2.2 Configurational Isomers Flashcards
Configurational isomers
can only change from on form to another by breaking and reforming covalent bonds
2 categories of configurational isomers
enantiomers & diastereomers
(both are “optical isomers”)
optical isomers
compounds with different spatial arrangement of groups which affects the rotation of plane-polarized light
chiral compounds
a compound whose mirror image cannot be superimposed on the original compound
lack an internal plane of symmetry
achiral compounds
have mirror images that CAN be superimposed
chiral centers
carbon atoms with 4 different constituents (these are chiral carbons)
characteristic of ALL chiral carbons
have 4 DIFFERENT groups connected
enantiomers (4)
2 molecules that are nonsuperimposable mirror images of each other
same connectivity but opposite configurations at EVERY chiral center
same physical and chemical properties (except optical activity and reactions in chiral environments)
rotate plane polarized light to the same magnitude but opposite directions
diastereomers
chiral molecules which have the same connectivity but are NOT mirror images of each other
must have multiple chiral centers (and differ at some but not all)
have different chemical properties
optically active compounds
can rotate plane-polarized light
optical activity
the rotation of plane polarized light by a chiral molecule
dextrorotatory (d-) compounds
rotate plane-polarized light to the right (clockwise)
labelled (+)
levorotatory (l-) compounds
rotate plane-polarized light to the left (counterclockwise)
labelled (-)
how is d- vs l- determined
EXPERIMENTALLY!!!
it can NOT be determined from a molecules structure
it is NOT related to a molecules absolute configuration (R vs S)
specific rotation formula
standard concentration and path length for determining optical activity
concentration = 1 g/ml
path length = 1 dm (10 cm)
racemic mixture
a mixture with equal concentrations of + and - enantiomers
no optical activity is observed (they “cancel” each other out)
will NOT rotate plane polarized light
number of possible stereoisomers
for a molecule with n chiral centers
a molecule with n chiral centers has 2^n stereoisomers
ex. I and II are enantiomers, III and IV are enantiomers, the other pairs are diastereomers of one another
cis-trans isomers (geometric isomers)
a subtype of diastereomers
substituents differ in their position around an immovable bond (ex. double bond) or a ring
cis: substituents are on the same side of the bond/ring
trans: substituents are on the opposite side of the bond/ring
nomenclature used for polysubstitued double bonds
E / Z
meso compounds
molecules that have chiral centers but are NOT optically active due to a plane of symmetry within the molecule
it IS superimposable on its mirror image
