Which compound is used for increase of octane rating?

Question

UnlockIAS · Accepted Answer

Correct — C, Tetraethyllead. Tetraethyllead, Pb(C₂H₅)₄, is the classical anti-knock additive of the petrol age. It was first synthesised by Carl Jacob Löwig in 1853, but it sat unused for seventy years until Thomas Midgley Jr., working in General Motors' research laboratory, found on 9 December 1921 that a tiny dose of it stopped engines knocking. Octane rating measures a fuel's ability to withstand compression without auto-igniting — the higher the number, the more the fuel can be squeezed before it detonates on its own instead of waiting for the spark. The scale is defined against two reference hydrocarbons: 2,2,4-trimethylpentane, called iso-octane, is fixed at 100 and n-heptane at 0, so a petrol rated 91 knocks like a blend of 91 parts iso-octane to 9 parts n-heptane. Tetraethyllead raises that number because it is thermally fragile: at combustion temperatures its lead–carbon bonds break and the finely divided lead and lead oxide formed mop up the peroxide and hydroperoxide radicals that drive the pre-flame chain reaction. Kill the chain carriers and the end-gas cannot self-ignite ahead of the flame front, so the charge burns smoothly from the spark. The commercial appeal was that a fraction of a gram per litre lifted the octane number by several points — far cheaper than rebuilding the fuel itself — which is why leaded petrol dominated for over fifty years and let compression ratios, and with them power and efficiency, rise. It was also, in hindsight, one of the most damaging products of the twentieth century: lead is a cumulative neurotoxin, especially of the developing brain, and it poisons the platinum-group catalysts in a catalytic converter, so lead and modern emission control cannot coexist. Phase-out began in the 1970s; India banned leaded petrol in March 2000, and UNEP declared the worldwide end of leaded petrol for cars on 30 August 2021 after Algeria, the last producer, stopped sales in July 2021. Note therefore that the question's present tense preserves a textbook association, not current Indian practice.

Why the other options are wrong — (a) The cleverest decoy on the paper, because it is chemically half right. Branched alkanes genuinely resist knock far better than straight chains — that is the whole point of refinery isomerisation — and trimethylhexanes are real C₉ isomers. But no trimethylhexane is a fuel additive, and the octane-100 reference compound is 2,2,4-trimethylPENTANE, a C₈. One syllable separates the option from the fact the candidate half-remembers.  (b) No compound of this name is used as a fuel additive, and the name does not parse in standard nomenclature — the simplest methyl ether is dimethyl ether, (CH₃)₂O. Its pull is that it sits one syllable from two real things: tetramethyllead, which genuinely was used as an anti-knock agent alongside tetraethyllead, and MTBE (methyl tert-butyl ether), the ether that helped replace lead after the phase-out.  (d) A true fact wrapped in a false name. Toluene itself is a standard octane booster — an aromatic with an octane number well above 100, blended into racing fuels and supplied naturally in refinery reformate, which is one reason reforming replaced lead. But the booster is plain toluene; 'triethyl toluene' is not a fuel chemical in commercial use, and adding fictitious alkyl groups to a real compound is a recurring BPSC construction.

Concept: Knocking is what a petrol engine does when the unburnt end-gas ahead of the advancing flame front reaches its own auto-ignition temperature and explodes on its own. Instead of one smooth pressure rise there are colliding pressure waves, heard as a metallic pinging, and sustained knock burns pistons and wrecks bearings. Because a higher compression ratio squeezes the charge harder, it extracts more work from each unit of fuel — but it also makes knock more likely, so knock resistance is the ceiling on engine efficiency. Octane rating is the number that measures that resistance. It is not a measure of energy content or of power: high-octane petrol does not carry more joules, it simply refuses to detonate under greater compression, which is why putting premium fuel in a low-compression engine buys nothing. Two laboratory numbers are quoted. The Research Octane Number is measured in a variable-compression test engine at 600 rpm; the Motor Octane Number uses the same engine at 900 rpm with a preheated charge and variable ignition timing, a harsher test, and typically comes out 8 to 12 numbers lower. India, Europe and most of the world advertise RON; the United States, Canada and Mexico advertise the Anti-Knock Index, (R+M)/2, which is 4 to 6 numbers lower for the identical fuel — the reason American '87 regular' and Indian '91 petrol' are much closer than they look.

State PCS 2025 Q137 — Science & Technology PYQ with Answer

State PCS 2025 Q137 — Science & Technology PYQ with Answer