Isooctane

Isooctane is a major additive to petrol, it defines the octane rating - pure isooctane has an octane rating of 100. While pure n-octane has a rating of 0.

The strict meaning of isooctane would refer to 2-methylheptane, but 2,2,4-trimethylpentane ended up taking the name historically due to its far higher importance.

Combustion of isooctane has a ∆G° of -1000 kJ / mol, yet can exist comfortably at room temperature if there are no spark sources about. It is a good example of a kinetically stable but thermodynamically unstable molecule.