Click here for a copy of the mechanism sheet used in lecture today.

Rules of the Day 4-21-08

1. Activating ortho,para directing groups operate by stabilizing an adjacent positive charge through resonance (for groups with lone pairs on atoms attached to the ring) or hyperconjugation (for alkyl groups). These are sometimes called electron releasing groups. THE GOOD

2. Deactivating, meta directing groups operate by destabilizing an adjacent positive charge through an inductive effect. These are sometimes called electron withdrawing groups and they usually have pi bonds to heteroatoms on the atoms attached to the ring or a highly positively-charged atom attached to the ring.THE BAD

3. Halogens are deactivators due to inductive effects, but ortho,para directors due to a resonance effect. THE UGLY

4. The bottom line is that a group already on an aromatic ring influences the location of an incoming electrophile, meaning that when multiple groups are added, the ORDER of adding groups matters!!!

5. A very useful reaction utilizes H2 and Ni catalysts to turn a nitro group (-NO2) into an amine group (-NH2) on an aromatic ring. Note that this turns a bad group in to a good group.

6. If there are two groups on the ring, and they predict different products, then just like in the movie, Good beats Ugly or Bad, and Ugly beats Bad. Bad always loses. Because of steric hindrance, electrophiles never add between meta substituents.

7. If an aromatic ring is loaded with electron withdrawing groups, it is no longer nucleophilic, but can react as an electrophile. In this case, strong nucleophiles can displace a halogen on an aromatic ring. We will only see one example of this very rare and special reaction, in which 2,4-dinitrofluorobenzene reacts with strong nucleophiles such as amines. This is called nucleophilic aromatic substitution, and is very, very rare compared to electrophilic aromatic substitution reactions.

Homework: Study for the exam next Thursday night. Note there will be a review session Tuesday evening (NOT MONDAY!!) and note the room WEL 1.308 (NOT 2.224!!).