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Knowing the meaning of ''iso-'' prefix, i could not understand why it is used in some cases. For instance

n-propyl and isopropyl

are named according to the carbon atom which has a lack of hydrogen. The same goes for

isobutyl

where branches define the common name as well. In the end my question is: What is the logic of using iso- prefix (especially in those cases), and how can i know when to use it?

p.s- if you know the answer to my question, it is mostly appreciated to state your source of answer!

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  • $\begingroup$ ISO is used when all carbons except last form a straight chain. $\endgroup$
    – JM97
    Commented Dec 7, 2016 at 1:34
  • $\begingroup$ It looks like verifying my question, but could you expand this a bit? For instance what do you mean exactly with 'straight chain'?. Besides, is there any exceptions for this king of application? $\endgroup$
    – alpy
    Commented Dec 7, 2016 at 18:07
  • $\begingroup$ You might have gotten the meaning wrong, there is no lack of hydrogen. More info in my answer. $\endgroup$
    – mykhal
    Commented Apr 23, 2017 at 8:04

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The IUPAC Organic Chemistry Nomenclature does not explicitly define the "iso-" prefix meaning, as it is not used in the systematic naming. However, it is mentioned on several places when listing acceptable semi-systematic names. In the section A-2 (1979 Blue book, some also in 1993 Recommendations, section R-9.1, table 19a) those are (not mentioning neo- compounds)

  • isobutane (CH3)2CH−CH3
  • isopentane (CH3)2CH−CH2−CH3
  • isohexane (CH3)2CH−CH2−CH2−CH3

And in the section A-2.25 (some also in R-9.1, table 19b), where it lists acceptable "isoalkyl group" names:

  • isopropyl (CH3)2CH−
  • isobutyl (CH3)2CH−CH2−
  • isopentyl (CH3)2CH−CH2−CH2−
  • isohexyl (CH3)2CH−CH2−CH2−CH2−

Some related isomeric alkyls mentioned in this section are

  • sec-butyl CH3−CH2−CH(CH3)−
  • tert-butyl CH3−CH(CH3)2−
  • tert-pentyl CH3−CH2−CH(CH3)2−

There are also some other, unsaturated "isoalkyl" group names mentioned:

  • isopropenyl CH2=C(CH3)−
  • isopropylidene (CH3)2C=

From the above we can induce that "iso-" prefix, when related to alkanes or alkyl groups, it means the simplest (n-)alkane/alkyl isomer branched at the end of the carbon chain.

Isopropyl happens to be secondary (sec) alkyl (there's lack of carbon atoms to create primary alkyl isomeric to the n-propyl), all higher isoalkyls are primary (no branching at the alkyl "attachment").

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