Chemical isomers are molecules of the same chemical composition, but different molecular structure. In other words, two isomers will have the same number and type of atoms, but a different arrangement in space. Although these molecules are of the same composition, this different arrangement can result in a difference of physical and chemical properties (such as boiling point, melting point, etc).
Isomers are very important for fuel choices when providing energy through combustion. Isomers of compounds burn differently, such as octane, which has 18 isomers. For most applications the goal when burning a fuel is to have a stable burn, and certain isomers accomplish this better. For octane, the more linear isomers as visible in Figure 1 burn much quicker than isomers that are "branched" like in Figure 2. The branched structures allow the fuel to burn slowly and evenly, therefore they are of more desire. Octane rating, which is the number visible on gas pumps, is a measure of how much iso-octane (Figure 2) is in a fuel mixture. The more iso-octane present in the mixture, the better the burn.
Figure 1. n-Octane, a more linear isomer of octane compared to its other forms.
Figure 2. iso-octane, a more branched isomer of octane, used in most fuels and gives the octane rating for a fuel.
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