# What are the conditions for using skeletal replacement ('a') nomenclature?

The Blue Book 2013 in P-63.2.4.1 gives “1-methoxy-2-(2-methoxyethoxy)ethane” as the IUPAC preferred name for $$\ce{CH3OCH2CH2OCH2CH2OCH3}$$ and comments,

Skeletal replacement (‘a’) nomenclature [method (4)] generates preferred IUPAC names, when the conditions for using this type of nomenclature are met (see P-15.4); otherwise substitutive nomenclature must be used.

It then names $$\ce{CH3OCH2CH2OCH2CH2OCH2CH2OCH3}$$ as “2,5,8,11-tetraoxadodecane.”

My question is why skeletal replacement nomenclature was used in the latter case but not the former—why the conditions for using skeletal replacement nomenclature are not met by the former molecule.

P-15.4 states that

“Skeletal replacement (‘a’) nomenclature is used in two ways:

“ (a) …

“ (b) to generate simpler names for heteroacyclic structures than those formed by following the principles of substitutive nomenclature, for example, in naming polyamines, polyethers, etc., by replacing carbon atoms in corresponding acyclic hydrocarbons” (89).

The former molecule seems like an instance of a polyether, and the name “2,5,8-trioxononane” seems simpler than the given preferred name, so it seems that this molecule meets the this criterion.

What factor led to the use of substitutive nomenclature where replacement nomenclature seemed to apply?