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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?

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Number 2 is not aromatic because transannular $\ce{C-H}$ 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., Ref 1).

References:

  1. E. Vogel, H. D. Roth, "The Cyclodecapentaene System," Angew. Chem. Int'l. Ed. 1964, 3(3), 228-229 (DOI: https://doi.org/10.1002/anie.196402282).
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    $\begingroup$ Yeah, there was also analogous ether IIRC. $\endgroup$
    – Mithoron
    Dec 13, 2017 at 0:58

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