I am having confusion on when to use iso- or sec- while naming the alkyl halides?


2 Answers 2


Preferably, you don’t use “iso” and “sec” at all when naming alkyl halides for two reasons:

  • According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the prefixes “isobutyl” and “sec-butyl” etc. are no longer recommended (“isopropyl” is still retained, but only for general nomenclature).
  • Preferred IUPAC names of alkyl halides are formed using substitutive nomenclature, i.e. using the prefixes “bromo”, “chloro”, “fluoro”, and “iodo” (e.g. iodomethane instead of methyl iodide).

If you cannot avoid using functional class nomenclature with traditional prefixes,

  • the “sec”-compounds are the simplest isomers that contain a secondary functional group, i.e. the halogen is bonded to the second carbon of the chain, e.g.
    sec-butyl chloride” is 2-chlorobutane,
    sec-pentyl chloride” is 2-chloropentane, etc.;

  • the “iso”-compounds are the simplest isomers that are branched at the end of the chain, i.e. all carbons except one form a continuous chain, e.g.
    “isobutyl chloride” is 1-chloro-2-methylpropane,
    “isopentyl chloride” is 1-chloro-3-methylbutane, etc.


iso- is used when the all carbons form a straight chain except one methyl at "2nd Carbon", like 2-methyl-R (where R refers to straight chain alkane)

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And, sec- is used when any functional group is linked to a secondary carbon.

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  • $\begingroup$ Then why is the name for iso-octane 2,2,4 trimethyl pentane rather than 2 methylheptane? $\endgroup$
    – H. Khan
    Commented Jun 17, 2018 at 3:04
  • $\begingroup$ look up isoctane it will show up as 2,2,4 trimethylheptane... is that incorrect? $\endgroup$
    – H. Khan
    Commented Jun 17, 2018 at 3:31
  • 1
    $\begingroup$ I am sorry, actually, 2-methyl-heptane should be the isooctane. But, since 2,2,4-trimethylpentane is the most important isomer (by application) of octane. Therefore, traditionally it is known as isooctane. Note: All other iso- compounds follows the general rule of 2-methyl-R. After all, exception is a common term in Chemistry ;) $\endgroup$ Commented Jun 17, 2018 at 3:49

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