Click here for a copy of the mechanism sheet used in lecture today.

Rules of the Day 4-09-08

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

2. The energy absobed 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.

3. According to the Huckel definition of aromaticity, monocyclic molecules will be aromatic if they are 1) flat, 2) all the ring atoms are sp2 hybridized (sp in rare cases) and 3) there are 4n + 2 pi electrons, where n = 0,1,2,3,4,5,6.......

4. 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.

5. Aromaticity stabilizes ions and atoms in molecules exist in hybridizaiton states that maximize aromatocity. The lone pair on a ring atom will be in a 2p orbital if that leads to aromaticity. This explains why pyridine is muchmore basic than pyrrole. You will need to understand this before the next test.

Homework: Start working on homework Set 9. Click here to download a copy. Read 21.4-21.5 Problems: 21.9 (you will still need to know this nomenclature for the MCAT, DAT, etc., 21.15, 21.36)