Ch. 5 – Stereochemistry Flashcards
What is stereochemistry?
3D structure of a molecule
What is a polymer?
large molecule composed of repeating smaller units (monomers) bonded covalently
What is starch?
main carbs of seeds, roots of plants
hydrolyzes to form glucose
(isomer of cellulose)
What is cellulose?
gives rigidity to stems/trunks
nature’s most abundant organic chemical
(isomer of starch)
How are starch and cellulose related?
isomers, differ in the position of the O atom that joins rings together
cellulose: O joins rings with 2 equatorial bonds
starch: O joins rings with 1 eq, 1 axial. bond
What are isomers?
different compounds with same chemical formula
What are stereoisomers?
different compounds, differ only in 3D space
Does a different 3D structure cause different properities?
YES
What are constitutional isomers?
differ in atom connectivities (also called structural isomers)
Do constitutional isomers have the same functional groups?
can be the same or different, usually different
Do constitutional isomer have same physical properties? Can they separated by physical techniques?
different phys properties so separable by phys techniques
Do constitutional isomers have the same or different chemical properties?
different
Do constitutional isomers have the same or different IUPAC names?
different
Do stereoisomers have the same or diff IUPAC names?
same (except for cis/trans prefixes)
Do stereoisomers have the same or diff functional groups?
same
What is a configuration?
a particular 3D arrangement
What does it mean to superimpose something?
align all parts of an object with its mirror image
What is a chiral molecule?
NOT superimposable on its mirror image
usually has stereogenic center and no plane of symmetry
What is an achiral molecule?
IS superimposable on its mirror image
usually has NO stereogenic center but HAS a plane of symmetry
How is chirality tested?
Draw 3D molecule
Draw mirror image
Try to align all bonds and atoms by rotation only (no breaking bonds)
What are enantiomers?
mirror image molecules that are NOT superimposable
What is a stereogenic center?
(for tetrahedral)
carbon atom with four different groups
- found in most chiral molecules
- interchange of 2 groups converts one enantiomer to another
If there is no st. center, the molecule is usually…
achiral
If there is one st. center, the molecule is…
always chiral
If there is two or more st. centers, the molecule…
might be chiral
What is a plane of symmetry?
mirror plane that cuts molecule in half
- achiral molecules usually have a plane of symmetry
T/F: Always omit all C atoms that can’t be tetrahedral (sp, sp2) when determining st. centers.
True, st. centers must be on sp3 carbons
How is an enantiomer pair drawn?
Place two bonds on a plane, one forward, one back
Arbitrarily place groups
Draw mirror image
OR
keep everything the same by convert wedges to dashes and vice versa
T/F: For any molecule with one tetrahedral st. center, it is a chiral compound and there exists a pair of enantiomers.
True
When labelling st. centers R and S, what is R and what is S?
R = clockwise S = counterclockwise
How do you label st. centers R and S?
1) Assign priorities 1-4 to atoms directly bonded to st. center in order of decreasing atomic number
2) Orient molecule with lowest priority group towards the back
3) Trace a circle from priority group 1-4 (if clockwise, R; if counterclockwise, S)
If two atoms on the same st. center are the same, what should be done to assign priority?
assign priority based on the atomic number of atom bonded to that atom or continue along a chain until a point of difference is reached
If 2 atoms bonded to same st. center are isotopes, what should be done?
assign priority in order of decreasing mass number
If an atom connected to a st. center is part of a multiple bond, how should priorities be assigned?
treat multiple bonded atom as = # of single bonded atoms
ex C=O is the same is C-O and C-O
T/F: A molecule with two or more stereogenic centers is always chiral/
false, it may or may not be chiral
For n number of stereogenic centers, what is the max number of stereoisomers?
2^n
n = 1, max = 2
n = 2, max = 4
How do we draw all possible stereoisomers for a compound with 2 st. centers?
1) Draw one stereoisomer by arbitrarily arranging subs around st. center
2) Draw mirror image (if chiral, need to draw two more)
3) Draw third possible isomer by switching positions of any two groups for ONE st. center
4) Draw mirror image
What are diastereomers?
stereoisomers that are NOT mirror images of eachother
What is R and S for two identical compounds?
same R and S designations
What is R and S for two enantiomers?
