For hydrocarbons with the same carbon number (i.e., same molecular size range), which structural class exhibits the minimum smoking tendency during combustion?

Introduction:
Smoking tendency reflects a fuel’s propensity to form soot and visible smoke during combustion. For hydrocarbon families of the same carbon number, structure (branching, ring formation, aromaticity) strongly influences smoke formation behaviors in burners or engine flames.

Given Data / Assumptions:

• Comparison is at a fixed carbon number (size) to isolate structural effects.
• Candidate families: normal paraffins, iso-paraffins, naphthenes, aromatics, olefins.
• We seek the class with the lowest (minimum) smoking tendency.

Concept / Approach:
Empirically, smoking tendency (lowest to highest) generally follows:
iso-paraffins < normal paraffins < naphthenes < aromaticsBranching promotes cleaner burning, while ring structures and aromaticity increase soot precursors. Thus, iso-paraffins produce the least smoke at the same carbon number.

Step-by-Step Solution:
1) Hold carbon number constant.2) Rank families by soot propensity based on structure.3) Identify the minimum: iso-paraffins.

Verification / Alternative check:
Smoke point testing and burner studies consistently show higher smoke for aromatics and lower for branched alkanes; refiners use isomerization to improve kerosene smoke points.

Why Other Options Are Wrong:
Paraffins (normal): Higher smoke than iso-paraffins.Naphthenes: Ring structures increase sooting vs paraffins.Aromatics: Highest smoking tendency among common families.Olefins: Often smoke more than paraffins due to unsaturation.

Common Pitfalls:
Assuming heavier = smokier without considering structural effects; aromatic content is a dominant driver of smoke formation even at the same carbon number.

Final Answer:
Iso-paraffins (branched alkanes)