Gasoline knock resistance: Among major hydrocarbon families, which generally exhibits the highest octane number (best knock resistance) in spark-ignition engines?

Introduction:
Octane number measures resistance to autoignition in spark-ignition engines. Hydrocarbon family strongly influences base octane. The question asks which family exhibits the highest octane among typical gasoline-range species.

Given Data / Assumptions:

• Comparison is across families: aromatics, isoparaffins, naphthenes, olefins, and n-paraffins.
• We consider typical gasoline boiling range compounds.

Concept / Approach:
Aromatic hydrocarbons (e.g., toluene, xylene) possess inherently high octane numbers due to ring stability and slower low-temperature chain branching. Isoparaffins also have high octane, but lower than aromatics on average. Normal paraffins are poorest. Olefins and naphthenes sit in between depending on structure.

Step-by-Step Solution:

Verification / Alternative check:
Gasoline blending literature consistently shows reformate (aromatic-rich) with very high RON, while straight-run paraffinic streams have low RON and require upgrading.

Why Other Options Are Wrong:

• Isoparaffins: High octane but typically less than aromatics.
• Naphthenes: Moderate octane.
• Olefins: Variable; usually below aromatics and top isoparaffins.
• n-Paraffins: Lowest octane in family comparisons.

Common Pitfalls:
Equating “clean burning” or stability with “highest octane”; octane is a knock metric, not a cleanliness metric.

Final Answer:
Aromatics