Structural Isomers
-least similar of all isomers
-also called constitutional isomers
-only share molecular formula (molecular weights are equivalent)
-Otherwise completely different molecules, with different chemical and physical properties
Physical Properties
-characteristics of processes that don't change the composition of matter
i.e: melting point, boiling point, solubility
Chemical Properties
-determine how the molecule reacts with other molecules
Stereoisomers
-have the same chemical formula
-same atomic connectivity
-only difference among stereoisomers is how the atoms are arranged in space.
Conformational Isomers
-also called "conformers"
-most similar
-same molecules, only at different points in their natural rotation
Newman Projection
-the line of sight extends along a carbon-carbon bond axis
Staggered Conformation
-No overlap of atoms along the line of sight
Anti Staggered Conformation
-most favorable type of staggered conformation
-two groups are antiperiplanar to eachother
Gauche Staggered Conformation
-occurs when the two groups are 60* apart.
-higher energy conformation
Eclipsed Conformation
-When the atoms overlap down the line of sight
-When the two groups overlap it is called "totally eclipsed".
Ring Strain
-Unstable conditions
-Arises from three factors:
1. Angle Strain
2. Torsional Strain
3. Nonbonded Strain (steric strain)
Angle Strain
-results when bond angles deviate from their ideal values
Torsional Strain
-results when cyclic molecules must assume conformations that have eclipsed interactions.
Nonbonded Strain
-results when nonadjacent atoms or groups compete for the same space.
-Dominant source of energy in flagpole interactions of the boat conformation.
Chair Conformation
Boat Conformation
Axial
-Perpendicular to the plane of the ring
Equatorial
-parallel to the plane of the ring
Ring Flip
-atoms/groups that were equatorial become axial and vice versa
Configurational Isomers
-Can only change from one form to another by breaking and reforming covalent bonds
-Two categories:
1. geometric isomers
2. optical isomers
Geometric Isomers
-compounds that differ in the position of substituents attached to a double bond or cycloalkane
Cis
-If substituents on the double bond are on the same side
Trans
-If the two substituents on the double bond are on opposite sides.
Z (zusammen)
-If the two highest-priority substituents on each carbon are on the same side of the double bond
E (entgegen)
-if the two highest-priority substituents on each carbon are on the opposite side of the double bond
Chiral Molecules
-a molecule that is not super-imposable upon its mirror image or lacks an internal plane of symmetry
Achiral Molecules
-have mirror images that CAN be superimposed
Enantiomers
-Nonsuperimposable mirror image molecules
-opposite configuration at all chiral centers
-identical physics properties
-most of the same chemical properties
exceptions: optical activity and how they react in chiral environments
Meso compound
-A molecule with a chiral center has a plane of symmetry
Configuration
-The spatial arrangement of the atoms or groups of a stereoisomer
Relative Configuration
-a chiral molecule's configuration in relation to another chiral molecule.
Absolute Configuration
-describes the exact spatial arrangement of a chiral molecule's toms or groups, independent of other molecules
R Configuration
-clockwise
-right
-put in parenthesis and separated from the rest of the name by a dash.
S Configuration
-counterclockwise
-left
-put in parenthesis and separated from the rest of the name by a dash.
How to determine the absolute configuration at a single chiral center
1. Assign Priority by atomic #
2. Orient the molecule with the lowest priority substituent in the back.
3. Draw a circle around the molecule from highest to lowest priority
4. Determine R or S
Fischer Projections
-Horizontal lines indicate bonds that project out from the plane of the page
-Vertical lines indicate bonds going into the plane of the page
-point of intersection = carbon atom
Optically Active Compounds
-if it has the ability to rotate plane polarized light.
-One enantiomer will rotate plane-polarized light to the same extent but in the opposite direction of its mirror image
Dextrorotatory
-rotates the plane of polarized light to the right (clockwise)
-(+)
-can only be determined experimentally
Levorotatory
-rotates the plane of polarized light to the left (counterclockwise)
-(-)
-can only be determined experimentally
Specific Rotation
Racemic Mixture
-Rotations cancel each other out and no optical activity is observed.
Diastereomers
-non-mirror-image configurational isomers
-when a molecule has two or more stereogenic centers and differ at some, but NOT all of these centers.
-Therefore, diastereomers are required to have MULTIPLE chiral centers
For any molecule with n chiral centers...
There are 2^n possible stereoisomers.