Isomers Flashcards
Structural isomers (constitutional)
- Same molecular formula, different structure
- different chemical and physical properties
Physical properties
Don’t change the composition of matter, such as melting point, boiling point, solubility, odor, color, and density
Chemical properties
Have to do with reactivity of the molecules and result in changes in chemical composition. Generally dictated by functional groups
Stereoisomers
Have same structural backbone but differ in how atoms are arranged in space.
For any molecule with n chiral centers, there are 2n possible stereoisomers.
Conformational isomers
- Are the same molecule but just differ at points in their natural rotation around single bonds
Straight chain conformations
- anti and gauche conformers
- Favors most stable conformer
Cyclic conformations
- Stability depends on ring strains: angle strain, torsional strain, steric strain
cyclobutane: puckered (v-shaped)
cyclopentane: envelope
cyclohexane: chair, boat, twist-boat
Angle Strain
Results when bond angles deviate from their ideal values by being stretched or compressed.
Torsional Strain
Results when cyclic molecules must assume conformations that have eclipsed or gauche interactions.
Steric strain
Results when nonadjacent atoms or groups compete for the same space
Configuration isomers
Can only change from one form to another by breaking and reforming covalent bonds.
- entantiomers and diastereomers
chiral
- mirror image cannot be superimposed on the original object
- NO plane of symmetry
- 4 different substituents bound to central carbon
enantiomers
nonsuperimposable mirror images
Diastereomers
- These molecules are chiral and share the same connectivity, but are not mirror images of each other.
- Differ at some, but not all stereogenic centers
Racemic mixture
- When both enantiomers are in equal conecntraions.
- the rotations of plane-polarized light cancels out and no optical activity is observed
Reacting two enantiomers with a single enantiomer of antoehr compound will, by definition, lead to two diastereomers.
Cis-trans isomers
Type of diasteromers in which substituents differ in their position around an immovable bond, such as a double bond, or around a ring structure.
Meso compound
A molecule with chiral centers that has an internal plane of symmetry.
- molecular equivalent of a racemic mixture
Relative configuration
the configuration of a chiral molecule in relation to another chiral molecule.
- Use to determine if molecules are enantiomers, diastereomers, or the same molecule
Absolute conformation
describes teh exact spatial arrangement of these atoms or groups, independent of other molecules
E & Z
Used for compounds with polysubstituted double bonds. Priority is assigned based on the atom bonded to double-bonded carbons
- Higher the atomic number, the higher the priority
R and S
Used for chiral (stereogenic) centers in molecules.
- Assign priority; highest atomic number
- Orient so lowest priority is in back
- Or swap group facing back with lowest priority
- Any time two groups are switched on a chiral carbon, the stereochemistry is inverted.
- Draw a circle
- Write the name
Finding R & S in Fischer Projections
- Move the lowest-priority group into the correct position (facing away)
- Switch the other two groups as well.
- molecule will have same designation as the initial molecule.
