Tetraethyl lead (TEL) has historically been used in the petroleum industry mainly as which of the following?

Introduction / Context:
This question comes from applied chemistry and environmental science. It asks about the main historical use of tetraethyl lead (TEL), a once common but now largely banned compound in fuels. Knowing its purpose and hazards helps you understand why many countries phased out leaded petrol to protect public health and the environment.

Given Data / Assumptions:

• The compound is tetraethyl lead, often abbreviated as TEL.
• We consider its primary historical industrial application.
• Options include uses as a fire extinguisher, gasoline additive, mosquito repellent, pain killer and food preservative.
• We assume modern awareness of TEL environmental and health impacts.

Concept / Approach:
Tetraethyl lead is an organolead compound that was added to gasoline to improve its octane rating, reducing engine knocking in internal combustion engines. It acts as an antiknock agent, allowing engines to run more smoothly and efficiently on lower cost fuel blends. However, combustion of leaded petrol released toxic lead compounds into the atmosphere, leading to serious health and environmental concerns. As a result, the use of TEL in motor fuels has been phased out in most parts of the world in favour of unleaded gasoline and alternative additives.

Step-by-Step Solution:
1) Recognise tetraethyl lead as a compound historically associated with motor fuels and the term “leaded petrol.”2) Its primary function was to act as an antiknock additive, raising the octane number of gasoline and preventing premature ignition of the fuel air mixture.3) TEL was never used as a pain killer or in pharmaceuticals because of its toxicity.4) It was not used as a fire extinguisher chemical or as a household mosquito repellent since lead compounds are hazardous.5) TEL definitely was not a food preservative; lead contamination in food is dangerous and tightly regulated.6) Therefore, the correct description of its main historical use is as a gasoline additive to increase octane rating and reduce knocking.

Verification / Alternative check:
Historical accounts of petrol formulations describe “ethyl gasoline” or “leaded petrol” that contained small amounts of tetraethyl lead. These fuels were marketed as premium products that allowed high compression engines to run without knocking. Over time, evidence accumulated linking environmental lead levels and health issues, especially in children, to emissions from leaded petrol. This prompted a worldwide movement to phase out TEL as a fuel additive and switch to unleaded fuel, confirming that its primary use was indeed as a gasoline additive, not any of the other roles listed in the options.

Why Other Options Are Wrong:
Fire extinguisher chemical: Fire extinguishers typically use substances like carbon dioxide, dry chemical powders or halons, not organolead compounds.
Mosquito repellent: TEL is highly toxic and would not be used in domestic insect control products.
Pain killer: Pharmacological pain relievers are carefully tested; toxic metal based compounds like TEL are not used as analgesics.
Food preservative: Lead compounds are poisonous; they are strictly excluded from food products.

Common Pitfalls:
Most students recognise tetraethyl lead from discussions of leaded petrol, but some may not recall the term “octane rating” or “antiknock agent.” A useful memory aid is that TEL allowed engines to operate with higher compression without knocking, which improved performance but at the cost of serious lead pollution. Remembering the link between TEL, leaded fuel, and environmental lead contamination helps you correctly identify its role as a gasoline additive in exam questions.

Final Answer:
Tetraethyl lead was historically used mainly as a gasoline additive to increase octane rating and reduce engine knocking.