R and S are exactly opposite
What is R and S for two diastereomers?
same R and S designation for at least one st. center
opposite R and S designation another st. center
What is a meso compound?
achiral compound that contains tetrahedral stereogenic centers
- has plane of symmetry
- identical isomers from enantiomers (C+D are identical but came from enantiomeric A and B)
- can rotate sigma bonds to see if molecule has symmetry
When does a meso compound occur?
when molecule with 2 st. centers has less than 4 stereoisomers (2 enantiomers, 2 identical)
T/F: Meso compounds usually have a plane of symmetry?
true
How do we draw all stereoisomers for a cycloalkane?
1) Draw mirror image of 3D isomer
2) Draw trans isomer (switch one dash/wedge)
3) Draw mirror image again
Are the chemical and physical properties of two enantiomers the same or difference?
the same (MP, BP, solubility), except when they react with chiral substances and how they react with plane-polarized light
What is plane-polarized light?
light that passes through a plane polarizer resulting in only one plane of light to come through
What is a polarimeter?
instrument that allows polarized light to travel through a sample tube that has slit that is rotated it determine plane direction
Are achiral compounds optically active?
no, light is unchanged
Are chiral compounds optically active?
yes, plane of polarized light is rotated through angle alpha
What is angle alpha?
(º) observed roation
What does it mean if rotation is dextrorotatory?
light is rotated clockwise (labeled d or +)
What does it mean if rotation is levorotatory?
light is rotated counterclockwise (labeled l or -)
What rotation is + ?
clockwise, dextrorotatory
What rotation is - ?
counterclockwise, levorotatory
How to do two enantiomers rotate light?
equal in magnitude, opposite in direction
What is a racemic mixture?
equal amount of two enantiomers; optically inactive
What is the boiling point or melting point of a racemic mixture containing A and B?
may be different from A and B
What is the optical rotation of a racemic mixture containing A and B?
0º
What is the optical rotation of A alone with respect to B?
equal mag, opposite direction
What does observed rotation depend on?
number of chiral molecules interacting with polarized light, concentration of sample, length of sample tube
What is the equation for specific ration [𝜶]?
[𝜶] = 𝜶 / (l • c) 𝜶 = observed rotation (º) l = length of sample tube (dm) c = concentration (g/mL)
T/F: Specific rotation is a physical constant.
true
What is enantiomeric excess (ee)?
tells how much more of one enantiomer there is in a mixture (in excess of racemic mixture)
also known as “optical purity”
What is the equation for enantiomeric excess (ee)? (in terms of %)
ee = % of one enantiomer - % of other enantiomer
What is enantiomeric excess (ee) equation? (in terms of specific rotation)
ee = ( [𝜶] mixture / [𝜶] pure enantiomer) • 100%
Are the physical properties of diastereomers the same or different?
different, including optical rotation (not mirror images)
Are the enantiomers able to be separated by physical techniques?
no, same physical properties
Are diastereomers and const. isomers able to be separated by common physical techniques?
yes, different physical properties
Do two enantiomers have the same chemical properties?
yes, except for reaction with chiral, non-racemic reagents
When are E and Z prefixes used?
for alkenes with greater than or equal to 3 substituents
How are E and Z prefixes assigned?
1) assign priorities to two sub groups at end of C=C using same priority rules as for R and S
2) Assign E or Z based on which sides have higher priority
When is E assigned to an alkene carbon?
when higher priority groups are on opposite sides
When is Z assigned to an alkene carbon?
when higher priority groups are on same side
What are Fischer projections?
“cross formula”
tetrahedron is tipped so horizontal bonds come forward and vertical bonds go behind (wedges are horiz, dashes are vertical)
How is R and S assigned for Fischer projections?
if lowest priority is vertical, then R = clockwise and S = counterclockwise
if lowest priority is horizontal, then R = counterclockwise and S = clockwise
