[orgo814]

Can someone explain this?

Was learning about hyperconjugation and why is it that a system with an adjacent carbonyl has less mixing (hyperconjugation) since it lowers energy level (thereby greater energy difference) but when we look at cyclopropene... that ch2 in the ring (which has more s character lowers energy also) increases overlap?

Thanks for any help

[zarhym]

There is huge ring strain in cyclopropene.

[orgo814]

Yes but that's not really answering my question. I'm confused more on the orbital side of things in particular why it goes lower in energy

[zarhym]

aromaticity
4n+2 when n=0 for cyclopropene 

[phth]

Cyclopropene is a great example of where the hybridiaiton rules change.  Cyclopropane is ~sp^2.5.  The orbitals are forces into conforming to different shapes because it lowers the enregy due to how much strain there actually is in the 3 membered ring.  Even more when there is an alkene.  An example is pi orbitals of fluorine is almost completely p in character.  It really depends on the structure.  To simply put it, the CH₂ group will distort itself to lower the energy of the system.  It's measured by spectroscopy.  Ansyln and Dougherty has a much better explanation than I in the back of the book discussing the quantum mechanical reasons why.  If you can't buy the book, then ask a professor/colleague/library for a copy.

 Hybridization is just some made up concept that says if we add the together we get 4 degenerate ones that form a tetrahedron--> explains the bond angle.  If i'm not mistaken, then most of the arguments are either derived form solid state nmr or XRD mesaurements of bond distance/angle and inference of the reason.
Reference:
http://www.amazon.com/Modern-Physical-Organic-Chemistry-Anslyn/dp/1891389319

[orgo814]

Thank you very much for your explanation- 5 stars!