We’re all to various degrees bumping into things in a dark room and gradually finding where the light switch is.

“Lakewood Avenue? That’s in Springfield. You’re in Dixon, son.”

Thanks for the commment. It is not a single resonant pi bond. There is a small *barrier* to rotation about the C2-C3 bond (about 2-3 kcal/mol) but this is not the same as breaking a pi bond. For breakage of a pi bond you would need to have a barrier to rotation of about 60 kcal/mol.

But on the diene, since the two pi bonds (as drawn on a Lewis diagram) are conjugated, isn’t it actually just a single resonant conjugated pi bond, stretching from C1-C4 in the case of butadiene?

Saying it another way, aren’t we really breaking one bond on the diene, not two?

Also, in terms of rotation on the diene, I thought that atoms participating in a conjugated pi bond cannot rotate because the p-orbitals need to be aligned for the conjugation to occur — so the alignment of the p-orbitals is preventing the rotation between the C2 and C3 carbons.

So if the s-cis and s-trans dienes are conformers, and there is rotation around the C2-C3 bond, then that means there can be rotation around at least certain conjugated pi bond, right?

Glad you’ve found it helpful Ian! Let me know if you find any typos/mistakes. Thanks

It is not immediately obvious. Relevant study: https://pubs.acs.org/doi/10.1021/acs.joc.5b00174