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I just began learning about ethers and I have noticed that pictures of crown ethers, no matter what size they are, always have two carbon atoms left and right of each oxygen atom before you reach another oxygen atom in the ring. It's almost like it is a poly-ethyl ether. So my question is can you have crown ethers with more than 2 carbon atoms between the oxygen atoms?

Image of 18-crown-6

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To be a crown ether, the compound in question must be an ether, i.e. must contain oxygen atoms which are bonded to two carbon atoms neither of which is bonded to another oxygen atom. Thus, if you had just one carbon between two subsequent oxygens you would no longer be dealing with a crown ether; if you wanted, you could call it a crown acetal. But it would have vastly different properties; most notably it would break down in aquaeous solution under acid catalysis.

To the best of my knowledge, there is no formal restriction other than that given in the first paragraph for crown ethers. 20-crown-5, which would have three $\ce{CH2}$ groups between each ether oxygen, would be a valid crown ether in my humble opinion. However, most cations that are coplexed by oxygen prefer five-membered rings for various reasons, resulting in many complexing agents — $\ce{en, dmg, m2g, phen}$ to name just a few examples — having exactly two other atoms between the ligating atoms. Thus, practically all crown ethers one hears of have two carbons between the oxygens.

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  • $\begingroup$ "must be an ether, i.e. must contain oxygen atoms which are bonded to two carbon atoms neither of which is bonded to another oxygen atom"... By this do you mean that ether by definition is a function group where O is attached to 2 carbon atoms. What about propyl ether then? It is still a ether. May I am reading this wrong? $\endgroup$ – TLo Feb 4 '17 at 20:08
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    $\begingroup$ Nobody would call 20-crown-5 by that name. It'd be cyclopenta (oxy-1,3-propylene). 1,3,5-Trioxan is rather stable in water, as is the linear polyoxymethylene, if you stabilise the end groups. $\endgroup$ – Karl Feb 4 '17 at 20:08
  • $\begingroup$ @Tlama You can built cyclic polyethers with (oxy-1,3-propylene) groups, mixed with ethyleneoxy groups, butylenoxy groups, whatever you like, but they are not usually called "crown ethers". $\endgroup$ – Karl Feb 4 '17 at 20:14
  • $\begingroup$ @Karl Yes to 1,3,5-trioxane, and the full acetal of polyformaldehyde but still less stable than 1,4-dioxane (6-crown-2? ;)) or poly-ethyleneglycol. My point was that even full acetals are decomposed more easily than ethers. Concerning ‘20-crown-5’: Please first show me who actually works with it and what they do with it. I am not aware of that type of cyclic ether being common and hence I consider the discussion which name is common academic. $\endgroup$ – Jan Feb 4 '17 at 20:14
  • $\begingroup$ @Jan POM is a widely used technical polymer. Those acetals are perfectly stable. Trioxan decomposes into formaldehyde if heated to >150°C in the presence of water (says de:wp, without reference). I have no idea what dioxane or PEG do under those conditions. $\endgroup$ – Karl Feb 4 '17 at 20:31

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