Chemical isomer - Energy Education

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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).^[1]

Isomers are very important for fuel choices when providing energy through combustion. Isomers of compounds burn differently, such as octane, which has 18 isomers.^[2] 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.^[2] 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.

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  Figure 1. n-Octane, a more linear isomer of octane compared to its other forms.^[3]

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  Figure 2. iso-octane, a more branched isomer of octane, used in most fuels and gives the octane rating for a fuel.^[4]

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• Hydrocarbon combustion
• Octane
• Octane rating
• Chemical
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References