Difference Between Octane and Cetane

The key difference between Octane and Cetane is that Octane has 8 carbon atoms whereas Cetane has sixteen carbon atoms per molecule. Furthermore, Octane is highly volatile while Cetane is not volatile.

Both these are hydrocarbon compounds that are used as references for the ranking of different engine fuels according to their performance (octane rating and cetane rating). Furthermore, both these are colorless liquids at room temperature that readily undergo ignition.

1. Overview and Key Difference
2. What is Octane
3. What is Cetane
4. Side by Side Comparison – Octane vs Cetane in Tabular Form
5. Summary

What is Octane?

Octane is a flammable hydrocarbon having the chemical formula C₈H₁₈. It is also an alkane (no double bonds or triple bonds present between two atoms). Since this compound is a low molecular weight hydrocarbon, it is highly volatile (the liquid phase easily converts into the vapor phase). This liquid is less dense than water; hence, it floats on water. That is because it is insoluble in water since it is a nonpolar liquid (water is a polar solvent). In addition, it can form an irritating odor via volatilization.

Chemical Properties Octane

Some chemical facts about this compound are as follows:

Chemical formula = C₈H₁₈
Molar mass = 114.23 g/mol
Melting point =-56.8 °C
Boiling point = 126°C
Appearance = colorless liquid
Odor = gasoline-like odor

Figure 01: Octane Molecule (unbranched)

Octane rating is a term used in ranking different fuel grades. It measures the antiknock property of a fuel, in other words, it measures the degree of knocking effect when it is in an engine. This rating system ranks a fuel according to a reference fuel; a mixture of isooctane and heptane. The knocking effect of isooctane is a minimal effect which leads it to rank isooctane as 100 (while heptane has a maximum knocking effect that leads it to rank heptane as 0). Therefore, higher the branching of octane molecule, higher the octane rating of it will be.

What is Cetane?

Cetane is a hydrocarbon having the chemical formula C₁₆H₃₄. The IUPAC name of this compound is Hexadecane. It is a highly flammable alkane. This compound is also less dense than water, hence it floats on water. However, it doesn’t mix with water due to its nonpolar property. Unlike octane, hexadecane is not volatile because it is a high molecular weight hydrocarbon.

Chemical Properties Cetane

Some important chemical facts about this compound are as follows:

Chemical formula = C₁₆H₃₄
Molar mass = 226.45 g/mol
Melting point = 18 °C
Boiling point = 287°C
Appearance = colorless liquid
Odor = gasoline-like odor

Key Difference Between Octane and Cetane

Figure 02: The Chemical Structure of Isocetane

Cetane number (or rating) is the measure of the combustion speed of a diesel fuel. In other words, it is a measure of the delay of ignition of diesel. Furthermore, it measures the compression required for the ignition of this fuel. A lower Cetane number indicates that the fuel has a longer ignition delay time and vice versa.

What is the Difference Between Octane and Cetane?

Octane is a flammable hydrocarbon having the chemical formula C₈H₁₈ whereas Cetane is a hydrocarbon having the chemical formula C₁₆H₃₄. Octane had 8 carbon atoms while Cetane has 16 carbon atoms. The molar mass of Octane is 114.23 g/mol. On the other hand, the molar mass of Cetane is 226.45 g/mol.

Considering the melting and boiling points of the two compounds, Octane has a melting point of -56.8 °C while it is 18 °C in Cetane. The boiling point of Octane is 126°C whereas it is 287°C in Cetane. Furthermore, Octane is highly volatile while Cetane is not volatile. And both these hydrocarbons are useful in rating fuels where Octane is used to rate the fuel and Cetane is used to number the diesel.

Summary – Octane vs Cetane

Octane and Cetane are readily flammable hydrocarbon compounds. Both these are colorless liquids at room temperature with a gasoline-like odor. The difference between Octane and Cetane is that Octane has 8 carbon atoms whereas Cetane has sixteen carbon atoms per molecule.

Reference:

1. “Octane.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine. Available here
2. “Octane.” Wikipedia, Wikimedia Foundation, 17 May 2018. Available here
3. “Hexadecane.” Wikipedia, Wikimedia Foundation, 17 May 2018. Available here