Read our privacy policy. Start your engines! Simon Cotton looks into one of the few chemical names known well to the public: Octane. Octane is one of the few chemical names that is part of everyday life and is recognised by the public. When you stop at a garage to fill up your car, you usually see the octane rating of the fuel displayed on the pump, 95 or 97, but what does it mean?
To understand that, you have to go back nearly a century. In the early days of the automobile, knocking or pre-ignition was a real problem. In a petrol engine, the gaseous mixture of air and hydrocarbon fuel is injected into the cylinder as the piston is on its downstroke, then gets compressed as the piston moves up.
At a particular point, a sparking plug fires a spark that ignites the mixture, and the resultant hot gases create the power that forces the piston down, driving the engine and powering the wheels. More sophisticated engines had higher compression ratios — the fuel-air mixture was compressed more before it was ignited, but this also made it more likely that the mixture would pre-ignite, causing the engine to misfire — knocking — and lose power.
In the mids an American chemist named Russell Marker , working for the Ethyl Corporation, made a systematic study of the knocking characteristics of different hydrocarbons. At best they have a zig-zag chain of carbon atoms, with attached hydrogens, though at room temperature the chains will be flexing around. Anyway, heptane caused really bad knocking. Next he chose what we call 2,2,4-trimethylpentane, which at that time was usually known as isooctane.
This had a branched chain of eight carbon atoms, but the great thing was that it did not cause any knocking. Yet when he tested ordinary octane, which had a straight chain of eight carbons, that was bad too. So scientists made up mixtures with varying amounts of heptane and isooctane in order to study their knocking characteristics. They have to be obtained from crude oil. Many millions of years ago, organic matter like plankton built up on the sea floor.
As time passed, it got buried deeper and deeper, so that eventually as the layers hardened and got turned into shale or mudstone, the organic molecules were subjected to increasing pressure and temperature. This caused the large molecules to split into a variety of smaller molecules with shorter chains.
He started with heptane, a seven-carbon molecule that was very similar to octane. Heptane caused really bad knocking, so next he tried 2,2,4-trimethylpentane, traditionally known as isooctane; it also has the formula C 8 H 18 so is an isomer of octane.
This has a branched chain containing eight carbon atoms, but does not cause any knocking. Mixtures were made with varying amounts of heptane and isooctane, so that their knocking characteristics could be studied. The higher the octane rating, the more the gas-air mixture can be compressed before it is ignited, and the more power you get when it burns. Yes, it means with the right fuel mixture, engines runs more smoothly, efficiently, and quietly.
And until the s, it was improved still further by the addition of tetraethyllead as an additional anti-knock agent. Nowadays, however, cars use 'lead-free' fuels due to issues with lead being found in the environment and the fact that it damages the catalytic converters that are fitted to modern cars see MOTM Jan It costs quite a lot of money to separate the isooctane from the other less desirable alkanes, so a mixture is used to keep costs down.
But octane fuel is used for some aircraft engines, and if you go back to the last century, high-octane fuel helped shape history. Back in , British seaplane aircraft won the Schneider trophy a speed race for airplanes for Britain three times, with the help of high-octane fuels developed by Air Commodore Rod Banks and his colleagues. The designer R. Mitchell later developed these seaplanes into the Supermarine Spitfire fighter aircraft.
Well, he became one of the few university professors not to have a PhD. Just over 10 years after his work on octane ratings, he found a way of degrading diosgenin, found in Mexican yams, into progesterone, which became known as the Marker degradation, an example of semisynthesis at work.
This ultimately led to oral contraceptives. People have won Nobel prizes for less valuable work. Several methods are due to famous chemists of the past. Herman Kolbe found that alkanes can be made by electrolysis of salts of carboxylic acids. So using sodium pentanoate results in octane and carbon dioxide at the anode — and sodium hydroxide and hydrogen at the cathode.
Explain it with Molecules. How do Drugs Work? Toggle navigation World of Molecules. Octane Molecule. Octane as a Fuel "Octane" is colloquially used as a short form of "octane rating" an index of a fuel's ability to resist engine knock at high compression ratios, which is a characteristic of octane's branched-chain isomers, especially iso-octane , particularly in the expression "high octane.
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