1. Chapter 20.3: Stereoisomerism
Stereoisomerism involves isomers which have different arrangements of atoms in space but do not differ in connectivity or bond multiplicity (i.e. whether single, double, or triple) between the isomers themselves.

2. Structural isomers: 2 compounds have different structural formulae (atoms not arranged in same order)
Stereoisomers: 2compounds have the same structural formula (spatial arrangements of atoms is different)

3. Configurational: what we will focus on here
Conformational: hard to tell difference due to interconversion via free rotation around bonds

4. Stereoisomers: Configurational Isomers
Permanent difference in geometry
(cannot be interconverted but can be separated)
Also called geometric isomers
Cis-trans
These isomers can be observed in 2 types of molecules:
Double bonded molecules
Cyclic molecules

5. Stereoisomers: Configurational Isomers cis-trans
The substituents do not have to be on adjacent carbons
The cis-trans configuration is relative to the plane of the ring

6. Stereoisomers: Configurational Isomers
Permanent difference in geometry
(cannot be interconverted but can be separated)
Also called geometric isomers
E/Z isomers
Used primarily for double bonds where the cis-trans terminology does not work: having different substituents on the carbons
Follow the rules of priority:
Higher atomic number has higher priority
If atoms are same, follow rule to next atom bonded (so longer hydrocarbons have higher priority)
C3H7 > C2H5 > CH3 > H and Br > Cl > F
If priority groups are on same side of double bond = Z isomer
If priority groups are on opposite sides of double bond = E isomer

7. Used primarily for double bonds where the cis-trans terminology does not work: having different substituents on the carbons
Follow the rules of priority:
Higher atomic number has higher priority
If atoms are same, follow rule to next atom bonded (so longer hydrocarbons have higher priority)
C3H7 > C2H5 > CH3 > H and Br > Cl > F

8. Used primarily for double bonds where the cis-trans terminology does not work: having different substituents on the carbons
If priority groups are on same side of double bond = Z isomer
If priority groups are on opposite sides of double bond = E isomer
This convention is much more useful than cis-trans

9. Stereoisomers: Optical Isomers (enantiomers)
- Two molecules have same structural formula but are mirror images (not superimposable on one another).
- This only occurs when there is a chiral* carbon present, a carbon bonded to four different substituents.
The chiral carbon is also known as asymmetric or a stereocenter (hence, stereoisomers)

10. Stereoisomers: Optical Isomers (enantiomers)
Two superimposable forms are called enantiomers
An equal mixture of these are called racemic mixture
Two isomers with the exact same structural formulas
Chiral carbon has four different substituents
Different mirror images are not superimposable
Therefore, NOT the same moleculez

11. Stereoisomers: Optical Isomers
Diastereomers are a type of isomer where some chiral centers have different configurations (but not all chiral centers)
Therefore, not mirror images of each other

12. Stereoisomers: Optical Isomers
Properties
Diastereomers usually differ in physical/chemical properties
Enantiomers have the same physical/chemical properties EXCEPT:
Optically different
Reactivity with other chiral carbons

13. Stereoisomers: Optical Isomers
Enantiomer differences: optically different
Polarimeter is an instrument to determine the manner in which an optical isomer rotates light
One enantiomer will rotate the light one direction, the other will rotate the opposite direction
A racemic mixture (equal amounts of both isomers) will effectively rotate both evenly, so the cancel each other and not by optically active

14. Stereoisomers: Optical Isomers
Enantiomer differences: RXTs w/ chiral molecules
If a racemic mixture of a compound RXTs w/ 1 enantiomer of a different compound
Different products produced
Different properties, so can be separated
This is called resolution