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While reading my organic chem book, I came across this compound:

enter image description here

Since this is an exercise question, I do not know the name of this compound. I am unable to visualize this compound (how it looks out of the paper) as I have never come across two rings joined by a bridge-like structure, my book says it is a methylene bridge. I appreciate your assistance in determining the structure of this compound.

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  • $\begingroup$ Here: chemistry.meta.stackexchange.com/questions/3736/… is something for beginners. $\endgroup$ – Mithoron Aug 6 '18 at 22:43
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    $\begingroup$ Even adding proper formula would be good 'cause I'm not even sure about bonding in this thing, looking at the pic. $\endgroup$ – Mithoron Aug 6 '18 at 22:46
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    $\begingroup$ Are you sure the structure is correct? It is very close to 1,6-methano(10)annulene (en.wikipedia.org/wiki/1,6-Methano(10)annulene). If so, I can post an answer with a picture of the crystal structure that might help. $\endgroup$ – jerepierre Aug 6 '18 at 22:50
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    $\begingroup$ @jerepierre I was thinking it may be this, otherwise it would be partially saturated ring. And from OP's comment, it indeed looks like it is the one. $\endgroup$ – Mithoron Aug 6 '18 at 22:55
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    $\begingroup$ @jerepierre You are right. It is 1-6methano(10)annulene since I verified some characteristics mentioned on the Wikipedia page with my book. $\endgroup$ – Parth Chauhan Aug 6 '18 at 22:57
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This compound is 1,6-methano[10]annulene, and a classic example of an unusual compound displaying aromaticity.

Aromaticity requires the pi-system to lie in a plane. One might speculate that [10]annulene (shown below) would be aromatic, because it contains a cyclic pi-system of 10 electrons, which appears to be planar. However, because the hydrogens indicated are trying to occupy the same space, the compound is forced out of planarity, and the compound it non-aromatic.

enter image description here

Bianchi and coworkers reported the crystal structure of 1,6-methano[10]annulene. In this compound, the hydrogens that clash in [10]annulene are replaced by a methylene (CH2) group. From directly above, the compound looks basically like naphthalene, with a little extra space in the middle. More interestingly, from the side, it's clear that the atoms of the 10-membered ring are nearly planar, with that methylene bridge oriented perpendicular to the plane.

enter image description here

Reference: Structure of 1,6-methano[10]annulene, R. Bianchi, T. Pilati and M. Simonetta, Acta Cryst. (1980). B36, 3146-3148, https://doi.org/10.1107/S0567740880011089

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