Iupac Nomenclature

AI Thread Summary
The discussion centers on the structural differences between 1-Bromo-2-(bromomethyl)butane and 2-Ethyl-1,3-propanediol, highlighting their respective functional groups: bromine and hydroxyl. The primary focus is on the naming conventions in IUPAC nomenclature, emphasizing that the longest carbon chain is not the sole determinant for naming. Instead, the priority of functional groups plays a crucial role. Hydroxyl (-OH) groups are prioritized over bromine (-Br) when determining the backbone of the molecule. This prioritization leads to the selection of a butane base chain for the compound with bromine substituents and a propane base chain for the compound with hydroxyl substituents. The discussion also references ChemSketch for consistency in naming and includes a resource on functional group priority in nomenclature.
neilparker62
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TL;DR Summary
Application of rules for determining the longest carbon chain
Consider the compounds

1-Bromo-2-(bromomethyl)butane​

1737271362747.png


and

2-Ethyl-1,3-propanediol​

1737271510114.png

1737275049359.png


As can be seen they are structurally similar compounds. The first has bromine substituents and the second has hydroxyl substituents.

My question is why does one have a butane base chain whilst the other has a propane base chain ?
 

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I am not entirely sure, but my understanding is that it is not strictly about the longest chain, even if in most cases that's how it works. First you need to prioritize the functional groups present and then select the longest continuous carbon chain to give the highest ranking group the lowest possible number. -OH is high, -Br is quite low on the list, even behind the carbon itself, so probably in the first case Br is ignored, and in the second diol is chosen as the backbone of the molecule.

Just checked that these names are consistent with what ChemSketch names these molecules.
 
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Borek said:
I am not entirely sure, but my understanding is that it is not strictly about the longest chain, even if in most cases that's how it works. First you need to prioritize the functional groups present and then select the longest continuous carbon chain to give the highest ranking group the lowest possible number. -OH is high, -Br is quite low on the list, even behind the carbon itself, so probably in the first case Br is ignored, and in the second diol is chosen as the backbone of the molecule.

Just checked that these names are consistent with what ChemSketch names these molecules.
Many thanks - looked up "priority of functional groups in iupac nomenclature" and found following ref which bears out what you say:

https://www.masterorganicchemistry....functional-group-priorities-for-nomenclature/
 
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