Stereochemistry: 3D Structures of Molecules Flashcards
Isomers
Different compounds with the same molecular formula.
Constitutional Isomer
Isomer with different atomic connectivity
Stereoisomers
Isomers with the same atomic connectivity but with different 3D arrangements
Chirality
is the ability of objects to exist as non-superposable mirror images of each other
Often originated from an atom that is connected to 4 different substituents
Objects that contain a mirror plane are
achiral
Objects that do not have a mirror plane are
chiral
Achiral
Can be superimposed with its mirror image
Chiral
Cannot be superimposed with its mirror image
Enantiomers
Chiral molecules and their non-superimposable mirror images
Stereogenic center
Chiral center or an atom that is connected to 4 other substituents
Enantiomers physical properties
They are identical physical properties including MP, BP, IR, NMR, density, etc.
Can be differentiated from each other;
- Interactions with other chiral molecules (ex. biological receptor sites only accept 1 enantiomer)
- Optical activity (interactions with polarized light). Means that they rotate plane-polarized light.
How do we test for enantiomers with polar light?
A light source is shown through a polarizing filter (ex. the polarized light is vertical).
Then this polarized light is shown through a sample of pure enantiomers and as it bounces of and hits these molecules it begins to rotate.
The angle of this rotation, denotated as α (aka optical rotation), can be measured.
How do we classify enantiomers?
Enantiomers polarize light by the same angle but in opposite directions (+ α and - α)
Optical Rotation α
the angle that a sample rotates plane-polarized light.
Dextrorotary
α is greater than 0 and so is positive
The light is rotated clockwise
Levorotary
α is less than 0 and so is negative
The light is rotated counterclockwise
Racemic Mixture
A 1:1 mixture of 2 enantiomers
Does not rotate polarized light, because the rotations are cancelled out
Absolute Configuration
a method of indicating the permanent arrangement of groups attached to the chiral center.
Denoted as either R or S
Determining whether a center is R or S
- Identify the chirality center
- Determine the priorities of attached groups (Cahn-Ingold-Prelog Rules)
- Rotate molecule to put the lowest priority group (4) at the back
- Determine the direction of the priority group numbers 1, 2, and 3
R
Clockwise
S
Counterclockwise
CIP (Cahn-Ingold-Prelog) Rules
Help assign priority to groups attached to a chiral center based on atoms directly attached to the center
*Higher Atomic number = Higher Priority
Diastereomers
Stereoisomers that are non-superimposable, non-mirror images
Difference between enantiomers and diastereomers
Changing all stereocenters (and substituents) at once give you enantiomers.
Changing some stereocenters gives you diastereomers
Diastereomers physical properties
Diastereomers have different physical and chemical properties (including BP, MP, IR, NMR, solubility, etc.)
How do you find the maximum number of stereoisomers
2^n
Where n = number of chirality centers
Note: the number of stereoisomers will be lower if molecules have symmetry elements
Meso Compounds
Molecules that have chirality centers but are achiral due to an internal mirror plane
The molecule and its mirror image are superimposable (so achiral)
Double bond Isomerism
If the two highest priority groups are opposite to one another (trans), the double bond is designated E (entgegen, German for “opposite”).
If the two highest priority groups are on the same side of the double bond (cis), the double bond has a Z configuration (zusammen, German for “together”).
