There are two key accounting tools for organic chemistry. The first is formal charge. Today is the second one… and it’s HUGELY important. It’s called “curved arrows”.
I say this to my students a lot: reactions are transactions of electrons between atoms [“electron economics”, if you will]. Imagine trying to read your bank statement or another financial document without all the transactions being neatly labelled as credits or debits. We take those conventions for granted now, but they had to be invented.
Curved arrows are the convention we’ve invented to show the movement of electrons.
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 (you can also use it for showing reactions)
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. Just make sure you don’t break the octet rule and also that you understand the key principles of what makes a resonance form more stable.
Thanks for reading! James
P.S. Rather see this in video form? Click here to watch. (and if you find it useful, please comment or up vote – thanks!)
PPS 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).