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

Rules of the Day 4-18-08

1. Neither the Freidel-Crafts alkylation nor the acylation will work if there is a strongly electron-withdrawing "bad" group on the ring, since the ring is too deactivated as a nucleophile. Friedel-Crafts reactions will occur if there is one ugly group on the ring, however.

2. The ketone produced by the acylation can be reduced (Wolff-Kishner, Clemmensen) to make alkyl benzenes that cannot be made in high yield via the direct alykylation.

3. To summarize electrophilic aromatic substitution: Aromatic rings react as weak nucleophiles, but only with wicked strong electrophiles. All the reactions described so far proceed via an arenium ion intermediate after attack of the wicked strong electrophile by the aromatic ring. The key feature of the arenium ion intermediate is that the positive charge is distibuted ortho and para to the location of where the electrophile is attached. You must be able to predict this ortho and para distribution by drawing all three appropriate resonance contributing structures.

4. Substituents already on the ring influence reactivity and orientation by interacting with the partial positive charges of the arenium ion intermediate. The groups are analyzed on the basis of pi donation (resonance of lone pairs on atoms attached to ring donated into aromatic pi system), hyperconjugation (alkyl groups donate electron density into aromatic pi system), and inductive effects (electron withdrawing groups remove electron density).

5. 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

6. 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

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

8. 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.

Homework: Read 22.2 (again) Problems: 22.21, 22.30, 22.33