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What is Octane Chemically?  

Octane is an organic molecule, with the chemical formula C 8H18. There are two forms of octane, each with a different structural formula.

Note that it is possible to rotate these 3D models by clicking and dragging on them, so that it is possible to see their structure more completely. The carbons are always at the intersection of four lines (except for carbon dioxide, in which the carbon is at the intersection of two lines), and hydrogens are at the ends of all white lines. Oxygens are represented by red lines. If you can't see the models, click on their labels.


Form 1: n-Octane (the 'n' stands for normal)




You can see that there are 8 Carbon atoms, and 2 Hydrogen atoms per Carbon atom, plus 2 Hydrogens on the end of the chain, hence C 8 H18. Another formula for this molecule could be CH3(CH2)6CH3.


Form 2: iso-Octane:




The structural formula for this molecule is more complex. On one side, there are three CH3- groups, joined by a Carbon atom, creating an ion of C(CH3)-. On the other side, there are only two CH3- groups, with one H-, all connected by a Carbon atom. The ion for this side is (CH3) 2CH-. These two sides are joined by a CH2 2+ group (Carbon can be a 4+ or a 4-). The logical combination of this information would be (CH3)3C(CH2)C(CH 3)2H. Yes, this does not follow any ordinary atomic ordering conventions, nor is it easy to read and work with; this is why C8 H18 is more commonly used.


Both of the forms of octane burn in oxygen and create CO2 and H2O. Incomplete burning is possible, causing toxins like CO to form; this will be discussed in detail in a future chapter in our textbook, and is not necesary to understand the importance of octane in gasoline.

Structures of CO2 and H2O:

Carbon Dioxide (CO2) Water (Dihydrogen Oxide; H2O)



Description Equations (factors in moles of atoms):
2C8H18 + 25O2 --> 16CO2 + 18H2 O    D-7,372kJ

(according to chapter 8 in the textbook, specifically p.'s 224-225)


The 'kJ' at the end of the equation is a concept that will be introduced in Ch. 8; it is a measure of the net enthalpy in the reaction. The equation is balanced, and energy is given off; this will all be explained in the future, and it is not all necesary for understanding this process. The kJ, or kilojoule, is the kilo- variant of the standard unit of energy (the Joule); 1kJ is, by definition, the energy required to accelerate a 2000kg mass by 1meter/second. The equation for energy is therefore Energy = 1/2 x Mass x Velocity2. Click here to see a calculator that uses this formula to calculate how much gasoline cars use in certain conditions.


Of course, gasoline is not pure octane, even though octane is the primary component of it; it is a homogeneous mixture of many different chemicals, the exact chemicals and their ratios regulated by the gasoline producing companies. The effect of octane on fuel will be discussed in more detail in the next section.



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