Rules of the Day CH310M/CH318M Fall 2007

 

Blasts from the past :

1. The "D" and "L" designations of amino acids refer to an old nomenclature scheme in which chirality is related to that of a reference molecule, D (i.e. "+") and L (i.e. "-") glyceraldehyde, the molecules on the cover of your book. Most amino acids are S, except for cysteine (R) and glycine (not chiral).

2. When establishing priority for either R vs. S or E vs. Z, count multiple bonds as being equivalent to that same number of bonds to the atoms taking part.

Moving on:

3. The most important consequences of the pi bond are that A) the double bond cannot rotate and B) electron density is above and below the bond axis.

4. Because alkenes have the electron density of the pi bond, they react as nucleophiles with molecules containing high partial positive charge (electrophiles).

5. Alkenes react with H-X (X = Cl, Br, I) in a mechanism that involves initial protonation of the pi bond to give a carbocation intermediate, that then adds X- to give an alkyl halide. (Time capsule: Markovnikov regiochemistry; stereochemistry mixed due to flat carbocation intermediate) Click here to see the reaction movie of H-Br adding to propene.

6. Reactions are like crimes, they need motive and opportunity.

A. A reaction has a motive if the products are lower in energy than the starting materials. That is, stronger bonds are made than broken in the reaction, and/or a weaker acid/weaker base is produced, and/or entropy is increased through the creation of a small gaseous fragment. Motive (also called thermodynamic driving force) determines position of equilibrium.

B. A reaction has an opportunity to take place if the mechanism contains no species that are higher in energy than the energies of a resonable fraction of starting material molecules at a given temperature. Opportunity determines rate of a reaction (also called reaction kinetics).

7. Heating up a reaction increases the rate by increasing the number of molecules with enough energy to get over an energy barrier

 

HOMEWORK The following readings and problems should be done before the next lecture so you are ready for it.

Read: Sections 6.3A-6.3C in the book

Do the problems in the book (but do not turn them in): 6.1-6.6, 6.14.

Start Working on Homework Problem Set 6 (You will turn this in on Oct. 19)