Fuel types and knock tendency in spark-ignition engines: Arrange the following in order of decreasing knock tendency (i.e., from most prone to knock to least prone): paraffins, naphthenes, aromatics.

Introduction / Context:
In spark-ignition (SI) engines, a fuel’s resistance to knock is characterized by its octane rating. Different hydrocarbon families have different inherent tendencies to knock, informing gasoline formulation and additive strategies.

Given Data / Assumptions:

• Hydrocarbon families considered: paraffins (alkanes), naphthenes (cyclo-alkanes), aromatics.
• We rank by knock tendency (inverse of octane quality).
• Operating conditions typical of SI engines.

Concept / Approach:
Generally, normal paraffins have the poorest knock resistance (lowest octane), naphthenes are intermediate, and aromatics often show the highest octane numbers (best knock resistance). Therefore, from most knocking to least: paraffins → naphthenes → aromatics.

Step-by-Step Solution:

Verification / Alternative check:
Reference fuel data show higher octane for many aromatics (e.g., toluene) and lower for straight-chain alkanes (e.g., n-heptane as low-octane reference), confirming the trend.

Why Other Options Are Wrong:

• Any order placing aromatics before paraffins (as more knocking) contradicts typical octane behavior.
• Orders swapping naphthenes and aromatics overlook the superior knock resistance of many aromatics.

Common Pitfalls:
Assuming all compounds in a family behave identically; branching and specific structure matter, but the family trend remains broadly valid.

Final Answer:
paraffins, naphthenes, aromatics