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Why does this molecule not exist

The bond angle in a normal pentagon is 108 degrees.
Apparently this structure would cause too much strain and would thus break open.
Why?

ImpossibleMolecule.png2.6KB

Reply 1

EierVonSatan

The ideal angle for C=C=C is 180 degrees, 108 degrees is a long way from that. You can bend bonds (an example would be cyclopropane) but bend them too much and they just break.

Reply 2

cpchem

I think you can just about make 6-membered cyclic allenes (they're obviously incredibly reactive), but I don't think 5 is manageable.

Reply 3

illusionz

Original post by cpchem

I think you can just about make 6-membered cyclic allenes (they're obviously incredibly reactive), but I don't think 5 is manageable.

Would make sense. If you can get benzyne then it would be logical that sp (ish) hybridisation is just about possible in a 6 membered ring.

(edited 11 years ago)

Reply 4

cpchem

Original post by EierVonSatan

The ideal angle for C=C=C is 180 degrees, 108 degrees is a long way from that. You can bend bonds (an example would be cyclopropane) but bend them too much and they just break.

And, of course - woo, triangles!

/excitedcyclopropanechemist

Reply 5

EierVonSatan

Original post by cpchem

And, of course - woo, triangles!

/excitedcyclopropanechemist

Ha, my masters sypervisor loves heterocylic strained rings. I was just looking up cyclic allenes/cumulenes since you mentioned it and yeah, 6 membered is the smallest I've found so far :teeth:

It wouldn't surpise me if some physical chemist hasn't made something 'silly strained' for a few femtoseconds :mmm:

Reply 6

ilovemath

Original post by EierVonSatan

Ha, my masters sypervisor loves heterocylic strained rings. I was just looking up cyclic allenes/cumulenes since you mentioned it and yeah, 6 membered is the smallest I've found so far :teeth: