Rules of the Day
Click here for a copy of my lecture notes from today's lecture
Click here for a copy of the handout used in class today
Featured Golden Rules of Chemistry: 3. There are two possible arrangements of four different groups around a tetrahedral atom. The two different arrangements are mirror images of each other, a property referred to as chirality and often compared to handedness.
1. Tetrahedral atoms such as carbon with four different substituents are chiral and are called chiral centers.
2. Name enantiomers as R or S by assigning Cahn-Ingold-Prelog "priority" to each atom attached to the stereocenter (higher atomic number AT THE FIRST POINT OF DIFFERENCE and watch those multiple bonds!), then arrange the molecule so you are looking down the chrial center-lowest priority bond. Count around the ring; clockwise is "R" and counterclockwise is "S".
3. Look at the handout on stereochemistry (click here for a copy) to help with definitions.
4. If there are "n" chiral centers in a molecule, there are 2 to the "nth" power possible stereoisomers, although symmetry can make this number lower (see "meso"). For a molecule with 2 chiral centers, that means 4 possible stereoisomers, for 3 chiral centers that means 8 possible stereoisomers, etc.
5. An enantiomeris any molecule that cannot be superimposed on its mirror image (it does not have a plane or center of symmetry).
6. Diastereomers are molecules that are stereoisomers but not enantiomers; a situation that arises when there are more than one chiral center in the same molecule.7. A meso compound is a molecule with two or more chiral centers yet is not chiral because it contains a symmetry element, usually a plane of symmetry.
8. To decide whether a pair of stereoisomers are enantiomers, diastereomers or even meso, assign R or S to all chiral centers and compare these. R,R and S,S are enantiomers, R,S and S,R are enantiomers, and all other combinations (i.e. R,R and R,S) are diastereomers. Meso compounds are always R,S = S,R for symmetric molecules.
9. You need to draw molecules in their most symmetric possible conformation when looking for symmetry, (do not worry about strain for this). Use eclipsed conformations if you need to. When analyzing ring compounds containing two or more chiral centers, look for planes of symmetry (when the cyclohexanes are drawn flat) to identify which compounds are chiral and which ones are not.
10. There are three situations in which you must be able to identify R or S configurations around a chiral center, when the 4th ranked group is in the back (no adjustment needed), facing toward you (reverse the direction) and in the plane (adjust paper and look down bond).
HOMEWORK:
Read: Sections 3.3-3.5 in the ebook textbook. This text is part of the Longhorn Textbook access program.
Take the Daily Quiz 8 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture. Together, they will count as 5% of your final grade.