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What is the priority order of the groups attached to the asymmetric carbon in 1,3-dibromobutane?
How do enantiomers differ in their interaction with chiral molecules?
What does the specific rotation of a compound depend on?
In a polarimeter, what does a counterclockwise rotation signify?
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What property is shared between enantiomers?
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Which of the following statements about plane-polarized light is true?
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What happens when one of the enantiomers of 2-butanol is placed in a polarimeter?
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When determining the configuration of enantiomers, what must be done first?
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What distinguishes the enantiomers of alanine from each other?
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Which of the following elements has the highest priority according to the Cahn–Ingold–Prelog system?
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When using the Cahn–Ingold–Prelog system, how should multiple bonds be treated?
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What configuration is assigned when the groups are arranged in a clockwise manner?
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Which of the following is true regarding trans-1,2-dichlorocyclopentane?
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How do you break ties when assigning priorities to groups around an asymmetric carbon?
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Which is true about the (R) and (S) configurations?
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What is the common name for both enantiomers of alanine?
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What is the relationship between the two chair conformations of cis-1,2-dibromocyclohexane?
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What must be true for an allene to be chiral?
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Which statement correctly describes a Fischer projection?
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What occurs during a 180° rotation of a Fischer projection?
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When naming (R) and (S) configurations from Fischer projections, what should you apply backward?
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What happens to the chirality of penta-2,3-diene?
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Why is a planar conformation of a biphenyl derivative considered conformationally locked?
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What effect does a 90° rotation have on the groups in a Fischer projection?
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What is the concentration of 2-butanol in g/mL when 6 g is diluted to 40 mL?
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Which statement is true regarding racemic mixtures?
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What is the specific rotation of (S)-2-iodobutane given that it is +15.90°?
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How is optical purity (o.p.) defined?
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What happens when optically inactive reagents combine to form a chiral molecule?
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What is the method used to determine the % composition of a mixture when the specific rotation is known?
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If the specific rotation of a mixture is -3.18° and the specific rotation of pure (S)-2-iodobutane is +15.90°, what is the observed optical purity of the mixture?
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What defines chirality in a molecule with conformers?
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What is the maximum number of stereoisomers for a compound with three chiral centers?
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Which of the following statements is true regarding meso compounds?
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How do diastereomers differ from enantiomers?
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What is the primary factor affecting the assignment of R and S configurations in Fischer projections?
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Which characteristic distinguishes diastereomers from enantiomers?
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Which is true regarding the absolute and relative configurations of molecules?
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In the context of stereoisomerism, what is the significance of the term 'internal mirror planes'?
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Which example illustrates the concept of cis-trans isomerism?
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Study Notes
Trans Cyclic Compounds
- Molecules with asymmetric carbons can have two enantiomeric forms
- Enantiomers are nonsuperimposable mirror images
- (R) and (S) configurations are used to describe the three-dimensional arrangement of groups around a chiral center
Cahn–Ingold–Prelog Convention
- This system is used to assign (R) or (S) configurations to chiral centers
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Step 1: Assign Priority
- Assign priority based on atomic number, higher atomic number = higher priority
- When there are ties, consider the next atoms along the chain
- Treat multiple bonds as if each bond is to a separate atom
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Step 2: Determine Configuration
- Rotate the molecule so the lowest priority group is pointing away from you
- Draw an arrow from the highest priority group to the second, then to the third and back to the highest
- Clockwise: (R) configuration
- Counterclockwise: (S) configuration
Configuration in Cyclic Compounds
- For cyclic compounds, treat the ring as if it were a chain, assigning priority to each group attached to the chiral center
- Draw the molecule with the lowest priority group pointing away from you
Properties of Enantiomers
- Enantiomers share some physical properties:
- Boiling point
- Melting point
- Density
- Refractive index
- Enantiomers differ in their interaction with chiral molecules, including:
- Enzymes
- Taste buds
- Scent receptors
Polarized Light
- Plane-polarized light oscillates in only one plane
- Enantiomers rotate plane-polarized light in opposite directions
Polarimeter
- A polarimeter measures the rotation of plane-polarized light
- Dextrorotatory (+): Rotates the plane of polarized light clockwise
- Levorotatory (-): Rotates the plane of polarized light counterclockwise
- Rotation direction is independent of (R) and (S) configurations
Specific Rotation
- The specific rotation of a compound is a measure of its optical activity under specific conditions, defined as:
- [α] = 𝛼(observed) / (c * l)
- 𝛼(observed) is the observed rotation in degrees
- c is the concentration in g/mL
- l is the path length in decimeters
Biological Discrimination
- Enzymes often display enantioselectivity, meaning they preferentially bind to only one enantiomer of a chiral molecule.
