WHOA COOL!
---At least I think so ;)
Here is an excellent chart to help you decide if something is Ortho-Para or Meta Directing!
Wtf does that mean? --- Well, when you are adding molecules to an aromatic ring, the structures already attached to that ring have a lot of say in how that addition will occur (if at all).
If a structure on the ring is deactivating, the addition will occur in the meta position
If a structure is activating, the addition will occur in the Ortho or para positions
*Why is this the case? First, make sure you understand what ortho, para, and meta means. A quick google search can clear this up. Next, draw out three aromatic rings with a methyl on one carbon. Place a carbocation in the ortho, para, and meta positions (one per aromatic ring). Now draw the resonance structures for each molecule.
Hopefully this will demonstrate that the ortho/para positioned carbocations place the carbocation right next to the methyl group.
Now that you see our resonance of the carbocation is depended upon ortho/para/meta placement, we can discuss deactivation/ activation.
ACTIVATION: Do carbocations want more or less electrons? They want MORE! So if they are able to resonate next to one of the electron donating groups, it will be happy.
What makes a group “electron donating”. An electron donating group will have an abundance of electrons. Atoms like Oxygen and Nitrogen are electron hogs, but they are donating because they are resonating their electrons towards the aryl carbon
DEACTIVATION: Carbocations do not like to be next to other electron hogs! They are hunting for electrons so they want to avoid electron withdrawing groups at all cost.
A structure will be electron withdrawing if they are stealing electron density away from the aryl carbon. Carbonyl groups are a good example of this because a resonance structure exists where the oxygen has ALL the electrons, leaving the neighboring carbocation super electron hungry! this makes the aryl carbon electron deficient, and the carbocation really hates to be next to him now.
This is why certain structures cause aromatic rings to undergo specific types of addition! Hope that helps.