How to apply electronegativity and resonance to understand reactivity

One thing has been missing from the discussion of resonance. What’s the point? Who cares if we can write out resonance structures? What does it matter if we can figure out the two or three most stable resonance structures? So what? Here’s the point: we can apply resonance (and electronegativity) to figure out the electron densities of molecules from first principles, and we can apply these electron densities toward understanding how a molecule will react. Put it another way: if you learn this skill, you will rely less on memorization for understanding reactions, because you’ll be able to figure out the chemical behavior of molecules you’ve never seen before. For instance: if you’re a non-chemistry major I can pretty much guarantee you’ve never seen this reaction before. But if you apply some of the principles in this post, you should be able to make some headway on it. Let’s look at these two aspects really quickly. Applying electronegativities. When you have a bond between two atoms with different electronegativities, there will be a dipole (two opposite charges separated in space). That dipole will give you a clue about the electron densities of those two atoms. For example in the molecule below, the oxygen is more electronegative than carbon which means that the C–O bond will be polarized towards oxygen (it will have a higher electron density). This is different than formal charge, which is where we have to assign a charge to an atom for “accounting” purposes. 1. Applying resonance: when you know the most stable two (or three) resonance forms, you’ll have a good idea of what the resonance hybrid looks like. The resonance hybrid also tells you electron densities, sometimes in a way that isn’t immediately apparent from electronegativity