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Questions and Answers
What characterizes enantiomers?
What characterizes enantiomers?
What is the maximum number of stereoisomers possible for a compound with 3 stereogenic centers?
What is the maximum number of stereoisomers possible for a compound with 3 stereogenic centers?
What distinguishes diastereomers from enantiomers?
What distinguishes diastereomers from enantiomers?
Which of the following describes a meso compound?
Which of the following describes a meso compound?
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In chiral synthesis, what is typically the outcome of most reactions producing chiral molecules?
In chiral synthesis, what is typically the outcome of most reactions producing chiral molecules?
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How are stereogenic centers assigned in naming compounds?
How are stereogenic centers assigned in naming compounds?
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What is true about the cis isomer of 1,3-dimethylcyclohexane?
What is true about the cis isomer of 1,3-dimethylcyclohexane?
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Which statement is correct regarding the physical properties of diastereomers?
Which statement is correct regarding the physical properties of diastereomers?
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What must be true for a molecule to be considered chiral?
What must be true for a molecule to be considered chiral?
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In the Cahn-Ingold-Prelog system, how are priorities assigned to groups attached to a stereogenic carbon?
In the Cahn-Ingold-Prelog system, how are priorities assigned to groups attached to a stereogenic carbon?
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If a molecule rotates and the groups descend in priority in a counterclockwise direction, how is the enantiomer designated?
If a molecule rotates and the groups descend in priority in a counterclockwise direction, how is the enantiomer designated?
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Which of the following statements about enantiomers is correct?
Which of the following statements about enantiomers is correct?
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What indicates that a molecule is not chiral?
What indicates that a molecule is not chiral?
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When assigning priorities for double or triple bonds within the Cahn-Ingold-Prelog system, how are these bonds treated?
When assigning priorities for double or triple bonds within the Cahn-Ingold-Prelog system, how are these bonds treated?
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What is the significance of chirality in biological systems?
What is the significance of chirality in biological systems?
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What experimental method can determine if two molecules are identical or enantiomers?
What experimental method can determine if two molecules are identical or enantiomers?
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What distinguishes enantiomers from diastereomers?
What distinguishes enantiomers from diastereomers?
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Which of the following criteria confirms a molecule is chiral?
Which of the following criteria confirms a molecule is chiral?
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What is a stereogenic center?
What is a stereogenic center?
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Which of the following statements is true regarding achiral molecules?
Which of the following statements is true regarding achiral molecules?
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Why do chiral molecules often have specific biological functions?
Why do chiral molecules often have specific biological functions?
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What happens when two groups at a tetrahedral center are switched?
What happens when two groups at a tetrahedral center are switched?
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Which example illustrates diastereomers?
Which example illustrates diastereomers?
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In the context of stereochemistry, what does it mean for a molecule to be nonsuperposable?
In the context of stereochemistry, what does it mean for a molecule to be nonsuperposable?
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Study Notes
Stereochemistry: Chiral Molecules
- Stereoisomers are isomers with the same molecular formula and connectivity, but different arrangements in space.
- Isomers are compounds with the same molecular formula, but different structural formulas.
- Constitutional isomers differ in the connectivity of their atoms.
- Stereoisomers have the same connectivity but differ in spatial arrangement.
- Enantiomers are stereoisomers that are nonsuperposable mirror images of each other.
- Diastereomers are stereoisomers that are not mirror images of each other. Examples include cis-trans isomers and diastereomers with more than one chiral center.
- Chiral molecules cannot be superimposed on their mirror images, and exist as a pair of enantiomers.
- Achiral molecules are superposable on their mirror images.
- 2-butanol is chiral.
- 2-propanol is achiral.
- A single tetrahedral carbon bonded to four different groups will always be chiral.
- A molecule with more than one tetrahedral carbon bonded to four different groups may or may not be chiral.
- Stereogenic centers are tetrahedral carbons that have four different groups attached.
- Interchange of any two groups on a stereogenic center leads to an enantiomer of the molecule with one stereogenic center.
- Carbons at a tetrahedral stereogenic center are designated by an asterisk (*).
The Biological Importance of Chirality
- Binding specificity for chiral molecules at a chiral receptor site is only favorable in one way.
- If the molecule or the biological receptor site had the wrong handedness, a physiological response would not occur.
- The interaction of a chiral molecule with a chiral receptor site is highly specific.
