[liliesx3]

Which of the following hydrocarbons would have the highest octane rating in a fuel?
A.iso-octane, (CH3)3CCH2CH(CH3)2
B.n-heptane
C.cumene, C6H5CH(CH3)2
D.n-hexane
E.1-pentene

A = 100
B = 0
C = 109
D = 25
E = 91

I know the answer is C but I was just wondering you figure out the octane number. I just looked up the values but I wanted to know the understanding behind it. I was told that I need to count carbons? How does counting carbons affect the octane number.

Thank you.

[Borek]

I can be wrong, but as far as I can tell octane number is something that is determined experimentally, I am not aware of any simple rules that can be used. Could be they exist, but if so they probably can be used to compare two fuels with assumption that everything else is identical - say n-heptane and n-octane, or more branched octane with less branched octane. They probably fail miserably when comparing two completely different compounds (like say n-hexane and branched octane). That's just an (more or less) educated guess, but that's what I would expect.

[Schrödinger]

From what I've heard, branching increases octane number(of course if the molecular masses are comparable, like Borek rightly said. No use comparing apples and oranges )

Aromatic hydocarbons have excellent anti-knock properties,and hence high octane numbers. So a good guess will be cumene.

[liliesx3]

thank you. this helps greatly.

[AWK]

http://en.wikipedia.org/wiki/Octane_number