Rules of the Day 320N

1. The 36 kcal/mol extra stability (unreactivity) of aromatic species derives from putting all the pi electrons in low energy molecular orbitals that extend over the sp2 hybridized ring atoms. These molecular orbitals involve overlap (in phase and out of phase) of the 2p orbitals on the sp2 hybridized ring atoms.

2. According to Huckel's rules to be aromatic, all ring atoms are sp2 hybridized, the ring must be flat, monocyclic and it must "4n + 2" pi electrons (2, 6, 10, 14, 20, 24 ....) where n = 0,1,2,3,4,5...).

3. Rings that otherwise satisfy the Huckel rules but have the WRONG number of pi electrons (i.e. 4, 8, 12, etc.) are referred to as "anti-aromatic" and are much less stable than expected compared to normal alkenes.

4 .Aromaticity stabilizes ions (both anions and cations)and atoms in molecules exist in hybridization states that maximize aromaticity. The lone pair on the ring will be in a 2p orbital if that leads to aromaticity. This explains why pyridine is much more basic than pyrrole. You will need to understand this before the next test.

5. The Hückel definitions, especially the 4n + 2 pi electron requirement, applies to monocyclic rings only. Also, only consider the ring atoms when counting pi electrons.

6. Fused benzenoid compounds are aromatic no matter how many pi electrons they have as long as the individual rings are aromatic.

7. You must learn how to use ortho, para, and meta.

8. 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.

Homework:

Read: Section 22.1 and 22.2 in the ebook textbook.

There are no quizzes or homeworks this week because of the exam on Thursday.