Rules of the Day 320N

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. Absorbance of a photon by a molecule corresponds to promotion of an electron from a filled molecular orbital to an unfilled molecular orbital. In the visible region, this usually involves pi bonding and antibonding molecular orbitals, and conjugation increases wavelength of absorption.

2. Molecules appear to our eye to be a combination of the wavelengths reflected (not absorbed).

3. The greater the number of pi bonds in conjugation, the smaller the energy difference between filled and unfilled orbitals, so the longer the wavelength of light that is absorbed (wavelength is inversely proportional to energy of light). That is why carrots are orange, for example.

4. The energy absorbed by molecules when they absorb a photon is usually converted to vibrational energy (they heat up). Fluorescence occurs when there are not vibrations possible (a rigid molecule) so the photon is emited as the electron goes back to ground state. Phosphorescence (glow in the dark) happens when the excited electron has flipped spins, and must reflip back before entering the original filled orbital while emitting a photon. Chemiluminescence (firefly light, "light sticks") happens when a chemical reaction produces an excited electron in a rigid molecule.

5. The Diels-Alder reaction works in part because the transition state can be viewed as being stabilized by the same considerations that make aromatic pi systems more stable (by 36 kcals/mol).

6. 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.

Homework:

Read: Section 22.1 in the ebook textbook. This text is part of the Longhorn Textbook access program.

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:

Take the Daily Quiz 17 before 10 PM Wednesday, April 2. Click here to access the quiz.

Finish working on the Homework 8 that will be due on April 2. Click here to get a copy. It is an important one, helping you truly understand all of the C-C bond-forming new reactions.