I got a problem with this:

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What if we stuck with the IUPAC name and called the isopropyl group a 1-methylethyl group? That would definitely reverse the alphabetical priorities; because the substituents here on the hydrocarbon are on equivalent positions, we must give the lower number to the substituent that comes first in the alphabet.

But that's the rub! (No isopropyl alcohol pun!)

Isopropyl - "I" comes before "M" as in methyl.

But if we name the isopropyl group systematically,

Methylethyl comes after just methyl in the alphabet.

So could the name of this also be:



No, 2,2-dimethyl-7-(1-methylethyl)bicyclo[3.2.0]heptane is not an acceptable name. Like the rule you quoted says, start numbering at a bridgehead carbon and first pass through the largest ring (in this case the 5-membered ring) and number that ring so that a substituent will have the lowest number. So this molecule would be named 2-isopropyl-7,7-dimethyl-bicyclo[3.2.0]heptane. Usage of "1-methylethyl" is not acceptable. If you weren't going to use the term "isopropyl", you could call the fragment "2-propyl". So you could write 2-(2-propyl)-7,7-dimethyl-bicyclo[3.2.0]heptane, but the term "isopropyl" is much more common usage.

Edit: After reading Greg E's comment, I looked around the internet and found that "1-methylethyl" is both correct and the official IUPAC name. Here is a paraphrasing of the relevant IUPAC rule...

These groups (those attached off the main chain) are always numbered so that the point of attachment to the rest of the molecule is C1, with the longest continuous chain beginning from the point of attachment taken as the parent.

Then all sites I visited would add something like this...

"For historical reasons, some of the simpler branched-chain alkyl groups also have nonsystematic (or common, or trivial) names."

Most of these sites seemed to (strongly) favor the trivial name. As one site put it, "Common names are used because they are easier to say, shorter, and save paper and ink in scientific publications when they have to be used repeatedly in manuscripts."

  • $\begingroup$ Are you sure the fragment is 2-propyl? It looks like a 1-methylethyl to me. $\endgroup$ – Dissenter May 16 '14 at 20:13
  • $\begingroup$ I think it is a usage thing. Compare the possibilities: 2-isopropyl-7,7- or 2-(2-propyl)-7,7- or 2-(2-methylethyl)-7,7- . The third item is not "wrong", it is just the longest, most cumbersome and least common usage of the 3. $\endgroup$ – ron May 16 '14 at 20:22
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    $\begingroup$ @JannisAndreska, I'm fairly certain that names like "1-methylethyl" are acceptable. Q.v., the section from the 1979 IUPAC "Blue Book" on acyclic branched alkanes, wherein names such as, e.g., "1-methylbutyl" appear. $\endgroup$ – Greg E. May 16 '14 at 22:41
  • $\begingroup$ @GregE.thank you for that comment; I've too seen 1-methyl ethyl used even by textbooks. So that changes the premise; 1-methylethyl should be correct. $\endgroup$ – Dissenter May 16 '14 at 23:35
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    $\begingroup$ According to current IUPAC recommendations, the preffered name is 7,7-dimethyl-2-(propan-2-yl)bicyclo[3.2.0]heptane. $\endgroup$ – user7951 Aug 16 '17 at 16:44

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