I'm struggling to understand why the following organic compound: picture of 2-ethylhexane-1,3-diol

is named systemically as 2-ethylhexane-1,3-diol (I think its common name is etohexadiol), if the longest carbon chain in the molecule is actually 7 carbons long, as labelled here: picture of 2-ethylhexane-1,3-diol with possible longest carbon chain labelled

Wouldn't the compound have a hydroxymethyl group on the 3rd carbon and a hydroxy group on the 4th? Or do I just have some big misconception about naming alcohols (very possible).

Thanks for any help anyone can give!

  • 3
    $\begingroup$ Welcome to ChemSE. The chain bearing the hydroxyls takes precedent over longest chain. $\endgroup$
    – user55119
    Nov 25, 2020 at 18:07
  • $\begingroup$ @user55119 Hi! Thanks for the response. Just to clarify - you name alcohols so that the parent chain is the chain with the most hydroxyl groups on it? And does this sort of rule apply for any other organic compounds? Thank you! $\endgroup$
    – kipf
    Nov 25, 2020 at 18:35
  • $\begingroup$ See this: masterorganicchemistry.com/2011/02/14/… $\endgroup$
    – user55119
    Nov 25, 2020 at 18:41

1 Answer 1


I'll be using the brief guide to the nomenclature of Organic Chemistry as a reference, because I could not gain access to the blue book. This source suffices for this nomenclature example.

Seniority of parent compounds

The systematic name is based on the name of the senior parent compound, which is chosen by applying the following criteria in the order described below and shown in Fig. 1, until a decision is reached. For a complete set of criteria see Ref. [8].

enter image description here

In the given example, both the chains contain the senior most group, which is the hydroxyl (alcohol) group. The smaller chain, however, contains 2 hydroxyl groups as compared to the larger chain, which contains one hydroxyl and one alkylhydroxyl group (which is a lower priority). Since this is the first point of difference, The smaller chain would be the parent compound.

After identifying the parent chain, it is easy to name the locants such that the seniormost groups get lower locants; As you have written, the name would finally be 2-ethylhexane-1,3-diol.

This image is possibly more detailed than this question requires, but the image and the linked reference are good resources, should you need more information on naming tricky compounds.


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