Pi Donation

At the beginning I said that the main theme of Org 2 is “resonance”. Well, here’s where it’s going to start getting used a lot more.

Here’s the most important new concept that stems from resonance. It’s called Pi donation, and it will be very prevalent throughout the rest of the course.

We’re used to thinking of atoms like oxygen or nitrogen as Scrooges of the periodic table: with their high electronegativity, they take electrons away from whatever they’re bonded to (“the inductive effect”). But they have a more giving side… like Santa.

When they are adjacent to a pi bond, atoms with a lone pair such as oxygen and nitrogen can donate a pair of electrons into the pi bond, making the carbon adjacent to the more electron rich (and forming a pi bond in the process). This is called “Pi donation”.

Important: the effect of donating electrons to the double bond is more powerful than the electron-withdrawing effects of induction.

So since increasing the elecron density means that we’re increasing nucleophilicity,  oxygen and nitrogen make the double bonds they are attached tobetter nucleophiles. Look out for this effect! You will see it again… and again… and again.

Tomorrow: let’s revisit the concepts of nucleophiles and electrophiles.

Thanks for reading! James