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The octane number of fuels are based on a set of primary reference fuels, which are mixtures of iso-octane and n-heptane. Now iso-octane being branched alkane has very less tendency to react and given ON=100 and n-heptane reacts easily and given ON=0. However n-octane that is the corresponding linear isomer of iso-octane has an ON<0. Now why does branching in a alkanes reduce its reactivity?

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The reaction desired in an automobile piston is a rapid burning, not an explosion (very very rapid reaction). If the air + gasoline vapor burns much faster than the camshaft is turning, the explosion causes a ping, or high pressure rise in a smaller volume (because the piston has not moved far enough). This can damage the piston, or at least add an unusual stress.

Burning is a radical reaction and the radicals produced from linear alkanes are quite reactive (i.e., fast reactors). Branched alkanes might lose a hydrogen, even break apart, but the free electron is more stable, and the reaction rate is slower.

There are compounds with a higher octane number than iso-octane. E.g., toluene has an octane number ~120, and was highly sought after for airplane fuel during WW II. The benzylic radical which is the likely first product of a radical reaction is quite stable (but not inert).

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