Chapter 2: Isomers Flashcards
Isomers
compounds with the same molecular formula as another, but differ in arrangement or connectivity of their atoms
Structural / constitutional isomers
share only their molecular formula (same atoms)
differ in their structures (connectivity of atoms)
different chemical and physical properties
Physical properties and examples
aspects of a compound that do not play a role in changing chemical composition
ex. melting point, boiling point, solubility, odour, colour, density
Chemical properties and examples
aspects of a compound that change chemical composition
have to do with the reactivity of the molecule with other molecules
dictated by the reactivity of functional groups
in organic chem, the chemical properties of a compound are generally dictated by the ——- in the molecule
functional groups
stereoisomers
same chemical formula AND same connectivity
differ in how these atoms are arranged in space (their wedge and dash pattern)
2 categories of stereoisomers
conformational isomers
configurational isomers
conformational isomers (conformers)
the same molecule, just at different points in their natural rotation around single (sigma) bonds
interconvert by simple bond rotation
recall: single bonds are free to rotate
configurational isomers
have differing connectivity and can only be interconverted by breaking bonds
Newman projections
a drawing that helps visualize the 3-dimensional structure of a molecule
there are drawn looking straight down a carbon-carbon bond
name the 2 conformations of straight chain conformational isomers
staggered and eclipsed
2 types of staggered conformation
anti conformation: the largest groups are opposite (180 degrees apart)
gauche conformation: the largest groups are close together (60 degrees apart)
Eclipsed conformation
this conformation has groups directly in front of each other
totally eclipsed: highest energy state, 2 largest groups are in the same plane on the same side
totally ecliped conformation
the two largest groups are directly in front of each other and strain is at a maximum
explain the potential energy vs degree of rotation plot of butane
higher potential energy = less stable
3 factors that can result in ring strain
angle strain, torsional strain, steric strain (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
steric strain (van der waals repulsion)
results when nonadjacent atoms or groups compete for the same space
ex. this is the main type of strain occuring in flagpole interactions
what is the dominant source of strain in flagpole interactions
non bonded strain / steric strain
what is the most stable conformation of cyclohexane
the chair conformation
what are the 3 most common conformations of cyclohexane
chair
boat
twist (scew-boat)
what occurs during a chair flip
the chair passes through the “half-chair” conformation
all equatorial groups become axial and vice versa
all dashes and wedges _remain the sam_e (components point UP stay pointing UP)
axial vs equatorial
axial (black): group is perpendicular to the plane of the ring (stick up or down)
equatorial (red): parallel to the plane of the ring (stick out)
which position do bulky groups favour in a boat conformation
equatorial
cis vs trans rings
cis: both groups are on the same side of the ring (up/up or down/down)
trans: both groups are on opposite side of the ring (up/down)
how to convert between ring and chair diagrams
wedges = UP
dashes = DOWN
ups alternate from equatorial to axial
drawing bonds for chair conformations
each carbon has an “up” and a “down”
the “up”s and “downs” of each type alternate
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
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
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
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
configuration
refers to the spatial arrangement of the atoms or groups in a molecule
relative configuration
of a chiral molecule
a chiral molecules configuration in relation to another chiral molecule
used to determine whether molecules are enantiomers, diastereomers, or the same molecule
absolute configuration
of a chiral molecul
describes the exact sptial arrangement of the atoms or groups in a chiral molecule, independent of other molecules
when is cis-trans used, and when is E/Z used
cis-trans: when there are only 2 substituents other than H
E/Z: when therea re 3 or more substituents other than H
check…
(E) and (Z) nomenclature is used for ….
compounds with polysubstituted double bonds (multiple substituents that are not just hydrogen)
how to determine (E) vs (Z) form
- find the groups with the highest priority on each side of the double bond
- (Z) if both are on the same side; (E) if both are on oppsite sides
absolute configuration: R and S forms (STEPS)
step 1: assign priority (by atomic number) to 4 substituents surrounding the chiral center
step 2: if the lowest priority group is on a dash, go directly to step 3; if the lowest priority group is on a wedge, the answer is the OPPOSITE of step 3
step 3: draw a circle connecting substituents 1→3 (clockwise = R, counterclockwise = S)
Fischer projections
a method to represent 3D molecules
horizontal lines represent wedges (out of the page)
vertical lines represent dashes (into the page)
