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What are enantiomers?
What are enantiomers?
Non-superimposable mirror image molecules.
Any molecule with one stereogenic center exists as a pair of enantiomers.
Any molecule with one stereogenic center exists as a pair of enantiomers.
True
What is a common convention for depicting a tetrahedron when drawing enantiomers?
What is a common convention for depicting a tetrahedron when drawing enantiomers?
Arbitrarily place the four groups on any bond to the stereogenic center and then draw the mirror image.
Stereogenic centers can only occur at carbon atoms in a linear chain, not in cyclic compounds.
Stereogenic centers can only occur at carbon atoms in a linear chain, not in cyclic compounds.
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Explain how to identify stereogenic centers on ring carbons.
Explain how to identify stereogenic centers on ring carbons.
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The enantiomers of 3-methylcyclohexene are non-superimposable mirror images.
The enantiomers of 3-methylcyclohexene are non-superimposable mirror images.
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Why are enantiomers of thalidomide important to consider in drug development?
Why are enantiomers of thalidomide important to consider in drug development?
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Paclitaxel (Taxol) is a well-known cancer treatment with a stereogenic center on a ring.
Paclitaxel (Taxol) is a well-known cancer treatment with a stereogenic center on a ring.
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What does R or S refer to in stereochemistry?
What does R or S refer to in stereochemistry?
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What is the first step in assigning R or S configuration to a stereogenic center?
What is the first step in assigning R or S configuration to a stereogenic center?
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If two atoms on a stereogenic center are the same, the priority is assigned based on the atomic number of the atoms bonded to those atoms.
If two atoms on a stereogenic center are the same, the priority is assigned based on the atomic number of the atoms bonded to those atoms.
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How do you assign a priority to atoms involved in a multiple bond during R/S configuration assignment?
How do you assign a priority to atoms involved in a multiple bond during R/S configuration assignment?
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What is the significance of assigning R or S configuration once the priorities are established?
What is the significance of assigning R or S configuration once the priorities are established?
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What is the correct step-by-step process for assigning R or S configuration? (Provide a brief summary of the steps.)
What is the correct step-by-step process for assigning R or S configuration? (Provide a brief summary of the steps.)
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What does the term racemic mixture refer to?
What does the term racemic mixture refer to?
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A racemic mixture is optically active.
A racemic mixture is optically active.
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A molecule with n stereogenic centers has a maximum of 2^n stereoisomers.
A molecule with n stereogenic centers has a maximum of 2^n stereoisomers.
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What are diastereomers?
What are diastereomers?
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When determining stereoisomers, it is crucial to find all possible configurations and then label them as enantiomers or diastereomers.
When determining stereoisomers, it is crucial to find all possible configurations and then label them as enantiomers or diastereomers.
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Enantiomers typically have similar physical properties like melting point and boiling point.
Enantiomers typically have similar physical properties like melting point and boiling point.
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How do you determine the relationship between two molecules? (Provide a brief summary of the process.)
How do you determine the relationship between two molecules? (Provide a brief summary of the process.)
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Enantiomers interact differently with plane-polarized light.
Enantiomers interact differently with plane-polarized light.
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What is a polarimeter used for?
What is a polarimeter used for?
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A compound that does not change the plane of polarized light is considered optically active.
A compound that does not change the plane of polarized light is considered optically active.
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Enantiomers rotate plane-polarized light to an equal extent but in opposite directions.
Enantiomers rotate plane-polarized light to an equal extent but in opposite directions.
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What are the two terms used to describe the rotation of polarized light? What do they represent?
What are the two terms used to describe the rotation of polarized light? What do they represent?
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There is a direct relationship between R and S prefixes and the (+) and (-) designations for optical activity.
There is a direct relationship between R and S prefixes and the (+) and (-) designations for optical activity.
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What is enantiomeric excess, or optical purity?
What is enantiomeric excess, or optical purity?
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Enantiomers can easily be separated by common physical techniques like distillation.
Enantiomers can easily be separated by common physical techniques like distillation.
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Diastereomers and constitutional isomers generally have very similar physical properties.
Diastereomers and constitutional isomers generally have very similar physical properties.
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What is the primary reason why enantiomers are important in drug development?
What is the primary reason why enantiomers are important in drug development?
