Intermediate Topic 774 of 1102 Organic Chemistry
Octane Number
A measure of a fuel's resistance to knocking in engines.
In-depth explanation of Octane Number
The octane number is a measure of a petrol's resistance to engine knocking, which is premature and uncontrolled ignition of the fuel-air mixture during compression. It is defined on a scale where 2,2,4-trimethylpentane, a highly branched isomer, is assigned 100 and n-heptane, a straight-chain alkane, is assigned 0. Branched alkanes, aromatic hydrocarbons and ethers all give high octane ratings because they resist knocking. Refineries raise the octane number of petrol by reforming and isomerization, and by adding oxygenates or lead-free additives.
Going deeper: Octane Number
Key ideas: Octane number measures resistance to knocking, defined by 2,2,4-trimethylpentane (isooctane) at 100 and n-heptane at 0. A fuel rated 90 behaves like a mix of 90 percent isooctane and 10 percent heptane. Branched alkanes, aromatics and alcohols rate high; unbranched chains rate low. Reforming and isomerisation raise octane number; lead and MTBE were additives.
Common mistakes and connections: Students think higher octane means more energy; it only means resistance to premature ignition. Another error is confusing octane number with cetane number for diesel, where straight chains are preferred. Links to chain isomerism, catalytic reforming and radical chain explosions in engines.
Visualization of Octane Number
Research octane numbers: straight chains knock easily (heptane is 0) while branched isooctane is 100 and aromatics and ethanol resist knocking. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Premium petrol is typically rated between 95 and 98 on the research octane number scale.
Frequently asked questions about Octane Number
What is Octane Number?
A measure of a fuel's resistance to knocking in engines.
Is Octane Number a beginner, intermediate or advanced topic?
Octane Number is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Octane Number?
Study these earlier topics in Organic Chemistry first: Combustion of Hydrocarbons, Cracking, Fractional Distillation of Crude Oil.
What comes after Octane Number?
The next topics in Organic Chemistry are Electrophilic Addition and Markovnikov's Rule.
Can I practise Octane Number with quizzes?
Yes. The interactive lesson includes a self-check question on Octane Number, and the Organic Chemistry quiz draws on every topic in this area.
Where Octane Number fits in Organic Chemistry
Octane Number is topic 46 of 148 in Organic Chemistry.