However, it is different in some important ways. We’ll cover their difference below.

Physical Properties

The following chart shows some of the isobutane’s physical properties. You can refer back to the chart as we explain the importance of the numbers in the following topics.

Note: Some numbers have been rounded.

What is Isobutane? Difference Between n-Butane and Isobutane

The difference between n-butane and isobutane is that isobutane (i-butane) is an isomer of normal butane (n-butane). Isobutane is a colourless gas, much like its other counterparts. But i-butane has distinct advantages over n-butane and propane.

As an isomer, it has the same chemical formula as butane – C4H10 – but has a different arrangement of its atoms, as you can see in the 3-D model images. (Isobutane molecule model shown)

Production Process

Isobutane is converted from butane in a production process called isomerisation. This isomerisation happens in something called a Butamer Isomerisation Unit and includes using platinum or another metal catalyst.

As with normal butane (n-butane), isobutane (i-butane) is a flammable hydrocarbon gas liquefied through pressurisation. However, it has different physical properties from normal butane (n-butane).

Isobutane is colourless and has a weak petrol odour. Both are very flammable, and gas/air mixtures can be explosive.

Isobutane vapour (gas) is heavier than air but weighs the same as butane since they have the same chemical composition. However, Isobutane has a lower vapour pressure than n-butane, which is roughly 35kPa to 45 kPa or 0.35-0.45 bar, than n-butane’s 50-60 kPa or 0.5-0.6 bar.

It is classified as LPG, along with propane, butane, and blends of these gases.

Check out this full compound summary from PubChem and FDA Food Additive Status for food safety

Common Uses for Isobutane

Gasoline-Petrol Additive

Isobutane is mainly used in refineries as a gasoline-petrol additive in 1-5% concentration. It’s processed through an alkylation unit to generate the alkylate necessary to produce isooctane.

Isooctane is a high-octane gasoline component that increases the octane rating of gasoline, which is responsible for its anti-knock properties. In fact, it is rated at 100 points on the octane rating scale.

Refrigerants

Aside from its use in fuel refineries, isobutane is also used in air-conditioning units as a refrigerant known as R600a. It is an excellent refrigerant because it is eco-friendly and doesn’t have CFCs’ ozone-harming properties.

Isobutane has very low global warming potential and insignificant ozone depletion potential. Hence, it replaces harmful CFC refrigerants, such as R-12, R-22, and R-134a.

It can be used as a replacement for R-12, R-22, R-134a, and other chlorofluorocarbon or hydrofluorocarbon refrigerants, in conventional refrigeration systems.

Both n-butane (R-600) and high-purity propane (>97.5%), known as R290, are also used as refrigerants.

Whilst it is flammable and poses known safety and chemical hazards when inhaled, there have been few problems in the millions of refrigeration units worldwide.

Check out the NIOSH pocket guide for isobutane for more information.

Other Uses

• Another important use of isobutane is as a feedstock for plastics.
• It is used to manufacture propylene oxide for use in making polyurethane plastics.
• Another use of isobutane is as a solvent.
• Isobutane is also used as an aerosol propellant

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Vapor Pressure & Use as Propellants

One of the other important differences between isobutane and other liquefied petroleum gases is vapour pressure.

Vapour pressure is the pressure exerted by the vapour (gas), in equilibrium with the liquid, against the walls of the cylinder or other closed container at a given temperature.

Isobutane has about 64% less vapour pressure than propane but about 44% more than butane (at 21ºC). This property allows isobutane to be used as a propellant for aerosols.

Combustion & Limits of Flammability

Isobutane’s limits of flammability are 1.8% to: 8.4% by volume. Assuming complete combustion, you get carbon dioxide and water:

2 C4H10 + 13 O2 → 8 CO2 + 10 H2O + Heat

However, with incomplete combustion, you get carbon monoxide and water

2 C4H10 + 9 O2 → 8 CO + 10 H2O + Heat

This would typically occur if the ratio of oxygen to isobutane were insufficient. Note that incomplete combustion gases, including carbon monoxide, are toxic when inhaled and may cause carbon monoxide poisoning.

Boiling Point: Turning from Liquid to Gas

Isobutane and butane have different boiling points, the temperature at which they go from liquid to gas (vapour).

Isobutane boils at -11.75°C, whereas butane boils at -0.4°C. This means you have a problem if you try to use pure isobutane when the temperature drops below -11.75°C.

No boiling, no vapourisation, equals no gas. So, when it gets cold, you could find yourself without gas for your heater and cooking appliances.

However, propane and butane are less costly, so isobutane is rarely used in large amounts as heating fuel.

In some areas, LPG suppliers provide just propane or a blend of propane and butane to address this problem. This can work well when there are temperatures both below and above freezing.

Needless to say, propane is the preferred choice for cold weather climates, as it boils at -42°C. Nevertheless, isobutane’s lower vapour pressure than propane and butane makes it superior for cold-weather applications.

LPG – Liquefied Petroleum Gas

Both isobutane, butane, and propane are hydrocarbon gases that fall under the broad label of “LPG” because they are all liquefied petroleum gases.

They are a group of flammable hydrocarbon gases liquefied through pressurisation and commonly used as fuel.

They are also called Natural Gas Liquids (NGL), along with ethane, pentane, and pentanes plus. Their common distinguishing characteristic is that they can be compressed into liquid at relatively low pressures.

All are used as fuel in combustion and for heat generation, but LPG has many other applications.

Final Thoughts

Many people have never even heard of isobutane, but it affects their everyday lives.

This odourless gas keeps cars from knocking (pre-ignition), is an environment-friendly refrigerant, and is a common propellant for hairspray, deodorant and other aerosols.

Isobutane also contributes to a variety of plastic products and solvents. Most importantly, it is a great LPG fuel for cold climates where butane and propane would not boil and produce sufficient vapour pressure.

So, whilst nearly unknown, it is important nonetheless.