Is this compound chiral?

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I can't tell where the chiral centres are because this polycyclic backbone is too confusing. I tried to draw a mirror image, but I got really confused and couldn't tell whether it was superimposable. And I don't have a molecular modelling set so I can't build a model.


Yes, this compound is chiral.

The polycyclic backbone is called adamantane. It has $T_\mathrm{d}$ symmetry, meaning that as far as chirality goes, it behaves like a perfect tetrahedron, somewhat like methane does.

It also has the interesting property that if you extend the C-F, C-Cl, ... bonds inwards, they will all meet at the same point. Those red dotted lines form a perfect tetrahedron.

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Now, since the adamantane backbone behaves exactly like a methane molecule - why don't we just replace it with one? [n.b. this is not 100% rigorous but works here.]

enter image description here

Hopefully you'll agree that this is chiral. It basically behaves like it has a chiral centre right in the middle of the adamantane backbone.

If you really wanted to be sure, you could build a mirror image and verify that they are non-superimposable. (Hint: they are.)

Each of the four substituted carbons is connected to four different groups, and theoretically one would expect $2^4 = 16$ stereoisomers. However, because three of the four groups (the polycyclic backbone) are held in a rigid, fixed geometry, this number is reduced to 2.

There is more discussion of this class of compounds here.

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  • $\begingroup$ I find it fairly obvious that every quaternary carbon (which is tetrahedrally coordinated) has all different substituents and thus breaks all symmetry. However, I find this answer very nicely illustrated, and it makes a good point for more complicated cases. $\endgroup$ – Martin - マーチン Sep 23 '16 at 5:10

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