Gasoline lead response: Among the listed gasoline streams, which typically shows the highest susceptibility to tetraethyl lead (TEL) octane improvement?

Introduction:
“Lead susceptibility” refers to the increase in octane number achievable by adding an organo-lead antiknock such as TEL. Different gasoline streams respond differently depending on composition (paraffins, olefins, aromatics).

Given Data / Assumptions:

• Streams considered: straight-run, reformate, FCC (catalytically cracked), polymer gasoline, natural gasoline.
• Legacy context where TEL usage was allowed (historical/technical).

Concept / Approach:
Lead response tends to be higher for olefinic and isoparaffinic streams and lower for highly aromatic reformates that already have high intrinsic octane.

Step-by-Step Solution:

Verification / Alternative check:
Textbook lead response curves show FCC gasoline with the steepest octane gain per millilitre of TEL.

Why Other Options Are Wrong:

• Straight-run: Lower olefin content, moderate response.
• Reformate: Already high octane from aromatics; limited incremental response.
• Polymer gasoline: Less response than FCC on average.
• Natural gasoline: Often low octane and variable response.

Common Pitfalls:
Confusing “highest octane” with “highest lead susceptibility” — they are not the same.

Final Answer:
Catalytically cracked gasoline (FCC gasoline)