Rules of the Day 320N

1. Because the Diels-Alder reaction is concerted: 1) the orientation of groups originating on the diene are determined by the s-cis conformation and 2) the configuration of the alkene (dienophile), either cis or trans, is maintained in the product. In addition (excuse the pun), carbonyl groups react in the so-called endo position "underneath" the double bonds of the diene. Click here to see a movie of a smiple Diels-Alder reaction. There are three more Diels-Alder movies, showing alternative views of a more complicate Diels-Alder reaction. Click here for a side view. Click here for an alternative view of the same reaction. Click here so see both views simultaneiously.

2. 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.

3. 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...).

4. 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.

5 .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.

6. 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.

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

Homework:

Read: Section 22.1 and 22.2 in the ebook textbook.

Many students have difficulty understanding how to think about "equivalents", how to balance certain equations like the Claisen condensation, what do we mean by H3O+ and knowing when to use hydroxide versus ethoxide. Here are some short videos to help you with any of these: How to think about "Equivalents" when using base in enolate reactions (7:17 minutes) Correct video for How to think about "Equivalents" when using base in enolate reactions (3:29 minutes) How to balance a Claisen reaction equation (5:06 minutes) What do we mean by H3O+ (2:52 minutes) When to use NaOH vs. NaOEt (7:35 minutes) Homework:

There is no Daily Quiz today!

Start 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.