Tests for Chirality: Planes of Symmetry
- An imaginary plane that bisects a molecule such that the two halves are mirror images of each other.
- A molecule with a plane of symmetry is achiral.
- 2-chloropropane has a plane of symmetry.
- 2-chlorobutane does not have a plane of symmetry.
Nomenclature of Enantiomers: The R,S System
- Also called the Cahn-Ingold-Prelog system.
- Priorities are assigned to the four groups attached to the stereogenic carbon from highest (a) to lowest (d).
- Atoms directly attached are compared.
- Atoms with a higher atomic number get higher priority. If a decision cannot be made from directly attached atoms, the next level of atoms out from the stereogenic center is examined.
- The molecule is rotated to put the lowest priority group on the back.
- If groups descend in priority in a clockwise direction the enantiomer is R.
- If groups descend in priority in a counterclockwise direction the enantiomer is S.
- Groups with double or triple bonds are treated as if all bonds to that atom were duplicated.
Properties of Enantiomers: Optical Activity
- Enantiomers have almost identical physical properties.
- Enantiomers rotate plane-polarized light in equal but opposite directions.
- The oscillation of the electric field of ordinary light occurs in all planes perpendicular to the direction of propagation.
- Plane polarized light emerges from a polarizer.
- A polarimeter is used to measure the rotation of plane-polarized light by an optically active substance.
Specific Rotation
- An empty sample tube or one containing an achiral molecule will not rotate the plane-polarized light.
- An optically active substance (e.g., a pure enantiomer) will rotate the plane-polarized light.
- The amount the analyzer needs to turn to allow light through is called the observed rotation (α). A standard value, called the specific rotation, [α] is determined by the equation [α] = α/(c * l), where a is the observed rotation, c = concentration in grams per ml, and l = length of the tube in decimeters.
- If the analyzer is rotated clockwise, the rotation is (+) and the molecule is dextrorotatory.
- If the analyzer is rotated counterclockwise, the rotation is (-) and the molecule is levorotatory.
Racemic Forms and Enantiomeric Excess
- A 1:1 mixture of enantiomers is known as a racemic mixture.
- A racemic mixture will have no net optical rotation.
- A racemic mixture is denoted as (±)
- Enantiomeric excess (ee) is a measure of the enrichment of one enantiomer in a mixture of enantiomers. The equation is [ee] = [(moles of one enantiomer/total moles of both enantiomers)] * 100%
The Synthesis of Chiral Molecules
- Most chemical reactions which produce chiral molecules produce them as a racemic mixture.
Molecules with More Than One Stereogenic Center
- The maximum number of stereoisomers for molecules with n stereogenic centers is 2n.
Meso Compounds
- Meso compounds are achiral despite having stereogenic centers.
- Meso compounds have a plane of symmetry.
- Sometimes a molecule with two or more stereogenic centers will have less than the maximum amount of stereoisomers.
Naming Compounds with More Than One Stereogenic Center
- Assign a priority to substituents at each stereogenic center.
- Use the R, S system to assign the stereochemistry at each stereogenic center.
Fischer Projection Formulas
- A 2-dimensional representation of chiral molecules.
- Vertical bonds project behind the plane of the paper.
- Horizontal bonds project out of the plane of the paper.
Stereoisomerism of Cyclic Compounds
- 1,4-dimethylcyclohexane: Neither the cis nor trans isomer is optically active.
- 1,3-dimethylcyclohexane: The cis isomer has a plane of symmetry (meso). The trans isomer exists as a pair of enantiomers.
Relating Configurations Through Reactions in Which No Bonds to the Stereogenic Carbon Are Broken
- A reaction which takes place in a way that no bonds to the stereogenic center are broken is said to proceed with retention of configuration.
Relative and Absolute Configuration
- Relative configuration describes the stereochemical relationship between two different molecules.
- Absolute configuration is the actual arrangement of atoms in space.
Chiral Molecules That Do Not Possess a Tetrahedral Atom with Four Different Groups
- Atropoisomers are conformational isomers that are stable.
- Allenes contain two consecutive double bonds.
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Description
Explore the fascinating world of stereochemistry with this quiz focused on chiral molecules. Discover the differences between stereoisomers, enantiomers, and diastereomers while testing your knowledge on molecular structures and their spatial arrangements. Perfect for chemistry students looking to solidify their understanding of this critical concept.
