CHE2100 Ch. 5
properties of enantiomers
- Same boiling point, melting point, and density - Same refractive index - Rotate the plane of polarized light in the same magnitude, but opposite directions - Different interaction with other chiral molecules ie smell/ tastes
diastereomer characteristics
- different physical properties - easily separated
enantiomerscharacteristics
- difficult to separate - differ only in reaction w/ other chiral molecules
Fischer Rules
- highest oxidized carbon hat the top - rotation of 90 degrees is not allowed bc it changes orientation of the vertical/horizontal groups
maximum number of isomers
2n (n= # of chiral carbons)
meso compound
A compound with chirality centers and an internal plane of symmetry causing it to be an achiral molecule
is called a racemic mixture and is optically inactive
A mixture of equal amounts of two enantiomers _
is called a racemic mixture and is optically inactive
A mixture of equal amounts of two enantiomers __________.
achiral
A molecule that is superimposable on its mirror image
The rotation of plane-polarized light by chiral molecules
A polarimeter is used to measure:
Allenes chirality
Allenes are chiral even though there is no sp3 carbon
biological discrimination
Biological systems commonly distinguish between enantiomers, and two enantiomers may have totally different biological properties. ie epinephrine
Constitutional Isomer
Compounds with the same molecular formula that differ in the order in which their atoms are bonded together but same bonding
cis-trans isomers (geometric isomers)
Diastereomers that differ in their cis-trans arrangement on a ring or double bond
5
How many chiral carbons are found in the structure of the vitamin shown below?
-18.5°
If (S)-alanine has a specific rotation of +18.5°, what is the specific rotation of (R)-alanine?
0.0°
If (S)-alanine has a specific rotation of +18.5°, what is the specific rotation of a racemic mixture of both enantiomers of alanine ?
breaks Cl2 into two Cl radicals
Light is required in the free radical chlorination reaction because it:
Fischer projection rule for H
Lowest priority (usually ) H comes forward
R and S configuration
S= counterclockwise R= clockwise Flip if 4 is NOT in the back
the enantiomer of the original
The mirror image of a chiral molecule is:
all of these
The property of chirality means that a molecule: a. is different than its mirror image b. does not have a plane of symmetry c. will rotate plane-polarized light d. all of these e. has a non-superimposable mirror image
Isomers
Two different molecules that have the same chemical formula
diastereomers
Two structures have the same IUPAC name except that one begins with (3R, 4S, 5R)- while the other begins with ((3R, 4R, 5S)-. What is the relationship between these molecules?
achiral meso compound
What description can be correctly applied to the molecule below?
R
What is the configuration for the chiral carbon whose sterochemistry is indicated in the molecule below?
R
What is the correct configuration for the chiral carbon found in the molecule below?
S
What is the correct configuration for the chiral carbon in the molecule below?
enantiomers
What is the relationship between the molecules shown below?
same molecule
What is the relationship between the two structures shown below?
chromatography
What method(s) below can be used to effectively separate enantiomers?
diastereomers
What term describes the structural relationship between cis-1,2-dimethylcyclopentane and trans-1,2-dimethylcyclopentane?
molecule II has free rotation of the C-C bond between rings; molecule I does not
When samples of molecules I and II below were analyzed in a polarimeter, only molecule I showed optical rotation. Which statement below offers a reasonable explanation?
diastereomers
Which of the following terms best describes the stereochemical relationship of the two compounds shown below in Fischer notation?
III
Which of the structures I-IV matches the structure in the Fischer Projection below?
all of these
Which of these are true for a meso compound? a. has 2 or more chiral carbons b. all of these c. mirror image is identical d. achiral e. has plane of symmetry
III
Which pair of similar molecules below are NOT diastereomers?
It will rotate plane-polarized light.
Which statement is true regarding a chiral molecule?
achiral (meso)
Which term best describes the chirality of the molecule below?
mixture of diasteromers
Which type(s) of mixture can be easily separated based on physical properties?
specific rotation
a = observed rotation / concentration (g/ml) * length (dm)
chiral carbon atom
a carbon atom bonded to four different groups is chiral
achiral
a molecule that has a plane of symmetry is
stereocenter
an atom, most commonly carbon, about which exchange of two groups produces a different stereoisomer
Polarimeter
an instrument that measures the rotation of plane-polarized light by an optically active compound
2n rule does not apply to
compounds w/ plane of symmetry
chiral
different from its mirror image (nonsuperimposable)
Fischer projection
flat representation of a 3-d molecule horizontal lines are forward vertical lines are behind
Stereoisomers
isomers that differ only in how their atoms are oriented in space
polarized light
light that vibrates in only one direction
optical purity equation
o.p= (observed rotation/rotation of pure enantiomer) x 100
Louis Pasteur
physically separated enantiomeric cysterals of salt (aka first artificial resolution of enantiomers)
optical purity
ratio of actual rotation to rotation of pure sample (o.p) or (e.e) which stands for enantiomeric excess
dextrorotatory
rotates plane-polarized light in a clockwise direction (+)
levorotatory
rotates plane-polarized light in a counterclockwise direction (-)
conformers
same molecule with single bond rotated
Diastereomers
stereoisomers that are not mirror images of each other
Enantiomers
stereoisomers with mirror image
optical activity
the rotation of plane-polarized light by a chiral molecule in opposite directions but the same # of degrees
racemic mixtures
with equal concentrations (50/50) of two enantiomers, will not be optically active because the two enantiomers' rotations cancel each other out
