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For example, if I were to draw the structure of 1,2,4-trimethylcyclohexane, then would one or both of these be wrong?

enter image description here

Or are both right? I'm wondering if it matters where I start to count from 1 in these cases, if there is some convention to it or if it doesn't matter much.

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Both of those are the same molecule (flip the one on the left over its horizontal axis, and they are identical). It does matter where you start counting from, because the numbers chosen for the functional groups should be as low as possible:

Numbering of the chain. This is done by first numbering the chain in both directions (left to right and right to left), and then choosing the numbering which follows these rules, in order of precedence:

  1. Has the lowest-numbered locant (or locants) for the suffix functional group. Locants are the numbers on the carbons to which the substituent is directly attached.
  2. Has the lowest-numbered locants for multiple bonds (The locant of a multiple bond is the number of the adjacent carbon with a lower number).
  3. Has the lowest-numbered locants for double bonds.
  4. Has the lowest-numbered locants for prefixes.

From IUPAC nomenclature

So, given the choice between 1,3,4-trimethylcyclohexane and 1,2,4-trimethylcyclohexane, you would choose the latter (for both molecules).

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  • $\begingroup$ I don't quite understand. I don't know what a locant is.. Are you saying I should count from the top, so that the first diagram is 1, 3, 4 and the second one is 1, 2, 4? $\endgroup$ – Threethumb Feb 26 '14 at 18:23
  • $\begingroup$ @Threethumb They are both 1,2,4-. Flip the diagram on the left over its horizontal axis. They are the same molecule (since all of the carbons of the hexane are "created equal" -- there are no double bonds there). The locants are the numbers of the carbons. $\endgroup$ – jonsca Feb 26 '14 at 18:26
  • $\begingroup$ Ah right, so technically $1, 3, 4-$ and $1, 2 4-$ is the same thing, but $1, 2, 4$ is the preferred way of writing it because it has the lowest numbers? $\endgroup$ – Threethumb Feb 26 '14 at 18:47
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    $\begingroup$ @Threethumb Exactly. Everything should always be as low as possible. $\endgroup$ – jonsca Feb 26 '14 at 19:03

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