page 13 Flashcards
stereoisomers are possible for 2,3-dibromobutane?
How many
A: Since 2,3-dibromobutane has two stereogenic centers, the maximum number of stereoisomers is 4.
What is the relationship between stereoisomers A and B of 2,3-dibromobutane?
A: A and B are enantiomers.
How do you find all stereoisomers of 2,3-dibromobutane?
A:
Arbitrarily arrange substituents to form one stereoisomer (e.g., A).
Draw its mirror image (B).
Repeat the process for other configurations to find all stereoisomers.
How can you verify if two stereoisomers are enantiomers?
A: Two molecules are enantiomers if they are nonsuperimposable mirror images of each other.
What is a meso compound, and how does symmetry affect the number of stereoisomers?
A meso compound is an achiral compound that contains tetrahedral stereogenic centers but is superimposable on its mirror image due to a plane of symmetry.
Meso compounds generally have two identical halves, created by their plane of symmetry.
Example: Compound C (2,3-dibromobutane) is a meso compound because it has a plane of symmetry.
This symmetry reduces the number of stereoisomers. Instead of four possible stereoisomers, there are only three because one stereoisomer is superimposable on its mirror image.
What are the stereoisomers of 2,3-dibromobutane, and how are they related?
Pairs of enantiomers: A and B are enantiomers (non-superimposable mirror images of each other).
Meso compound: Compound C is a meso compound with a plane of symmetry, making it achiral despite having stereogenic centers.
Pairs of diastereomers:
A and C are diastereomers.
B and C are diastereomers.
Enantiomers (A and B) are related by mirror images, while diastereomers (A vs. C, B vs. C) are not mirror images and have different physical properties.
What are the stereoisomers of cis-1,3-dibromocyclopentane, and how are they classified?
Maximum stereoisomers: The compound has two stereogenic centers, leading to a maximum of four stereoisomers.
Cis vs. Trans isomers:
Cis isomer (A): Both bromine atoms are on the same side of the ring.
Trans isomer (B): Bromine atoms are on opposite sides of the ring.
Relationship:
Cis and trans forms are stereoisomers, specifically diastereomers, as they are not mirror images of each other.
What is the significance of the cis isomer of 1,3-dibromocyclopentane in terms of chirality?
The cis isomer of 1,3-dibromocyclopentane is superimposable on its mirror image, making the two images identical.
This property classifies the cis isomer as an achiral meso compound.
A meso compound has a plane of symmetry and does not exhibit optical activity despite having stereogenic centers.
Why are there only three stereoisomers for 1,3-dibromocyclopentane instead of four?
The trans isomer of 1,3-dibromocyclopentane is not superimposable on its mirror image.
The mirror images, labeled B and C, are enantiomers (non-superimposable mirror images).
One stereoisomer (cis isomer) is a meso compound, which is superimposable on its mirror image and thus does not contribute an additional unique stereoisomer.
Therefore, there are three stereoisomers in total: the meso compound and the pair of enantiomers.
What are the two main types of isomers, and how do they differ?
Constitutional Isomers: Isomers with the same molecular formula but different connectivity (atoms bonded to different atoms).
Stereoisomers: Isomers with the same molecular formula and connectivity but differ in the 3D arrangement of atoms.
Enantiomers: Stereoisomers that are non-superimposable mirror images.
Diastereomers: Stereoisomers that are not mirror images.
How do you determine the relationship between two nonidentical molecules?
Check molecular formula:
If different: Not isomers.
If the same: Proceed to step 2.
Compare connectivity:
If different: Constitutional isomers.
If the same: Proceed to step 3.
Check naming (e.g., cis, trans, R, S):
If the same: Proceed to step 4.
If different: Stereoisomers.
Check mirror image relationship:
If they are mirror images: Enantiomers.
If not: Diastereomers.
What is optical activity and how is it measured?
Optical activity refers to the ability of chiral molecules (enantiomers) to rotate plane-polarized light.
Enantiomers have identical physical properties, except in their interaction with plane-polarized light.
Plane-polarized light:
Light oscillating in a single plane.
Produced by passing ordinary light through a polarizer.
Polarimeter:
Measures the degree of rotation of plane-polarized light as it passes through a sample.
Used to distinguish optical activity in organic compounds.
What happens when plane-polarized light interacts with achiral compounds?
Back:
Plane-polarized light: Light oscillating in a single plane.
Achiral compounds:
Do not change the plane of polarization.
The light that exits the sample tube remains unchanged.
Such compounds are referred to as optically inactive.
Illustration: Light passes through a polarizer, becoming plane-polarized, and remains unchanged when passing through an achiral compound.
How does plane-polarized light interact with chiral compounds?
Interaction:
Chiral compounds rotate the plane of polarized light by an angle α.
Angle Measurement:
The angle α is measured in degrees (°) and is called the observed rotation.
Optical Activity:
Compounds that rotate polarized light are optically active.
Illustration Explanation:
Plane-polarized light passes through a chiral compound, causing the light’s plane of polarization to rotate before exiting the sample tube.
