#ochem2

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. 

DECARBOXYLATION: 

This is an important mechanism in most ochem two classes. Decarboxylation is what it sounds like- the removal of a carboxyl group. A fun example of where this mechanism is commonly used is the conversion of THCA (the inactive compound in marijuana) into THC (the active compound in marijuana). This decarboxylation takes place under applied heat, hence why weed must be burned or baked to receive its maximum effects. 

I am going to give a step-by-step description of how the decarboxylation in the third image works. 

1. First the molecule orients in a way that will allow the OH group on the carboxyl to associate with the carbonyl

2. The carbonyl double bond attacks the hydrogen. This breaks the carbonyl bond. The electron shared with the hydrogen is pulled back into a double bond with that carbon. That creates the CO2 molecule!

***The carbonyl bond was very stable, thus heat needed to be applied to break it***

3. The alcohol that formed is attached to the double bond that formed. Is this a stable formation? NO! This is called an enol. An enol will never be your final product because the carbon-oxygen association is tooooo strong. So we need to form that carbonyl--- the electrons on the oxygen will push down and the double bond will attack any available hydrogen.