Chapter 5: Stereoisomers Flashcards
What is an isomer?
Molecule with the same molecular formula, and a different structure
What is a constitutional isomer?
Molecule with the same molecular formula, but different connectivity
What is a stereoisomer?
Molecule with a different orientation in space
What are the types of stereoisomers?
- cis and trans
- conformations (rotating around a sigma bond)
- mirror image isomers (enantiomers and diastereomers)
Achiral
Identical (superimposable) to its mirror image
Chiral
Nonsuperimposable to its mirror image
Enantiomers
Each isomer of a mirror image
Stereocenter
Source of chirality (carbon w 4 different substituents)
Diastereomer
Nonsuperimposable non-mirror image
Formula for number of possible stereoisomers
2^x (# of stereocenters)
Something that is optically active Is …
Chiral
Enantiomers have identical physical properties, such as…
boiling point, melting point, density
How do we tell enantiomers apart?
Plane polarized light, performed by a polarimeter
What is the specific notation for optical rotation?
[d]
How do enantiomers rotate light?
In equal and opposite directions
(+) Detrorotatory
Right/clockwise
(-) Levoratatory
Left/counter clockwise
What does optical rotation tell us?
How pure our sample is (optical purity)
What is produced when no optical rotation is present?
50-50 mix of enantiomers; known as a racemic mix
If one enantiomer is present, it is referred to as …
Enantiopure
What is it called if enantiomer is in excess (ee)?
Enantiomeric Excess (xs)
There is no correlation between optical rotation and …
Structure
Rectus
Right or clockwise
Sinister
Left or counterclockwise
What are the rules for determining absolute configuration?
- Assign priority to substituents on chiral center
- Place lowest priority in the back
- Draw an arrow from high to low (1-4)
Cahn Ingold Prelog Rules (CIP)
- Higher priority from higher atomic #
- Tie? Use the first point of difference
- Treat pi bonds as multiple bonds to the same atom
What do enantiomers and diastereomers have in common?
Both are non superimposable to their mirror image
Describe properties of enantiomers
-is the mirror image
-configuration changes
-R to S or S to R
Describe properties of diastereomers
-not the mirror image
-only some invert and some remain constant
Do all chiral molecules have to have a chiral center?
No, we need a 3-D shape that is not the same as its mirror image
Are all molecules w chiral centers optically active?
No, can have some point of symmetry or plane of symmetry
Are all molecules with a specific shape chiral?
No
List some manipulation types
1.) Rotate: hold one thing constant and rotate other 3
2.) Flip over
3.) Turn
