L.3 Stereochemistry Flashcards
ISOMERS MAP
Physical Properties (6) VS Chemical Properties (2)
Physical Properties
- melting point
- boiling point
- solubility
- odor
- color
- density
Chemical Properties
- functional group
- how a molecule reacts with other molecules
Definition of ISOMERS
each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties.
Constitutional
What are Structural Isomers?
- share only molecular formula
- have different physical and chemical properties
Stereoisomers
Conformational Isomers
Different only by a single sigma bond
Newman projections
Cyclic Strain
Chair Conformations
Newman Projections
(Stereoisomers - Conformational Isomers)
A Newman projection, useful in alkane stereochemistry, visualizes the conformation of a chemical bond from front to back, with the front atom represented by a dot and the back carbon as a circle.
Name and define 3 types of cyclic Conformations Strain
(Stereoisomers - Conformational Isomers)
Angle Strain
stretching or comprising angles from their normal size
Torsional Strain
eclipsing conformation
Nonbonded Strain (van der Waals repulsion)
interactions between substituents attached to non-adjacent carbons
Chair conformations
(Stereoisomers - Conformational Isomers)
Cyclic molecules will adopt nonplanar shapes to minimize their strain
Axial; sticking up or down (create more nonbonded strain)
Equatorial; in the plane of the molecule (largest substituents take this place to minimize strain)
Stereoisomers
Configurational Isomers
6 Main Concepts
Can only be interchanged by breaking and reforming bonds
Enantiomers
Diastereomers
Optical Activity
Racemic Mixtures
Cis-trans
Meso Compounds
Diastereomers vs Enantiomers
Diastereomers
- non-mirror images, they differ at some, but not all, chiral centers, they have different physical and chemical properties
- Cis-trans; cis-trans isomers are a subtype of diastereomers in which groups differ in position about an immovable bond
(such as a double bond or cycloalkane)
Enantiomers
- non-superimposable images and thus have opposite stereochemistry at every chiral carbon, have same chemical and physical properties except for rotation of plane-polarized light and reactions in chiral environments.
- Racemic Mixtures, with equal concentrations of enantiomers, will not be optically active because the two enantiomers rotations cancel each other out.
What is a Chiral Center?
have four different groups attached to the central carbon
What does it mean when a molecule has Optical Activity?
How do we annotate this?
The ability of a molecule to rotate plane-polarized light
d- or + rotate light to the right
l- or - rotate light to the left
Relative vs Absolute Configurations
Relative; gives the stereochemistry of a compound in comparison to another molecule
Absolute; gives stereochemistry of a compound without having to compare to other molecules
- uses the Cahn-Ingold-Prelog priority rules; in which priority is given by looking at the atoms connected to the chiral carbon or double bonded carbons; highest atomic number wins.
What are E/Z assignations on an alkene?
An alkene is (Z) if the highest priority substituents are on the same side of the double bond
and (E) if on the opposite.
What are R & S designations on a stereocenter?
Stereocenter
is R if it’s clockwise
It is S if it’s counterclockwise
Fisher Diagrams
- DASHES; into the plane of the paper
- WEDGES; out of the plane of the paper
- Switching one pair of substituents inverts the stereochemistry of the chiral center - Switching two pairs retain the stereochemistry
- Rotating a 90 degrees inverts the stereochemistry - Rotating 180 retains stereochemistry
