Stereoisomers and Isomers Quiz

Questions and Answers

What are stereoisomers?

Compounds that differ from each other only in the three-dimensional, spatial arrangement of their atoms, but not in the connectivity of their atoms.

What are isomers?

Compounds that are constructed from the same atoms (same molecular formula) but that differ from each other.

What are the two types of isomers?

Constitutional isomers and stereoisomers.

What are constitutional isomers?

Same molecular formula but different constitution (order of connectivity of atoms). Signup and view all the answers

What are stereoisomers?

Same molecular formula and constitution but different spatial arrangement of atoms. Signup and view all the answers

Do double bonds (pi-bonds) experience free rotation like single bonds (sigma bonds)?

No, because pi-bonds are formed by the overlap of two p orbitals. Signup and view all the answers

What does trans mean?

Groups on the opposite side. Signup and view all the answers

What does superimposable mean?

That the mirror image of an object is identical to the actual object. Signup and view all the answers

What are chiral objects?

Objects that are not superimposable on their mirror images. Signup and view all the answers

What is the most common source of molecular chirality?

Presence of a carbon atom bearing four different groups. Signup and view all the answers

What is an enantiomer?

A chiral object's mirror image. Signup and view all the answers

How to determine if a molecule is clockwise or counterclockwise?

Assign priority numbers based on atomic numbers, arrange the molecule so that the fourth priority group is behind, and then observe the sequence. Signup and view all the answers

If 1-2-3 is counterclockwise, how is the molecule designated?

S = counterclockwise. Signup and view all the answers

What does [alpha] stand for?

Observed rotation. Signup and view all the answers

What is specific rotation dependent on?

Concentration (g/mL) and pathlength (dm). Signup and view all the answers

Are racemic mixtures optically active?

No, it will be optically inactive because the two equal parts of enantiomers cancel each other out. Signup and view all the answers

What does it mean to be optically pure?

A solution only contains a single enantiomer. Signup and view all the answers

Do diastereomers have the same physical properties (BP, MP, etc.)?

No. Signup and view all the answers

What is the maximum number of stereoisomers?

2^n, where n is the number of chirality centers. Signup and view all the answers

Study Notes

Stereoisomers and Isomers

Stereoisomers: Compounds that differ only in the three-dimensional arrangement of their atoms, not in atomic connectivity.
Isomers: Compounds with the same molecular formula but different structures.
Two main types of isomers: Constitutional isomers (different connectivity) and Stereoisomers (same connectivity, different arrangement).

Characteristics of Isomers

Constitutional isomers: Same molecular formula, different order of connectivity.
Stereoisomers: Same molecular formula and connectivity, but different spatial arrangements.

Chiral and Achiral Molecules

Chirality: A property of a molecule that is non-superimposable on its mirror image, indicated by a chiral center (a tetrahedral carbon with four different groups).
Achiral molecules: Superimposable on their mirror images.

Rotation and Optical Activity

Optically active compounds: Chiral compounds that rotate plane-polarized light.
Enantiomers: Chiral molecules that are mirror images of each other, rotating light in opposite directions.
Racemic mixture: Equal amounts of two enantiomers, resulting in no optical activity.

Assigning Configuration

Use the Cahn-Ingold-Prelog Rules for assigning priority based on atomic number.
With clockwise sequence (1-2-3), the designation is "R" and for counterclockwise, it is "S".

Physical Properties and Behavior

Boiling point (BP) and melting point (MP) are not affected by the configuration of chiral centers.
Intermolecular interactions determine physical properties; enantiomers typically have similar interactions.

Measuring Optical Activity

Polarimeter: Device used to measure the rotation of polarized light.
Observed rotation is dependent on both concentration (g/mL) and pathlength (dm).
Specific rotation: [α] = α/(c*l), standardized rotation measurement.

Relationships Between Molecules

Diastereomers: Not mirror images and have different physical properties (BP, MP).
Enantiomeric excess: A measure of the purity of one enantiomer over another, calculated as % ee = (observed rotation / α of pure enantiomer) x 100%.

Stereoisomer Count

Maximum number of stereoisomers = (2^n), where n is the number of chirality centers.
Three chirality centers produce four pairs of enantiomers, while four chirality centers produce eight pairs.

Special Cases

Meso compounds: Compounds with multiple stereocenters that possess symmetry, thus being achiral.
Cis and trans isomers: Their relationships with chirality; trans tends to be chiral due to symmetry while cis may have planes of symmetry.

Additional Key Terms

Inversion: Reflection about a point.
Chiral vs. achiral: Chiral is optically active; achiral is optically inactive.

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Description

Test your understanding of stereoisomers and isomers, including their characteristics and differences. Explore concepts like chirality, optical activity, and the classification of various types of isomers. Challenge your knowledge with a variety of questions designed to deepen your grasp of these important concepts in chemistry.