A few weeks ago I said resonance was the key theme of Org 2. It’s probably a good idea at this time to go back and refresh on one of the key skills you’ll need to master for this part of the course: drawing curved resonance arrows.
Curved arrows are how we show the movement of electrons. They’re one of the two key accounting tools for organic chemistry (the other being formal charge).
Every curved arrow has a tail (the source of electrons) and a head (which is the destination of the electrons). A curved arrow says “take a pair of electrons from here (the tail) and move them there (the head)!”
We can use curved arrows to show the conversion of one resonance form into another.
There are three legal moves for a resonance curved arrow (and only 3).
- Break a pi bond and move the pair of electrons to a lone pair.
- Move a pair of electrons from a lone pair to form a new pi bond.
- Break a pi bond and move the pair of electrons to form a new pi bond.
Here they are:
Every resonance structure of a molecule can be built through a combination of these three moves. Every. Single. One.
Two more iron rules for resonance structures:
- Don’t ever exceed the octet rule for C, O, N, F, etc.
- Only break Pi bonds, not single bonds.
One more key guideline:
- When breaking a Pi bond between two atoms of unequal electronegativity, it’s best to move the electrons toward the more electronegative atom.
Tomorrow: we’ll start applying curved arrows when we talk about “directing groups” in aromatic substitution reactions.
Thanks for reading! James
PS One last thing to note. Curved arrows also tell you how to change formal charge. Since electrons are moving from the tail to the head, the tail is donating electrons and the head is accepting electrons. You want to make the formal charge at the tail more positive by one (since it’s losing a negatively charged electron) and make the formal charge at the head more negative by one (since it’s adding a negatively charged electron).
PPS Further reading: In Summary – Resonance
PPPS Common Resonance Mistakes