Isomerisation product octane: The (unleaded) octane number of gasoline produced via isomerisation of butane-range material is approximately:

Introduction:
Isomerisation rearranges straight-chain paraffins to more branched isomers with higher octane. Although butane itself is a LPG component, its isomer (isobutane) is central to downstream high-octane blending via alkylation; exam conventions often associate butane isomerisation pathways with high-octane blend contributions.

Given Data / Assumptions:

• “Unleaded octane” refers to base blending octane without lead additives.
• Isomerisation increases octane substantially versus normal paraffins.

Concept / Approach:
Isomerisation of light paraffins produces highly branched species or enables downstream manufacture of very high octane components (e.g., isobutane to alkylate). The canonical exam value for the effective octane associated with such isomerate/derived blendstock is near 90, clearly higher than 45–70 and plausible for unleaded high-quality components.

Step-by-Step Solution:

Verification / Alternative check:
Refinery training material often quotes C5/C6 isomerate in the 80s–90s RON; alkylate from isobutane routes blends well above 90 RON.

Why Other Options Are Wrong:

• 45/55: Too low for isomerised/paraffinic high-branched material.
• 70: Understates isomerisation benefit.
• 100: More typical of special components; not a baseline exam value for this question.

Common Pitfalls:
Assuming isomerisation alone always yields triple-digit octane; actual values depend on carbon number and downstream blending.

Final Answer:
90