Racemic Mixtures
- Racemic mixtures contain equal amounts of both enantiomers
- Notation: (d,l) or (±)
- Racemic mixtures are optically inactive
- Racemic mixtures may have different boiling points and melting points compared to pure enantiomers
Racemic Products
- Racemic mixtures can be formed when optically inactive reagents react to form a chiral molecule
Optical Purity (Enantiomeric Excess)
- Optical purity measures the amount of one enantiomer present in excess in a mixture:
- o.p. = (observed rotation / rotation of pure enantiomer) * 100
- Also known as enantiomeric excess (e.e.)
Chirality of Conformers
- Conformations that interconvert rapidly are not chiral because they average out to a non-chiral form.
- The most symmetrical conformer should be used to determine chirality
- Cyclohexane can be considered planar on average
Chirality of Conformational Isomers
- Conformations that are locked in a specific form can be chiral
- Examples include biphenyl derivatives where rotation around the central C-C bond is restricted
Allenes
- Allenes can be chiral, even though they don’t have a chiral carbon
- The central carbon is sp-hybridized
- For an allene to be chiral, the groups at the end carbons must be different
Fischer Projections
- A flat representation of a 3-D molecule
- The chiral carbon is at the intersection of horizontal and vertical lines
- Horizontal lines are forward, out of the plane
- Vertical lines are behind the plane
Fischer Projection Rules
- The carbon chain is on the vertical line
- The highest oxidized carbon is at the top
- Rotation of 180° in the plane does not change the molecule
- Rotation of 90° is not allowed
Fischer (R) and (S)
- Assign priority based on the Fischer projection, but treat the lowest priority group (usually H) as if it were coming towards you
- Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R)
Diastereomers: Cis-Trans Isomerism on Double Bonds and Rings
- Diastereomers are stereoisomers that are not mirror images of each other
- Cis-trans isomers on double bonds are diastereomers
- Cis-trans isomers on rings are diastereomers
Diastereomers
- Molecules with two or more chiral carbons have stereoisomers that are not mirror images
Two or More Chiral Carbons
- When a compound has two or more chiral centers, it can exist as enantiomers, diastereomers, or meso isomers
- Enantiomers: Opposite configurations at each chiral carbon
- Diastereomers: Some matching and some opposite configurations at each chiral carbon
- Meso compounds: Have internal planes of symmetry
- Maximum number of isomers is 2^n, where n is the number of chiral carbons
Meso Compounds
- Meso compounds have a plane of symmetry
- They are achiral, even though they have chiral centers
Absolute and Relative Configurations
- Absolute configuration: The exact 3D arrangement of all atoms in a molecule
- Relative configuration: The relationship between the configurations of two molecules
Number of Stereoisomers
- The 2^n rule does not apply to compounds with planes of symmetry
- Example: 2,3-dibromobutane only has 3 stereoisomers, not 4
Optical Purity and % Composition
- Optical purity is another name for enantiomeric excess
- Optical purity is calculated by dividing the observed rotation of a mixture by the specific rotation of the pure enantiomer
- Optical purity is a measure of the percent of one enantiomer in excess relative to the other enantiomer.
- Example: A mixture that is 80% optically pure has an 80% excess of one enantiomer and 20% of the other enantiomer
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Description
Explore the fascinating world of trans cyclic compounds and the configuration of chiral centers. This quiz covers the concepts of enantiomers, the Cahn–Ingold–Prelog convention, and how to determine (R) and (S) configurations. Test your understanding of molecular stereochemistry and how asymmetric carbons influence chemical properties.
