Ch 3- Stereoisomerism And Chirality Flashcards
What does it mean when an object is achiral?
The object lacks chirality; it has no handedness
What does it mean when an object is chiral?
The object and its mirror image are not superposeable
An object will be achiral if _____.
it has one or more of certain elements of symmetry, the most common being the plane and center of symmetry
stereoisomer
Isomer that has same molecular formula and same connectivity of atoms but different orientation of their atoms in space
configurational isomer
Isomer that differs by the configuration of substituents on an atom
enantiomer
Stereoisomer nonsuperposeable mirror images of each other; refers to a relationship btwn pairs of objects
chiral center
a tetrahedral atom, mostly carbon, that is bonded to 4 different groups
stereocenter
Atom about which exchange of 2 groups produces a stereoisomer; chiral centers are one type of stereocenter
diastereomer
Stereoisomers that are not mirror images of each other; refers to relationships among 2 or more objects
conformational isomer
Type of stereoisomer; not a configurational isomer because bonds do not need to be broken to interchange atoms; the bonds only need to be rotated
atropisomer
Enantiomer that lacks a chiral center and differs because of hindered rotation
R, S System
A set of rules for specifying absolute configurations about a chiral center
absolute configuration
Which of the two possible isomers an enantiomer is (left or right handed)
R
Used in R,S convention to show that the order of priority of groups in a chiral center is clockwise
S
Used in R,S convention to show that the order of priority of groups on a chiral center is counterclockwise
Priority is based on _____.
atomic number; the higher the atomic number, the higher the priority (the highest priority is assigned a priority of 1)
When 2 or more groups on a chiral center are the same, priority assignment is made _____.
at the first point of difference
When assigning priorities, atoms in double bond or triple bonds are considered to be bonded to _____.
an equivalent number of “phantom atoms” by single bonds, bonded to no other atoms
What are the four steps of assigning an R or S configuration to a chiral center?
- Locate chiral center, identify its 4 substituents, and assign a priority from 1 to 4 to each substituent
- Orient molecule so that substituent of priority 4 is pointing away from you
- Read 3 groups in order from 1 to 3
- If clockwise, R. If counterclockwise, S.
OR use Harran’s RHR
When 2 or more stereoisomers exist in a molecule, _____.
multiple stereoisomers are possible
For a molecule with n chiral centers, the max number of stereoisomers possible is _____.
2^n
meso compound
An achiral compound possessing 2 or more chiral centers that also have chiral isomers
2 stereoisomers will be enantiomers if _____ and the stereoisomers will be diastereomers if _____.
all of the chiral centers present are reversed; only some of the chiral centers are reversed
Enantiomers have _____ (identical/different) physical and chemical properties in an achiral environment.
identical
achiral environment
solvents that have no chiral centers
Diastereomers have _____(identical/different) physical and chemical properties.
different
Fischer projection
a 2D projection of a molecule; groups on the left and right are in front while those at the top and bottom are to the rear
If 2 compounds have the same molecular formula, they are _____ (isomers/not isomers).
isomers
If 2 compounds have the same connectivity of atoms, they are _____ (constitutional isomers/stereoisomers).
stereoisomers
If 2 compounds do not have the same connectivity of atoms, they are _____ (constitutional isomers/stereoisomers).
constitutional isomers
If 2 compounds can interconvert by rotations about single bonds they are _____ (configurational/conformational) isomers.
configurational
If 2 stereoisomers cannot interconvert by rotations about single bonds they are _____ (configurational/conformational) isomers.
conformational
If 2 stereoisomers are nonsuperposeable mirror images of each other, they are _____ (diastereomers/enantiomers).
enantiomers
If 2 stereoisomers are not nonsuperposeable mirror images of each other, they are _____ (diastereomers/enantiomers).
diastereomers
If conformational isomers that are also enantiomers readily interconvert at ambient temps, they are _____.
atropisomers
optically active
refers to a compound that rotates the plane of polarized light
plane-polarized light
light oscillating in only a single plane; the vector sum of left and right circularly polarized light, which are enantiomers- each component interacts in an opposite way with chiral molecules
observed rotation
the number of degrees through which a compound rotates the plane of polarized light
dextrorotatory
refers to a substance that rotates the plane of polarized light to the right
levorotatory
refers to a substance that rotates the plane of polarized light to the left
specified rotation [α]
observed rotation in degrees/(length in dm • concentration in g/mL)
For any pair of enantiomers, one is _____ and the other is _____.
dextrorotatory; levorotatory
For enantiomers, the absolute value of the _____ is the same but the sign is opposite.
specific rotation
racemic mixture
a mixture of equal amounts of 2 enantiomers
The specific rotation of a racemic mixture is _____, because _____. Alternatively, we say that a racemic mixture is _____.
zero, because there are equal numbers of dextrorotatory and levorotatory molecules; optically inactive
