Iupac Nomenclature

[neilparker62]

TL;DR Summary

Application of rules for determining the longest carbon chain

Consider the compounds

1-Bromo-2-(bromomethyl)butane​

and

2-Ethyl-1,3-propanediol​

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 ?

 

[Borek]

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.

 

[neilparker62]

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/