Which of the following compounds is aromatic?

From the 4n+2 rule, it must be either option 2 or 4. The answer given in my book is option 4. Why? I think It might be due to one being planar and the other not, but to me both seem to be planar. Is there a way to check if one is planar or not?


Number 2 is not aromatic because transannular C-H interactions cause the molecule to be non-planar although it satisfies 4n+2. This problem was solved by the synthesis of number 4, which, while a little less than planar but rigid, displays aromatic properties (NMR, etc.). E. Vogel, H. D. Roth, Angew. Chem. Int'l. Ed. 1964, 228.

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    $\begingroup$ Yeah, there was also analogous ether IIRC. $\endgroup$ – Mithoron Dec 13 '17 at 0:58

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