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What is the common characteristic of many drugs that makes them important to study from a stereochemical perspective?
What is the common characteristic of many drugs that makes them important to study from a stereochemical perspective?
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Study Notes
Enantiomers
- Enantiomers are non-superimposable mirror image molecules.
- Any molecule with one stereogenic center exists as a pair of enantiomers.
Drawing Enantiomers
- To draw both enantiomers of a chiral compound (like 2-butanol), use the tetrahedron convention.
- Arbitrarily place the four groups (H, OH, CH3, CH2CH3) on any bond to the stereogenic center for the first enantiomer.
- Draw the mirror image to create the second enantiomer.
Stereogenic Centers in Cyclic Compounds
- Stereogenic centers can occur on ring carbons.
- Draw rings as flat polygons.
- Look for tetrahedral carbons bonded to four different groups.
Stereogenic Centers
- Examples of enantiomer pairs are presented visually.
Some Biologically Active Molecules with Stereogenic Centers on Rings
- Stereogenic centers can occur in large biologically active molecules, which are not presented chemically.
- Specific examples include molecules related to anti-nausea drugs, teratogens, and anticancer agents like paclitaxel and sucrose (a table sugar).
Labeling Stereogenic Centers with R or S
- Enantiomers are distinguished using prefixes R or S.
- Rule 1: Prioritize groups in decreasing order of atomic number (highest atomic number gets priority 1)
- Rule 2: If atoms on a stereogenic center are the same, prioritize based on the atoms bonded to those atoms, assigning priority to the atom with the higher atomic number.
- Rule 4: For multiple bonds, treat a multiply bonded atom as the equivalent number of singly bonded atoms
Assigning Priorities for R and S–Multiple Bonds
- Multiple bonds are treated as equivalent singly bonded atoms.
- For example, C=O is treated as C bonded to two O atoms.
- Other multiple bonds (C=C and C≡C) are similarly treated.
Assigning Priorities to Stereogenic Centers
- Examples illustrate assigning priorities to stereogenic centers.
- The stereogenic center is the atom bonded directly to four different atoms or groups.
How To Assign R or S
- Step 1: Assign priorities from 1 to 4 to each group bonded to the stereogenic center according to the precedence rules
How To Assign R or S (Step 2)
- Orient the molecule with the lowest priority group (4) placed back (on a dashed wedge), and visualize the remaining three groups.
- Look towards the lowest priority group
How To Assign R or S (Step 3)
- Trace a circle from priority group 1→2→3.
- Clockwise is R, counterclockwise is S.
How To Assign R or S (further details)
- The letter R or S precedes the IUPAC name of the molecule.
- Visualize stereocenters and identify types, (R) or (S)
Using Models to Visualize R,S Enantiomers
- Models enable a visual representation of R and S enantiomers.
- Color-coded models are utilized to represent atoms.
Prioritize Groups
- Groups are color-coded to aid model visualization.
- Prioritization follows rules 1-4
Put the lowest priority to the back
- The lowest priority group is placed behind the molecular plane.
- The orientation is viewed based on the order and direction of priority groups.
- The orientation is determined based on the priority groups.
The other should be R
- This reiterates the procedure for determining if the isomer should be R (or S).
Diastereomers
- A molecule with multiple stereogenic centers might have more than two stereoisomers, which may not be mirror images.
- Diastereomers are non-mirror-image stereoisomers. - These are stereoisomers that are not enantiomers.
Stereocenters of 2,3-dibromopentane
- An example method/steps determine stereoisomers of 2,3-dibromopentane.
Finding All possible Stereocenters
- The procedure demonstrates how to discover all possible stereoisomers for a compound with two stereogenic centers.
Finding All Possible Stereocenters
- If the atoms do not align, the compound has two nonsuperimposable mirror images- i.e. enantiomers
- This process identifies if compounds have two enantiomers
Finding All Possible Stereocenters
- Switching the position of two groups on one stereogenic center results in a new stereoisomer.
Summary of Stereoisomers of 2,3-dibromopentane
- A and B are enantiomers; C and D are enantiomers.
- A and C are diastereomers, A and D are diastereomers.
- B and C are diastereomers, B and D are diastereomers.
