IUPAC naming - choosing the main branch doubts

  • #1
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Main Question or Discussion Point

What are the rules for choosing main branch ?

Wikipedia states -

"Identification of the parent hydrocarbon chain. This chain must obey the following rules, in order of precedence:

1 It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.
2 It should have the maximum number of multiple bonds
3 It should have the maximum number of single bonds.
4 It should have the maximum length."

I don't understand how 3 and 4 differ. Which single bonds are we counting ? only carbon-carbon single bonds in the main chain right?

Also , i heard there is a rule that says if you've two equal length chains , then choose the more substituted one. Why is it not given here?

And now now imagine you've two choices for a chain , like this -
GDk2s.jpg

The chemdoodle program (http://web.chemdoodle.com/demos/iupac-naming) gives the name as 2-(1-Hydroxybutyl)-3-methylhexanoic acid . I simply do not understand why the methyl substituted chain is chosen instead of hydroxy substituted one and how that fits with iupac rules given in wikipedia.

Please help.
 

Answers and Replies

  • #2
DrClaude
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"Identification of the parent hydrocarbon chain. This chain must obey the following rules, in order of precedence:

1 It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.
2 It should have the maximum number of multiple bonds
3 It should have the maximum number of single bonds.
4 It should have the maximum length."

I don't understand how 3 and 4 differ. Which single bonds are we counting ? only carbon-carbon single bonds in the main chain right?
As you quoted, this is for the parent hydrocarbon chain, so the bonds considered are C-C. I guess you can have two chains with the same number of C-C bonds, but with one longer than the order because of the presence of heteroatoms.

Also , i heard there is a rule that says if you've two equal length chains , then choose the more substituted one. Why is it not given here?
It is: "It should have the maximum number of substituents."

I simply do not understand why the methyl substituted chain is chosen instead of hydroxy substituted one and how that fits with iupac rules given in wikipedia.
The rule #1 mentions the "suffix functional group." If you look up the order of precedence of groups, you will find that the carboxylic acid has precedence over the alcohol. Therefore, this is going to be an "-oic acid."
 
  • #3
Bystander
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why the methyl substituted chain is chosen instead of hydroxy substituted one
The IUPAC alphabet (odd at first glance, but there is some method to its madness) assigns precedence in alphabetical order, and (methyl) alkyl precedes hydroxy alphabetically. Halo will precede hydroxy whether it be bromo-, chloro-, fluoro-, or iodo-.
 

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