PHARM 201 Final Exam -Questions with Correct
Solutions
An object is chiral if:
- no plane of symmetry
- cannot be superimposed on its mirror image
- at least one asymmetric atom (ex. carbon)
A molecule is chiral if:
It contains chiral carbon centers.
chiral atoms need to attach to ___ different atoms
4
A carbon atom with 3 different heteroatoms attached to it cannot be chiral because:
When the mirror image is flipped 180 degrees, it ends up being the same molecule (can be
superimposed).
Apart from carbon, these other 3 asymmetric molecules can be chiral:
N, P, S
pro-chiral drug
a drug with non-chiral centers that can be transformed into chiral centers (ex. by addition of
atoms to molecule).
Carbons that contain double bonds are usually not:
,chiral
(fewer than 4 groups, lone pair constantly switching)
Stereoisomers
Compounds possessing the same molecular formula, but differ only in the configuration of at
least 1 chiral center.
Two types of stereoisomers
1. Enantiomers
2. Diastereomers
Enantiomers
Two compounds that are non-superimposable mirror images of one another.
Enantiomers must contain at least one chiral center as part of their structure.
(non-superimposable = more than 3 different groups = chiral)
In enantiomers, the stereocenters become:
flipped (ex. dashed line hydrogen instead of solid line hydrogen)
The chemical and physical properties of enantiomers are:
identical.
Enantiomers differ solely in:
The direction in which they rotate plane polarized light.
+ isomer (dextrorotary)
,The enantiomer that rotates plane polarized light clockwise (to the right).
- isomer (laevorotary)
The enantiomer that rotates plane polarized light counterclockwise (to the left).
(Both + and - isomers do not provide information about activity -> only direction of light.
Cannot determine + or - just by looking at structure.)
D/L configuration
Look at the highest numbered chiral carbon.
-OH on right = D-sugars, -OH on left = L-sugars.
Mainly used for amino acids and sugars.
There is no direct relationship between (+)(-) designation and either:
D/L designation or R/S designation.
A complete stereochemical designation requires both:
(+)(-), as well as D/L or R/S.
The atom with the lowest priority is rotated ____________ from the viewer. Most of the
time, this group will be:
away, hydrogen
S is (clockwise/counterclockwise), whereas R is (clockwise/counterclockwise).
, counterclockwise, clockwise
(based on priorities)
Atoms ordered, based on priorities:
I (highest atomic number)
Br
Cl
S
P
F
O
N
C
H (lowest atomic number)
What is the rule in assigning priorities with double bonds?
A double-bonded carbon will take priority over a single bonded carbon. (double bonds are
duplicated, meaning two carbons count instead of one).
Triple bonds have an even stronger priority.
Swap rule
If the hydrogen is in the front instead of the back, assign priorities like you normally would then
switch the R with the S, or vice versa.
Solutions
An object is chiral if:
- no plane of symmetry
- cannot be superimposed on its mirror image
- at least one asymmetric atom (ex. carbon)
A molecule is chiral if:
It contains chiral carbon centers.
chiral atoms need to attach to ___ different atoms
4
A carbon atom with 3 different heteroatoms attached to it cannot be chiral because:
When the mirror image is flipped 180 degrees, it ends up being the same molecule (can be
superimposed).
Apart from carbon, these other 3 asymmetric molecules can be chiral:
N, P, S
pro-chiral drug
a drug with non-chiral centers that can be transformed into chiral centers (ex. by addition of
atoms to molecule).
Carbons that contain double bonds are usually not:
,chiral
(fewer than 4 groups, lone pair constantly switching)
Stereoisomers
Compounds possessing the same molecular formula, but differ only in the configuration of at
least 1 chiral center.
Two types of stereoisomers
1. Enantiomers
2. Diastereomers
Enantiomers
Two compounds that are non-superimposable mirror images of one another.
Enantiomers must contain at least one chiral center as part of their structure.
(non-superimposable = more than 3 different groups = chiral)
In enantiomers, the stereocenters become:
flipped (ex. dashed line hydrogen instead of solid line hydrogen)
The chemical and physical properties of enantiomers are:
identical.
Enantiomers differ solely in:
The direction in which they rotate plane polarized light.
+ isomer (dextrorotary)
,The enantiomer that rotates plane polarized light clockwise (to the right).
- isomer (laevorotary)
The enantiomer that rotates plane polarized light counterclockwise (to the left).
(Both + and - isomers do not provide information about activity -> only direction of light.
Cannot determine + or - just by looking at structure.)
D/L configuration
Look at the highest numbered chiral carbon.
-OH on right = D-sugars, -OH on left = L-sugars.
Mainly used for amino acids and sugars.
There is no direct relationship between (+)(-) designation and either:
D/L designation or R/S designation.
A complete stereochemical designation requires both:
(+)(-), as well as D/L or R/S.
The atom with the lowest priority is rotated ____________ from the viewer. Most of the
time, this group will be:
away, hydrogen
S is (clockwise/counterclockwise), whereas R is (clockwise/counterclockwise).
, counterclockwise, clockwise
(based on priorities)
Atoms ordered, based on priorities:
I (highest atomic number)
Br
Cl
S
P
F
O
N
C
H (lowest atomic number)
What is the rule in assigning priorities with double bonds?
A double-bonded carbon will take priority over a single bonded carbon. (double bonds are
duplicated, meaning two carbons count instead of one).
Triple bonds have an even stronger priority.
Swap rule
If the hydrogen is in the front instead of the back, assign priorities like you normally would then
switch the R with the S, or vice versa.
