Isomers: The Arrangement of Atoms in Space University of California, Chapter 4 Isomers: The Arrangement of Atoms in Space Paula Yurkanis Bruice University of California, Santa Barbara

Contents of Chapter 4 Conformational and Configurational Isomers Chirality Centers, Enantiomers Optical Rotation Isomers with More Than One Chirality Center Separation of Enantiomers Stereochemistry of Reactions Chapter 6

Kinds of Isomers Chapter 6

Conformational Isomers Chapter 6

Configurational Isomers: Cis-Trans Diastereomers Chapter 6

The E,Z System for Designating Geometric Isomers

The E,Z System for Designating Geometric Isomers

The E,Z System for Designating Geometric Isomers If the atoms attached to the sp2 carbon are the same, the atoms attached to the tied atoms are compared; the one with the greater atomic number belongs to the group with the higher priority.

The E,Z System for Designating Geometric Isomers If an atom is doubly bonded to another atom, treat it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, treat it as if it were singly bonded to three of those atoms. Cancel atoms that are identical in the two groups; use the remaining atoms to determine the group with the higher priority.

Isomers with One Chirality Center A chirality center arises when four different substituents are bonded to a carbon Only two isomers are possible, an R isomer and an S isomer. Chapter 6

Chirality R and S enantiomers are mirror images of each other, just as your right hand is the mirror image of your left hand Chapter 6

Chirality Any object that has a plane or point of symmetry is achiral (not chiral). Chapter 6

Examples chiral achiral Chapter 6

Mirror Trick Whenever two structures can be positioned around a symmetry plane if they aren’t identical they’re enantiomers. Chapter 6

Drawing Enantiomers Fischer Projections N and S are down into plane, E and W are up out of plane of page or screen. Chapter 6

Naming Enantiomers: The R,S System of Nomenclature Rank groups by atomic number of the atom bonded to the chirality center. Use the same system that was used for the E and Z isomers of alkenes Chapter 6

Naming Enantiomers: The R,S System of Nomenclature Orient molecule so that group (or atom) of lowest priority is directed into plane. 3. Draw a curve from group of highest priority through the group of second priority to group of third priority Chapter 6

Naming Enantiomers: The R,S System of Nomenclature R (Latin rectus) = right turn S (Latin sinister) = left turn 1 2 3 4 (S)-2-bromobutane Chapter 6

Swap Trick If for any reason you ever wish a group were in a different position simply swap it with another group. Swap simply reverses the chirality. R for switched compound implies S for actual compound Chapter 6

Low Priority Group Up Trick If the low priority group in figure points up rather than down simply draw circular arrow and reverse chirality. R for reversed chirality implies S for actual chirality Chapter 6

Rotation Trick If one group is in the same position in two different structures and the other three groups are ALL in different positions then the structures are identical. These 3 structures interconverted by rotation of bottom 3 groups around vertical bond attached to top group Chapter 6

Compounds with More Than One Chirality Center Two Pairs of Similar Groups Chapter 6

Compounds with More Than One Chirality Center Three Pairs of Similar Groups Chapter 6

Meso Compounds Chapter 6

Drill on Meso Compounds Does the following compound have a stereoisomer that is a meso compound? Plane of symmetry Meso form possible Chapter 6

Relative and Absolute Configurations (–) amphetamine is known to have the R- configuration Therefore the (+) form has S configuration Chapter 6

Separation of Enantiomers Racemic lactic acid can be reacted with a naturally occurring chiral base, such as morphine or strychnine The product is a pair of diastereomers Properties of diastereomers are sufficiently different from each other to allow separation After separation, each diastereomer can be reacted with hydrochloric acid, yielding an optically pure R or S acid Chapter 6

Reaction Stereochemistry A regioselective reaction is one in which multiple constitutional isomers possible, but more of some formed than others. Chapter 6

Syn Addition When the two substituents add to the same side Addition of H2 is a syn addition Chapter 6