Rules of the Day
4-8-2025
Featured Golden Rule of Chemistry:
5. Delocalization of charge over a larger area is stabilizing.
6. Delocalization of unpaired electron density over a larger area is stabilizing.
7. Delocalization of pi electron density over a larger area is stabilizing.1
Click here for a copy of the lectures notes I wrote in class
Click here for a copy of the handouts I used in class today
1. You must learn how to use ortho, para, and meta.
2. Atoms with a positive charge, a negative charge or an unpaired electron are all highly stabilized by resonance delocalization when attached directly to an aromatic ring. Phenols are extra acidic because the phenoxide anion is stabilized by resonance delocalization. It is important that you can draw all of the contributing resonance forms of the phenoxide anion, and thus you understand why the negative charge is on the ortho and para positions.
3. Knowing when to use phenyl vs. benzyl can be tricky!
4. As the result of the benzyl radical stability, it is easy to carry out free radical halogenation of the benzyl position using X2 and light or heat.
5. Alkyl groups on benzene rings are oxidized all the way to carboxylic acids using Na2Cr2O7/H2SO4 (H2CrO4) as long as there is at least one benzylic hydrogen atom on that alkyl group.
6. 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.
7. Make sure you know the reagents needed for each reaction, as well as the nature of the "wicked strong" electrophile on the handouts from class. Click here for the first handout, and click here for the second. Students often forget when to use FeX3 vs. AlX3 as catalysts. Remember that the FriedAL-Crafts used ALuminum (AlX3). OK, so I had to spell Friedel wrong, but I am trying to help here!
8. Both Friedel-Crafts reactions (alkylation and acylation) will not work if there is a strongly deactivating (BAD) group on the ring. The Friedel-Crafts alkylation is also a slow reaction, meaning the carbocation intermediates will rearrange if possible. In fact, primary haloalkanes will always rearrange to more stable secondary carbocations during the reaction, that then add to the aromatic ring by attaching through the secondary carbon. Rearrangement prone secondary carbocations will also only give rearranged products. The Friedel-Crafts acylation does NOT rearrange.
9. A workaround to avoid primary haloalkane rearrangemet in the Friedel Crafts alkylation reaction is to use the Friedel-Crafts acylation to create a ketone (with no rearragement) followed by a Wolff-Kischner or Clemmensen reduction to remove the ketone carbonyl group and give the desired alkane group on the ring.
11. 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).
Homework: No assigned reading
Take the Daily Quiz 18 before 10 PM Wednesday, April 9. Click here to access the quiz.
Finish working on the Homework 9 that will be due on April 9. Click here to get a copy. It is an important one, helping you truly understand all of the C-C bond-forming new reactions.