Stereoisomers of 2,3-dibromobutane
- Determine all stereoisomers of 2,3-dibromobutane, using an example with two stereogenic centers.
- The maximum number of stereoisomers is four.
Stereoisomers of 2,3-dibromobutane
- Form a first stereoisomer and its mirror image. This process illustrates how to create and identify stereoisomers when there are two stereogenic centers.
Stereoisomers of 2,3-dibromobutane: Determining other Possible Stereoisomers
- Switch and replace two groups, and generate the mirror image of the newly-formed structure
Meso Compounds
- Compound C contains a plane of symmetry, making it achiral.
- Meso compounds possess a plane of symmetry, resulting in overall achirality.
Stereoisomers of 2,3-dibromobutane: Figure 5.9
- A and B are enantiomers.
- A and C are diastereomers, similarly.
- Pairs of enantiomers: A and B.
- Pairs of diastereomers: A and C; B and C
R and S Assignments in Compounds with Two or More Stereogenic Centers
- Assign R and S configurations to each stereogenic center of a compound with more than one steregenic center.
- Illustrations of (2S,3R)-2,3-dibromopentane.
- Identical compounds have the same R, S designations.
- Opposite R and S configuration are found in enantiomers.
- Same or opposite R and S designation for at least one, are diastereomers.
1,3-Dibromocyclopentane Stereoisomers
- Two stereogenic centers; maximum of four stereoisomers possible
- Illustrations of cis and trans isomers.
- Cis-isomer is a meso compound; only three isomers exist.
- Trans-isomer has an enantiomer pair.
Summary-Types of Isomers
- Isomers are different compounds with the same molecular formula.
- Constitutional isomers have atoms bonded to different atoms.
- Stereoisomers have a difference in 3-D arrangement.
- Enantiomers are mirror images.
- Diastereomers are not mirror images.
Determining the Relationship Between Molecules
- Determine if molecules are isomers or not, based on their molecular formulas
- Decide relationships between structures based on naming prefixes and properties
- Determine if they are mirror images, and classify based on those characteristics.
How do properties of enantiomers differ from each other
- Enantiomers have the same chemical/physical properties except in how they interact with chiral substances.
- Optical activity - ability to rotate a plane-polarized light is the key difference.
Optical Activity
- Plane-polarized light has an electrical vector that oscillates on a single plane.
- A polarimeter is an instrument used to measure the degree to which an organic compound rotates plane-polarized light.
Plane-Polarized Light
- Achiral compounds do not change the plane of polarized light, making them optically inactive.
Rotation of Plane-Polarized Light
- Chiral compounds change the plane of polarized light.
Optical Activity Summary
- Rotation of polarized light can be clockwise or counterclockwise.
- Clockwise is d(+) or dextrorotatory.
- Counterclockwise is l(-) or levorotatory.
- The amount of rotation is equal but opposite for enantiomers.
- No relationship between the R/S system and the (+) / (-) system.
Racemic Mixtures
- An equal amount of two enantiomers is called a racemic mixture or a racemate.
- Racemic mixtures are optically inactive because the rotations cancel out.
Racemic Mixtures
- The physical properties can differ between individual enantiomers, and a racemic mixture when combined.
Enantiomeric Excess
- The enantiomeric excess (ee) measures how much an enantiomer is present in excess of the racemic mixture.
- The calculation is based on the relative percentages of the enantiomers within a mixture.
Physical Properties of Stereoisomers
- Enantiomers have identical physical properties, making separation difficult.
- Diastereomers and constitutional isomers have different properties, allowing for separation using common techniques.
So why are enantiomers, etc important
- Enantiomers react differently with chiral, non-racemic reagents, potentially leading to different outcomes.
Chemical Properties of Enantiomers
- Some drugs are chiral and require a specific chiral receptor or enzyme to be effective.
- The different outcomes illustrate why determining enantiomeric configurations is crucial
- Some examples of specific applications in the chemical and pharmaceutical industry are presented.
End chapter 5
- List of homework assignments is given at the end.
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Description
This quiz explores the concepts of enantiomers, stereogenic centers, and the methods for drawing enantiomers using chiral compounds. It highlights the importance of stereogenic centers in cyclic compounds and provides examples of biologically active molecules. Test your understanding of these fundamental topics in stereochemistry.
