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29 Cards in this Set

  • Front
  • Back


• When a molecule's isomer isn't superimposable upon itself

• Molecule must be asymmetrical

• Molecule must have stereocenters!


• When a molecule's mirror image can be superimposed upon itself

• Molecule must have symmetry

• Molecule may have no stereochemistry = a meso compound

Which are chiral and which are achiral? Why?




Chiral = enantiomers and diastereomers

Achiral = meso



Chiral or not?


Ratio in a mixture?

• A stereoisomer which not superimposable upon the original molecule

• Has a chiral center and is chiral

• All chiral centers have inverted, so that the isomer is a mirror image

• Usually produces equal 50:50 racemic mixtures



Chiral or not?


Ratio in a mixture?

• A stereoisomer which is not superimposable upon the original molecule

• Must have multiple chiral centers and is chiral

• Some chiral centers have inverted and some haven't - doen't produce a mirror image

• Usually DOES NOT produce equal 50:50 racemic mixtures

Meso compounds

• A stereoisomer which IS superimposable upon the original molecule

• Has a plane of symmetry, image could be folded onto itself

• Has a chiral center and but ISN'T chiral b/c of its symmetry and superimposability


2 examples

• An atom at which the interchange of two groups gives a stereoisomer

• Ex. cis-trans isomers over double bonds

• Ex. chiral centers

Chiral center


• A type of stereocenter

• An asymmetric sp3 atom with 4 different single bonded groups

Which is chiral and which is achiral:

Cis or trans cyclic compounds

Chiral = trans cyclic compounds b/c the groups are on opposite sides, so that if the compound were folded onto itself the groups would not match up

Achiral = cis cyclic compounds b/c the groups are on the same side, giving it a plane of symmetry on which the molecule can be folded in half to make the groups match up

Configurations vs. conformations

Configurations = the spatial arrangements of bonded groups: chiral or achiral

Conformations = the different positions a molecule can twist into, while still maintaining that configuration

How is priority assigned to atoms in a configuration?

Highest priority = highest atomic number

What is the letter designation when priority numbers ascend:



Clockwise = R

Counterclockwise = S

If the lowest priority group isn't in the back, how is the configuration letter found out?

• Switch the positions of the lowest priority and the one which is in the back

• Figure out the configuration -> this is the isomer b/c 2 groups have been switched!

• Take the opposite of the isomer's which you had found

Do they have the same or different physical properties (BP, MP, density):



Diastereomers have different physical properties, and so they can be separated out via distillation or recrystallization

Enantiomers have the same physical properties, and so they can't be separated out

What is the relationship enantiomers have with one another in regards to rotation of the plane of polarized light?

Enantiomers rotate the plane to the same magnitude (º), but in opposite directions (+/-)

Dextrorotatory vs. levorotatory

Dextrorotatory = rotates the plane of polarized clockwise +

Levorotatory = rotates the plane of polarized counterclockwise -

Specific rotation vs. observed rotation

What is the symbol of each?

Specific rotation = a constant that is unique to each compound under std conditions

• symbol = [α]

Observed rotation = angle of rotation of polarized light which is found experimentally

• dependent upon the concentration of molecules in the polarimeter cell and the length of the polarimeter cell

• symbol = α

What is the equation that relates specific rotation to observed rotation, concentration, and cell length?

[α] = specific rotation ➞ a constant that is unique to each compound under std conditions

α = observed rotation

c = concentration of molecules in polarimeter cell (std = 1g/ml)

l = length of polarimeter cell (std = 1 dm)

[α] = specific rotation ➞ a constant that is unique to each compound under std conditions
α = observed rotation
c = concentration of molecules in polarimeter cell (std = 1g/ml)
l = length of polarimeter cell (std = 1 dm)

What is a racemic mixture?

• A 50/50 (1:1 ratio) mixture of d- and l-enantiomers

• Has no optical activity b/c the observations cancel each other out so that the activity = 0

How is optical purity calculated?

What is another name for it?

AKA enantiomeric excess (e.e.)

AKA enantiomeric excess (e.e.)

How is the percent composition of an enantiomeric mixture calculated?

% of major enantiomer found first:

% major = (e.e. x 50) + 50

% minor = 100 - % major

How do you know which is the major enantiomer in a mixture, and which is the minor?

• The major and minor enantiomers both have the same value for their specific rotations, but different signs

• The major enantiomer has the same sign as the observed specific rotation, and the minor has the opposite

Conformational isomers

• Two isomers which can be converted into one another by rotating around a single bond, or if a cyclo flipping the conformation

• They are nonsuperimpossible

Chirality of conformational isomers

When there is equilibrium between these isomers, the molecule is not chiral

What is an allene? (2)

Is it chiral or achiral? (3)

• Allenes are functional groups with the conformation C=C=C

• central carbon is sp2 hybridized

• Allenes have NO chiral carbon BUT are chiral when the end carbons have two different groups on them

• This is b/c the groups twist the molecule so that the mirror image isn't superimposable

How to draw a fischer projection from a model

1) Rotate the molecule so that two groups are coming out and two groups are going back

2) Draw the two groups coming out as a straight horizontal line, and the two groups going back as a straight vertical line


How to determine R or S configuration from a Fischer projection

1) Assign priorities

2) Switch group #4 with a vertical molecule so that #4 is in the back

3) Find the configuration BUT take the opposite of this as the answer (b/c you had switched the groups)

How to calculate the maximum number of isomers of a molecule

What is the exception to this?


n = # of chiral carbons

• Doesn't apply to compounds that have a meso compound w/ a plane of symmetry as an enantiomer

• B/c the meso is its own enantiomer

Why are enantiomers difficult to separate?

How can separation be done?

• Enantiomers are difficult to separate b/c they have the same physical properties

• They can be separated by first converting them into diastereomers with different physical properties