Isomers Flashcards
Isomer Definition
compound of same molecular formula, but different molecular structures
Structural/Constitutional Isomers
only thing they share is molecular formula/weights
least similar of all isomers as they have different chemical and physical properties
Same Connectivity? Yes/No
Yes -> Stereoisomer
No -> Structural Isomer
Require Bond Breaking to interconvert? Yes/No
Yes: Configurational
No: Conformational
Nonsuperimpossible mirror images? Yes/No
Yes: Enantiomer
No: Diasteriomer
Physical Properties
characteristics that don’t change composition of matter
melting, boiling points, solubility, odor, color, density
Chemical Properties
reactivity of molecule with other molecules resulting in changes in chemical composition
Stereoisomers
share same molecular formula/weights, but also share same connectivity/molecular backbone
Conformational Isomer vs Configurational Isomer
ConFOR: differ in roation around single sigma bonds
ConFI: can be interconverted only by breaking bonds
Of all isomers ___ are the most similar
Conformational Isomers
Conformational Isomer
they are same molecule but different points in their natural rotation around a single bond
Staggered Conformation
lowest energy state, no steric repulsion
Anti Conformation
for staggered where two largest groups are antiperiplanar (same plane, opposite sides)
Gauche Conformation
staggered conformation where the two largest groups are 60 degrees apart
Eclipsed Conformation
staggered with 120 degrees
Totally Eclipsed Conformation
staggered with 0 degrees, highest energy form, least favorable as largest groups are synperiplanar (next to each other on same plane)
The higher the energy of the conformation the…
less time it will stay int he conformation
Conformational transitions at Room/Low Temp
Room: easily overturned
Low: interconversion occurs slowly due to lack of energy
Ring Strain
Cycloalkanes due to 3 reasons:
angle strain, torsional strain and steric strain
Angle Strain
bond angles deviate from what they should be
Torsional Strain
cyclic molecules take gauche or eclipsed interactions
Steric/van Der Waals Strain/Repulsion
nonadjacent atoms/groups compete for space
common in flagpole interactions for steric strain
Cyclohexane Conformations
Boat, twist and chair
Chair Conformation
most stable, lowers all 3 strains for cyclohexane
Axial
atoms/hydrogens standing perpendicular to plane of ring (up and down)
Equatorial
atoms/hydrogens sticking out parrallel (outwards) from thr ring
Chair Flip bonds…
equatorial and axial flip but wedges stay wedges and dashes stay dashes
Interconversions can be slowed by…
Bulky groups!!
Bulky Groups in Cyclohexanes
tert-butyls for example
favor equatorial to reduce nonbonded strain
Cis
bulky groups are on same side of ring
Trans
bulky groups on opposite sides of rings
Configurational Isomer
only change form by breaking and reforming covalent bonds
Optical isomers
enantiomers and diasteriomers
affect rotation of plane-polarized light
Chiral
if its mirror image cannot be superimposed on the original object; lacks internal plane of symmetry
Achiral
mirror image can be superimposed
Chiral center
4 different substituents on a carbon atom
Enantiomers
two molecules that are nonsumerimposible mirror images of one another
same connectivity but opposite configurations at every chiral center
HAVE SAME PHYSICAL AND CHEMICAL PROPERTIES
only difference: optical activity and reactions in chiral enviornments
Diasteriomer
share same connectivity and molecules are chiral but not mirror images
DIFFERENT PHYSICAL AND CHEMICAL PROPERTIES
often because they differ at some of their chiral centers
Optically Active Compound
if it can rotate plane-polarized light
Ordinary light is__
unpolarized
At the molecular level, one enantiomer will rotate plane-polarized light to the same magnitude but ___ direction of its mirror image
opposite
Dextrorotary (+)
compound that rotates polarized light to the right
Levorotatory (-)
compound that rotates polarized light to the left
Amount of light rotation depends on…and depends on…
number of molecules a light wave encounters
concentration of compound and length of tube which light passes
Standard Conditions For Optical Measurement
1dm (10 cm) and 1 g/ml concentration
Specific Rotation Equation
[alpha] = [alphaobs]/(c*l)
alphaobs is rotation observed , c is concentration, l is 1 dm
Racemic Mixture
Both + and - held in equal concentrations ro make no optical activity observed
Separating Enantiomers
react with a single enantiomer to produce 2 diasteriomers which are physically different by nature and use lab techniques such as crystallization, filteration, etc. to seperate the diasteriomers and bring back original enantiomers
For any molecule with n number chiral centers…
there are 2^n possible steroisomers
Diasteriomers can rotate plane-polarized light but can’t tell…
the rotations of the other diasteriomer
Cis-Trans Isomer
type of diasteriomer
substituents differ in position around an immovable bond (double/cyclic)
Meso Compound
contains plan of symetry causing it to lose optical properties even if it contains chiral centers
equivalent of meso compounds
Relative/Absolute Configuration
R: configuration of one chiral molecule in relation to another
A: exact spatial orientation of atoms and groups independent of other molecules
Cahn-Ingold-Prelog Priority Rules
priority of E/Z is assigned based on the atom bonded to the double bonded carbons: the higher the atomic number, the higher the priority
**if atomic numbers are similar, compare the next atom
E vs Z
E: when highest priority substituent to double bond on each side faces different direction
z: when highest priority substituent to double bond on each side faces the same direction
R and S Form Procedure
1) Assign Priority
2) Invert The Stereochemistry
3) Draw a circle
4) Write the name
RS Assigning Priority
highest atomic number gets priority over the lowest one
RS: Invert the Stereochemistry
if lowest priority substituent isn’t in the back, switch it to the back with the other substituent and remember to switch RS for our final answer
RS: Draw a Circle
Draw a circle going from 1->2->3
if CLOCKWISE it is R (r is right!)
if COUNTERCLOCKWISE it is S
RS: Write the name
place R or S in parenthesis and then a hypen to the name
ex: (R)-
if multiple chiral centers, remove the hyphen and location number is within parenthesis before the R/S
ex: (3R) ….
Fisher Projection: Horizontal Lines
wedges, project OUT of plane
Fisher Projection: Vertical Lines
dashes, project INTO the plane
Assigning Priority to Fisher projection
same rules as with RS before, just note rotating 90 degrees leads to changing of R/S
**note by rotating two pairs 90 degrees to let lowest go into the plane, you don’t have to change R/S, original stereochemistry is kept!
Assigning Priority to Fisher Projection: 0 Switches
ignore lowest priority, give numbers, make circle, write the opposite configuration
Assigning Priority to Fisher Projection: 1 Switches
swap lowest priority group with vertical group, make circle, write the opposite configuration
Assigning Priority to Fisher Projection: 2 Swtiches
swap lowest priority group to vertical, swap the remaining two, make circle, take the configuration found
