Chapter 6 Principles of Stereochemistry ***Bring Your Model Kits to Class!***

Stereoisomers: compounds that have the same connectivity, but a different arrangement of atoms in space Enantiomers: molecules that are non-superimposable (non-congruent) mirror images

Enantiomers

Enantiomers are said to be “chiral” They possess the property of “chirality” Greek: “hand” or “handedness” No plane of symmetry

If a molecule is superimposable on its mirror image it is said to be “achiral” Has a plane of symmetry

Chiral Carbons Many chiral molecules contain one or more asymmetric carbons Called “Chiral Carbons” Attached to four different groups Denoted by an asterisk Source of stereoisomerism Interchange of groups = other stereoisomer Stereocenters

Problems Identify, with an asterisk, the chiral carbons in the following molecules:

How do we differentiate between these two enantiomers How do we differentiate between these two enantiomers? What do we call them?

Nomenclature of Enantiomers Identify your chiral carbon Prioritize your substituents on the chiral carbon according to the Cahn-Ingold-Prelog priority rules used to determine E and Z for alkenes Highest priority = 1, Lowest Priority = 4

Priority Assignment Rules Identify the atoms directly attached to the C and rank them according to atomic #. Higher atomic # = higher priority Higher isotopic mass = higher priority

If the first atoms connected to the chiral carbon are the same, continue moving outward until the first point of difference

Multiple-bonded atoms are equivalent to the same number of single bonded atoms

Point the lowest priority substituent away from you and look at the 3 remaining groups in a plane Count  1, 2, 3 If you go CW = R (rectus, Latin “proper”) If you go CCW = S (sinister, Latin “left”)

Name your enantiomers by placing an “(R)-” or an “(S)-” in front of the molecule’s IUPAC name

Problems Identify whether the following molecules are R or S. Identify and name the following molecules: