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

Rules of the Day 4-16-08

1. Electrophilic aromatic substitution involves wicked strong electrophiles reacting with the aromatic pi system to create a resonance stabilized arenium ion intermediate that then loses a proton to give the substitution product.

2. The arenium ion intermediate has partial postive charge located otho and para to the position of the incoming electrophile, but not meta.

3. The above mechanism applies to halogenation (X2/FeX3), Nitration (H2SO4/HNO3), Sulfonation (H2SO4/SO3), Friedel Crafts Alkylation (RX/AlCl3) and Friedel Crafts Acylation (ROCl/AlCl3).

4. Friedel-Crafts alkylation and acylation reactions are important because they make carbon-carbon bonds. The wicked strong electrophiles are a carbocation and an acylium ion for the alkylation and acylation reactions, respectively. The alkylation reaction is prone to rearrangement, but the acylation is not.

5. 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.

6. 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.

Homework: Read 22.2 (again) Problems: 22.4-22.5, 22.7, 22.19, 22.20