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While learning nomenclature of organic compounds I found the common names to be a bit difficult to learn than the IUPAC ones. One of these common name stuff is the iso-,neo-, sec-, etc. So I searched for the ways to differentiate between these on the YouTube and found this video (note that the anchor of the video uses hindi for communication so it might not be intelligible to most people here). To summarize what the anchor said:

Prefix-iso: is used when all carbon atoms except one form a continuous chain.

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They have a structure like this: General Structure for iso- compounds and an example

Now in the same video the anchor says that iso-octane is an exception to this rule, as it has the following shape: Iso-octane structure

Before this came in the video I formed the view that these are just the ways of naming compounds with similar structure but now I think that that isn't the case. So:

  • Why iso-octane has shape different than the other iso-s?

The anchor says that the structure of iso-octane was determined experimentally. This implies that there is some way to identify these compounds without knowing their structures beforehand so:

  • How do we know that whether some given compound is iso-, neo-, etc?

  • What might be the most general guidance for using iso-, neo-, sec- and tert- prefixes?

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    $\begingroup$ Do not try to learn chemistry from youtube videos of questionable background. $\endgroup$
    – Karl
    Commented Feb 16, 2020 at 14:17
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    $\begingroup$ "secondary" and "tertiary" (+"primary", "quarternary") are clearly defined (by IUPAC) structural descriptors. "iso" and "neo" are not. $\endgroup$
    – Karl
    Commented Feb 16, 2020 at 14:24
  • $\begingroup$ @Karl thanks for the suggestion. Actually this was the first time I watched a video from that channel and thought that it might be a legitimate one given that there are 100k+ subscribers. Though you are right and I do try to avoid such things but for now I wasn't able to find any other resources online so I had to reside for that video. $\endgroup$
    – user88957
    Commented Feb 16, 2020 at 15:02
  • $\begingroup$ This would be one that is reasonably reputable: en.wikipedia.org/wiki/Alkane#Nomenclature $\endgroup$
    – Karl
    Commented Feb 16, 2020 at 16:00
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    $\begingroup$ Looks like a slang term that got commonly recognized. It's definitely unpleasant to the eye, much like $`` \mathrm{C} 0{}_{2} ".$ $\endgroup$
    – Nat
    Commented Feb 17, 2020 at 8:32

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The usage of non-standard prefixes such as "iso" has never been defined with hard, unambiguous rules such as those in the IUPAC recommendations. Instead, their usage has often more been "by convention". In Organic Chemistry (1st ed.) by Clayden et al., the authors write (p 315):

Strictly speaking, if the standard meaning of ‘iso’ is followed, the name isooctane should be reserved for the isomer 2-methylheptane. However, 2,2,4-trimethylpentane is by far the most important isomer of octane and so, historically, it has been assigned this name.

(This phrase is also found at Wikipedia, where it is copied wholesale without any indication that it's a direct quote. Please don't do this in your work. Even if you provide a citation, you should either paraphrase the information, or clearly indicate that it's a quote, e.g. by enclosing it in quotation marks.)

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  • $\begingroup$ If I remember right from ~25 years ago quoting my chemistry teacher "Iso-" [equal] indicates a mixture of carb-molecules where the longest chain is the name-giver .. as in iso-octane being an 8-carbon chain. .. Other possible molecules use side-chains of 1 or 2 carbons length. $\endgroup$
    – eagle275
    Commented Feb 17, 2020 at 10:10

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