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Cycloalkane numbering

  1. Aug 4, 2015 #1
    1. The problem statement, all variables and given/known data
    In one internet site it is 1-propyl-2-methylcyclopentane and in other it is 1-ethyl-2-propylcyclohexane

    2. Relevant equations

    3. The attempt at a solution
    I know when two different substituents could receive the same number, number them by alphabetical order. So I can understand 1-ethyl-2-propylcyclohexane but it contradict with 1-propyl-2-methylcyclopentane. Are they both right?
  2. jcsd
  3. Aug 4, 2015 #2


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    Staff: Mentor

    Wouldn't hurt to see the molecule in question (especially taking into account cyclopentane and cyclohexane are two completely different backbones).
  4. Aug 4, 2015 #3
  5. Aug 4, 2015 #4


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    Staff: Mentor

    These are two different molecules.
  6. Aug 4, 2015 #5
    They both are cycloalkanes, I wanted to know how are they numbered for nomenclature.
  7. Aug 4, 2015 #6
    I learned alphabetical order or by complexity. Your first example uses neither. Maybe the rules have been liberalized.
  8. Aug 5, 2015 #7
    If there are two substituent consecutively in a cycloalkane, I think it shouldn't matter which ever substituent is numbered 1, because it wouldn't change the molecular structure.

    That is,
    1-propyl-2-methylcyclopentane or 1-methyl-2-propylcyclopentane,
    1-ethyl-2-propylcyclohexane or 1-propyl-2-ethylcyclohexane

    Also if there were more than two substituent, they can be numbered , from left or from right.
  9. Aug 5, 2015 #8
    As long as it's unambiguous, I guess it's OK....fewer rules to have to remember.
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