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!
Answer
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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