As an organic chemistry student you will be required to study many alkene reactions. Many students fall into the trap of memorization and regurgitation and set themselves on the path of doom. In this article I will help you understand alkene reactions by explaining a key concept in reactions: Why Pi Bonds Attack
I don’t want YOU to fall into the trap of confusion due to lack of understanding
So let’s break it down
A single or sigma-bond is a direct connection between one atom and the next. The electron density will be concentrated between the two bonds in question
The pi bond is different. If you picture a p-orbital in the y-plane of a molecule you will realize that the p bond sits very high above, and very low beneath the molecule.
The electrons moving around the pi bond are much further away from the molecule compared to a sigma bond. Think of this as walking a curious puppy on a long leash. Sure the puppy is ‘attached’ to your hand, but the long leash allows him to run around and sniff everything right and left. In fact, if you let go of the leash he’d be off and running away following his nose. Why? because he’s so curious and all these attractive smells are calling to him
Pi bonds are like that too. They are very electronegative and will be easily ‘distracted’ by partially positive or full positive charges nearby. This negativity makes the pi electrons very nucleophilic
Quick recap of nucleophile and electrophile
Nucleophile = nucleus loving = positive seeking thus partially/fully negative
Electrophile = electron loving = negative seeking thus partially/fully positive
When an electrophile is brought close enough to the pi bond, the pi electrons will be so ‘curious’ and actually break away from the carbon atom. Imagine this as the moment you let go of the puppy’s leash. He’s off…
The pi electrons don’t break away from each other though. Instead they will simply break away from one of the 2 carbon atoms holding the pi bonds. This is the first step in your reaction mechanism
In doing so the former pi electrons are now sitting as a bond between one of the former sp2 (pi bond wielding) carbon atoms and the attacked electrophilic atom
The other carbon is now deficient having lost its pi bond and carries a positive charge. This carbocation is the reason for another nucleophile to attack in the next step
