Chemistry Formulas

Octane Formula

Octane is a straight alkane chain which comprises of 8 carbon atoms. It has a role as a xenobiotic. It has a chemical formula \(C_{8}H_{18}\). Octane is a hydrocarbon and most hydrocarbon groups are found in petroleum. Petroleum is an oil that is found in nature and under the surface of the earth. It is a mixture of crude oil and natural gas. Octane has many structural isomers. But they differ depending on the amount and location of branching in the carbon chain. It is a colorless liquid with a density of 0.703 g \(cm^{-3}\). Octane is also used to describe the term “octane rating”. Octane became popular in America around the mid- and late 1960s when gasoline companies boasted of “high octane” levels in their gasoline advertisements. It has 18 structural isomers. Let us learn the octane formula.

Octane Formula


Octane Formula

What is Octane?

Octane is a colorless liquid with a gasoline-like smell. It has a melting point of −57.1 to −56.6 °C; −70.9 to −69.8 °F and boiling point of 125.1 to 126.1 °C; 257.1 to 258.9 °F. Its refractive index is 1.398. Its density is lesser than water and very much insoluble in water. Octane is a component of gasoline (petrol). Octane is also volatile and very flammable like all other low-molecular-weight hydrocarbons.

The related term with octane is octane rating or octane number. We define it as a standard measure of the performance of an engine or aviation fuel. So fuels with a higher octane rating are useful in high-performance gasoline engines that require higher compression ratios.

Among its isomeres, the isooctane (2,2,4-trimethyl pentane) is most important, because its octane rating is 100. So it is an important constituent of petrol. Octane was found in the crude oil discovered by Edwin L. Drake in August 1859. He was the first to drill for oil. The octane rating was developed in 1927 by Graham Edgar.

The 18 Isomers of octane are:

  1. Octane
  2. 2-Methylheptane
  3. 3-Methylheptane
  4. 4-Methylheptane
  5. 2,2-Dimethylhexane
  6. 2,3-Dimethylhexane
  7. 2,4-Dimethylhexane
  8. 2,5-Dimethylhexane
  9. 3,3-Dimethylhexane
  10. 3,4-Dimethylhexane
  11. 3-Ethylhexane
  12. 2,2,3-Trimethylpentane
  13. 2,2,4-Trimethylpentane
  14. 2,3,3-Trimethylpentane
  15. 2,3,4-Trimethylpentane
  16. 2-Methyl-3-ethylpentane
  17. 3-Methyl-3-ethylpentane
  18. Tetramethylbutane

What is the Formula of Octane?

The chemical formula of octane is \(C_{8}H_{18}\), and the condensed structural formula is \(CH_{3}\left ( CH_{2} \right )_{6} CH_{3}\). Other names are n-octane, 111-65-9 Oktan, Oktanen. Its molar mass is 114.23 \(\frac{g}{mol}\).

When we burn octane in air, chemical reaction with oxygen gas \(O_{2}\) occurs to produce carbon dioxide \(CO_{2}\) and water \(H_{2}O\). The reaction is as follows:

\(2C_{8}H_{18} + 25O_{2}  \rightarrow 16CO_{2} + 18H_{2}O\)

Combustion reactions are the rapid reactions that produce the flame. A gas with a lower octane number ignites at lower temperatures and is as such more flammable. Higher octane number gas requires more compression and higher temperatures to burn efficiently.


Octane prevents the air-fuel mixture from igniting before the spark plug does it. The sparking or firing of the air-fuel mixture at the proper time gives you the maximum power an engine is designed for. Using gasoline of a higher octane number than your engine is designed is only a wastage of your money.

Solved Examples

Q.1.What is the molar mass of Octane?

Ans- Molar mass of \(C_{8}H_{18} = 114.22852 \frac{g}{mol}\).

The atomic mass of hydrogen is 1.00794

The atomic mass of carbon is 12.0107

No of atoms of hydrogen is 18

Molecular weight calculation:

\(12.0107 \times  8 + 1.00794 \times  18 = 114.22852  \frac{g}{mol}\).

Q.2. What is the octane booster?

Ans- An octane booster protects against low octane level issues by increasing the octane rating, and it improves performance by helping to ensure that fuel in a cylinder ignites exactly when it should, even in the high-compression engine.

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