Isopropyl chloride should have been called secondary propyl chloride, right? The first iso must be isobutyl chloride, methinks.

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    $\begingroup$ Trivial names can't be wrong by definition. $\endgroup$ Commented Jan 11, 2019 at 7:21
  • $\begingroup$ Oh, it must be a matter of common acceptance, right? $\endgroup$ Commented Jan 11, 2019 at 14:24
  • $\begingroup$ Yeah, that's right. $\endgroup$ Commented Jan 11, 2019 at 14:28

1 Answer 1


In this case sec-propylchloride and isopropylchloride coincide. The prefix “iso” has several interpretations, but generally means that a terminal carbon (methyl group) is moved down the chain by one, which for propylchloride means that a methyl group is attached to what was carbon 1 (the one with the chloride).

So you could call it either sec-propylchloride or isopropyl chloride, neither would be wrong, but it is desired to standardise names when possible, and isopropyl was chosen. AFAIK the choice of iso- over sec- was arbitrary, but as Neretin says, trivial names can’t be wrong by definition. I suspect, although I don’t know, that iso was chosen because it was known that the molecule was similar to propylchloride or had the same formula (thus an isomer) before the actual structure was known.

One should be aware that “iso” is sometimes also used in a looser sense to refer to any member of the class of isomers of a compound.

  • $\begingroup$ I follow your point; yet i feel if we standardise the prefixes 'iso' and 'neo', "isoalkane" can be generalized as the isomer having one additional methyl group on the second carbon and "neoalkane" having two methyl groups on the second carbon; also substituted alkanes like "isoalkyl halide" & neoalkyl halide"have halogen at the distant end of the parent chain. To make this point more convincing, first 'neo' alkane is neopentane and first neoalkyl chloride is neopentyl chloride; first iso alkane is isobutane and so first isoalkylchloride must be isobutylchloride. $\endgroup$ Commented Jan 11, 2019 at 14:22
  • $\begingroup$ As a teacher, i used to explain students that way and ask them to remember isopropyl to be an exception; then they are quite convinced. $\endgroup$ Commented Jan 11, 2019 at 14:33
  • $\begingroup$ Isopropanol and isopropyl chloride are not exceptions if you make the slight adjustment that “iso” compounds have the methyl at the far end moved down by one to the second-to-last carbon, rather than the second carbon. $\endgroup$ Commented Jan 13, 2019 at 11:46
  • $\begingroup$ You can think of “sec-“ and “tert-“ as adjusting the functionalised (business end) of the molecule, and “iso-“, “neo-“ as adjusting the other end. A good example is sec-isoamyl borane (siamyl)₂BH, the bulky reducing agent where “amyl” means 5-carbons. “Sec” moves the boron up from C1 to C2, and “iso” moves the last carbon C5 from being attached to C4 down to C3. $\endgroup$ Commented Jan 13, 2019 at 11:52

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