Straight-chain octane ($\ce{CH3(CH2)6CH3}$) is also called n-octane. What does the n stand for, and where did the term originate?

For reference, see the PubChem entry for octane (beside "Chemical Names:")


1 Answer 1


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. For example, 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.

Isomers of octane

  • $\begingroup$ Okay I found a source for this information (under "Etymology"): en.wiktionary.org/wiki/n-octane $\endgroup$
    – Mörre
    Commented Aug 6, 2014 at 14:55
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    $\begingroup$ Amusingly, 2,2,4-trimethylpentane is colloquially called iso-octane, which totally flies in the face of the n/sec/tert/iso convention. $\endgroup$
    – Lighthart
    Commented Aug 7, 2014 at 0:14
  • $\begingroup$ @Lighthart Iso-octane would mean an isomer of octane, which 2,2,4-trimethylpentane qualifies. So why do you think it is wrong? $\endgroup$
    – LDC3
    Commented Aug 7, 2014 at 4:04
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    $\begingroup$ Iso typically refers to a 1-methylethyl motif. $\endgroup$
    – Lighthart
    Commented Aug 7, 2014 at 16:24
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    $\begingroup$ Please note that the completely correct, exhaustive, preferred (and only?) IUPAC name of ‘n-octane’ is octane. Current IUPAC Nomenclature of Organic Compounds does not even mention the ‘n-' prefix anywhere (if I did not miss something). $\endgroup$
    – mykhal
    Commented Oct 16, 2018 at 11:29

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