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Straight-chain octane is also called n-octane.

https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=356 ("Also known as:")

I don't see any purpose for the "n"?

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up vote 8 down vote accepted

In this case, n means "normal", i.e., straight chain. This is somewhat of a holdover from the olden days, but in the case of octane in particular, it makes sense to specify n-octane if that is what you have. The "octane" used as a standard gasoline engine fuel against which gasolines are rated (by definition, 100 octane) is actually not n-octane, but rather 2,2,4-trimethylpentane--a branched isomer of octane--in contrast to the straight chain version.

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Okay I found a source for this information (under "Etymology"): en.wiktionary.org/wiki/n-octane –  Mörre Aug 6 at 14:55
    
Amusingly, 2,2,4-trimethylpentane is colloquially called iso-octane, which totally flies in the face of the n/sec/tert/iso convention. –  Lighthart Aug 7 at 0:14
    
@Lighthart Iso-octane would mean an isomer of octane, which 2,2,4-trimethylpentane qualifies. So why do you think it is wrong? –  LDC3 Aug 7 at 4:04
    
Iso typically refers to a 1-methylethyl motif. –  Lighthart Aug 7 at 16:24

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