At some point you might have to ask yourself: what’s a conjugated system?
A conjugated system is a system of connected p orbitals.
Any time there’s a group of three or more adjacent p orbitals that can all line up in the same plane, this is a conjugated system (or pi system). It’s a little bit like a pi bond, but extended over more than two carbons – kind of like a row of men on a foosball table. Conjugation is a stabilizing effect: electrons can spread themselves out over a larger area, which results in a lowering of energy.
What can be part of a conjugated system? Any atom that is capable of donating a p orbital. This includes:
- atoms that are part of a double bond
- atoms with lone pairs
- atoms with empty p orbitals (such as carbocations)
- atoms with half-filled orbitals (radicals)
A short cut to figure out if it’s conjugated: ask yourself – can you draw a resonance form that puts a pi bond on that atom? If so, then it can be part of the conjugated system.
Here are four examples of conjugated systems (and one that is not!)
Thanks for reading! James
PS – related post : Are these alkenes conjugated?