Naming cycloalkenes #1.21

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Isha Bagga 3J
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Joined: Fri Sep 26, 2014 2:02 pm

Naming cycloalkenes #1.21

Postby Isha Bagga 3J » Sun Mar 08, 2015 10:48 pm

For #21 in chapter 1, why is part a) 5-Methylcyclopenta-1,3-diene? If you start numbering at the bottom left carbon, couldn't it be named 3-Methylcyclopenta-1,4-diene, which would result in smaller values? I don't think I understand the priority in this case.

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Re: Naming cycloalkenes #1.21

Postby Chem_Mod » Sun Mar 08, 2015 11:16 pm

Hi Isha, having lower numbers on double- or triple-bonds takes priority over having lower numbers on prefixed substituents.

To number a parent carbon chain, follow these rules in this order of precedence:
1. Lowest numbers for the suffix group (-ol, -one, -al, -amine, -oic acid etc)
2. Lowest numbers for multiple bonds (tiebreak with double bonds if needed)
3. Lowest numbers for prefixed groups (methyl-, ethyl-, chloro-, bromo- etc)

In this problem, there are no suffix groups so we skip rule 1. There are multiple bonds, so we follow rule 2 and number them as (1,3). Since the starting-point and the direction of numbering has now already been decided, we do not go on to rule 3, and the prefix "methyl-" must settle for whatever number it gets.

Justin Le 2I
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Re: Naming cycloalkenes #1.21

Postby Justin Le 2I » Mon Mar 09, 2015 9:33 pm

I think the above post would be a great addition to the course reader.

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