Isomers Flashcards
structural (constitutional) isomers
share only their molecular formula
physical properties
don’t change the composition of matter; i.e. melting point, boiling point, solubility, odor, and density
chemical properties
have to do with the reactivity of the molecule with others and result in changes in chemical composition
stereoisomers
share the same atomic connectivity; have the same structural backbone
conformational isomers (conformers)
differ in rotation around single bonds
configurational isomers
can be interconverted only by breaking bonds
staggered conformation
no overlap of atoms along the line of sight
anti conformation
the two largest groups are antiperiplanar (in the same plane but on opposite sides)
gauche conformation
occurs when the two largest groups are 60 degrees apart
eclipsed conformation
must pass through this to convert from anti to gauche; two methyl groups are 120 degrees apart and overlap with the hydrogen atoms on the adjacent carbon
totally eclipsed
two methyl groups directly overlap each other; highest-energy state
ring strain
depends on angle strain, torsional strain, and non bonded strain
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
non bonded strain (steric strain)
results when nonadjacent atoms or groups compete for the same space
chair conformation of cyclohexane
most stable because it minimizes all three types of strain
axial hydrogens
perpendicular to the plane of the ring (stick up or down)
equatorial hydrogens
parallel to the plane of the ring (stick out)
chair flip
one chair form is converted to the other; all axial Hs become equatorial and vice versa
conversions with large groups
bulky groups slow conversions; favor the equatorial position to reduce strain
cis
both substituents on the same side
trans
substituents on opposite sides
optical isomers
different spatial arrangement of groups in these molecules affects the rotation of plane-polarized light
chiral
mirror image can not be superimposed on the original object; molecule lacks an internal plane of symmetry
achiral
mirror images that can be superimposed
chiral center
carbon with four different substituents
enantiomers
non superimposable mirror images of each other; same connectivity but differ at every chiral center; identical properties except optical activity and reactions in chiral environments
diastereomers
chiral and share same connectivity but are not mirror images of each other; differ at some but not all chiral centers
optical activity
refers to the rotation of plane-polarized light by a chiral molecule; enantiomers rotate light to the same magnitude but in opposite directions
dextrorotary (d-)
labeled (+); rotates plane of polarized light to the right
levorotatory (l-)
labeled (-); rotates plane of polarized light to the left
specific rotation
(A)=Aobs/c*l A=specific rotation in degrees Aobs= observed rotation in degrees c=concentration in g/ml l=path length in dm
racemic mixture
when both (+) and (-) enantiomers are present in equal concentrations; do not rotate plane polarized light
2^n rule
for any molecule with n chiral centers, it can have 2^n stereoisomers
cis-trans isomers
specific subtype of diastereomers in which substituents differ in their position around an immovable bond, such as a double bond or a ring structure
meso compounds
a molecule with chiral centers that has an internal plane of symmetry; optically inactive
Cahn-Ingold-Prelog priority rules
used to determine E/Z; priority is assigned based on the atom bonded to the double-bonded carbons; the higher the atomic number, the higher the priority
Z alkene
two highest priority substituents are on the same side of the double bond
E alkene
two highest priority substituents are on opposite sides of the double bond
steps to determine absolute configuration at chiral center
- assign priority by atomic number
- Arrange molecule with lowest priority sub in the back (or invert stereochemistry by switching two subunits)
- Draw a circle around the molecule from highest to lowest
- Clockwise =R, Counterclockwise = S
Fischer projection
horizontal lines(wedges)=bonds going out from the plane vertical line(dashes)=bonds going into the plane
