Rules of the Day 320N

1. Think of electron density as waves, in which you can get extra stability when waves add constructively, and you lose stability when waves cancel each other (add destructively).

2. You generate as many new molecular orbitals as atomic orbitals used to create them. Half are bonding molecular orbitals (waves add constructively) and are filled with electron density, and half are antibonding molecular orbitals (waves add destructively) and are not filled with electron density.

3. Provided all the atoms are in the same plane, all adjacent 2p orbitals overlap, allowing the pi electron density to delocalize into all the adjacent 2p orbtials, their wave functions adding constructively to provide for extra stability. (See Golden Rule #7). This situation is officially referred to as conjugation, although in class I will call it a "pi-way". For a "pi-way" to exist, there can be no sp3 hybridized atoms between the overlapping 2p orbitals, and all the overlapping 2p orbitals must be in the same plane.

4. Conjugated dienes equilibrate between the s-cis and s-trans geometry, referring to the conformation of the single bond between the double bonds. The s-trans geometry predominates.

5. 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.

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

7. 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.

8. 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.

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)

Take the Daily Quiz 15 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture.

Continue working on Homework 8 that will be due Wednesday, April 2nd at 10 PM. Note, there are no Aktiv learning problem sets for the rest of the semester.

Click here for the Gradescope Homework Problem Set 8