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29 Cards in this Set
- Front
- Back
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Chiral |
• When a molecule's isomer isn't superimposable upon itself • Molecule must be asymmetrical • Molecule must have stereocenters! |
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Achiral |
• When a molecule's mirror image can be superimposed upon itself • Molecule must have symmetry • Molecule may have no stereochemistry = a meso compound |
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Which are chiral and which are achiral? Why? Meso Diasteromers Enantiomers |
Chiral = enantiomers and diastereomers
Achiral = meso |
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Enantiomers Superimposable? Chiral or not? Inversion? 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
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Diastereomers Superimposable? Chiral or not? Inversion? 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 |
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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 |
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Stereocenter 2 examples |
• An atom at which the interchange of two groups gives a stereoisomer • Ex. cis-trans isomers over double bonds • Ex. chiral centers |
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Chiral center 2 |
• A type of stereocenter • An asymmetric sp3 atom with 4 different single bonded groups |
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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 |
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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 |
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How is priority assigned to atoms in a configuration? |
Highest priority = highest atomic number |
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What is the letter designation when priority numbers ascend: Clockwise Counterclockwise |
Clockwise = R Counterclockwise = S |
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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 |
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Do they have the same or different physical properties (BP, MP, density): Diastereomers Enantiomers |
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 |
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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 (+/-) |
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Dextrorotatory vs. levorotatory |
Dextrorotatory = rotates the plane of polarized clockwise +
Levorotatory = rotates the plane of polarized counterclockwise - |
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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
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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)](https://images.cram.com/images/upload-flashcards/09/01/87/7090187_m.jpg)
[α] = specific rotation ➞ a constant that is unique to each compound under std conditions |
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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 |
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How is optical purity calculated? What is another name for it? |
AKA enantiomeric excess (e.e.) |
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How is the percent composition of an enantiomeric mixture calculated? |
% of major enantiomer found first:
% major = (e.e. x 50) + 50
% minor = 100 - % major |
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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 |
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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 |
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Chirality of conformational isomers |
When there is equilibrium between these isomers, the molecule is not chiral |
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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 |
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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
PRACTICE FROM 2D IMAGES |
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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) |
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How to calculate the maximum number of isomers of a molecule
What is the exception to this? |
2^n
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 |
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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 |
