# Do crown ethers have to have two carbons between the oxygens? If so, why?

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?

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.