Cycloalkane Numbering Confusion: Which is Right?

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Discussion Overview

The discussion revolves around the nomenclature of cycloalkanes, specifically addressing the numbering of substituents in two different examples: 1-propyl-2-methylcyclopentane and 1-ethyl-2-propylcyclohexane. Participants explore the rules of naming and the implications of different numbering conventions.

Discussion Character

  • Homework-related
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant notes the discrepancy in naming conventions between two sources and questions whether both names can be correct.
  • Another participant suggests that visualizing the molecules could clarify the differences, emphasizing that cyclopentane and cyclohexane have distinct backbones.
  • It is pointed out that the two names refer to different molecules, which may affect how they are numbered for nomenclature.
  • A participant mentions learning that substituents should be numbered by alphabetical order or complexity, suggesting that the rules may have changed or become more flexible.
  • One participant argues that if two substituents are adjacent in a cycloalkane, the numbering may not affect the molecular structure, allowing for different valid numbering schemes.
  • Another participant expresses that as long as the naming is unambiguous, fewer rules might be preferable.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to numbering substituents in cycloalkanes, with no consensus reached on whether both naming conventions can be considered correct.

Contextual Notes

Participants highlight the importance of molecular structure in determining nomenclature, but there are unresolved aspects regarding the application of naming rules and potential changes in those rules over time.

Who May Find This Useful

This discussion may be useful for students and practitioners in organic chemistry, particularly those interested in nomenclature and structural representation of cycloalkanes.

rajeshmarndi
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Homework Statement


In one internet site it is 1-propyl-2-methylcyclopentane and in other it is 1-ethyl-2-propylcyclohexane

Homework Equations

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?
 
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Wouldn't hurt to see the molecule in question (especially taking into account cyclopentane and cyclohexane are two completely different backbones).
 
Borek said:
Wouldn't hurt to see the molecule in question (especially taking into account cyclopentane and cyclohexane are two completely different backbones).
cycloalkane.jpg
 
These are two different molecules.
 
Borek said:
These are two different molecules.

They both are cycloalkanes, I wanted to know how are they numbered for nomenclature.
 
I learned alphabetical order or by complexity. Your first example uses neither. Maybe the rules have been liberalized.
 
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.
 
As long as it's unambiguous, I guess it's OK...fewer rules to have to remember.
 

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