Stereochemistry and Symmetry in Molecules

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Questions and Answers

What characteristic of allenes contributes to their enantiomerism despite lacking a chiral center?

Presence of chiral axis (correct)

Presence of cyclic structures

Presence of a chiral center

Presence of double bonds between non-adjacent carbons

Which factor is crucial for the chirality of spiranes?

Presence of symmetry in the rings

Proper substitution on the terminal carbon (correct)

Existence of a double bond

Presence of a chiral center

What type of isomers are biphenyls that arise due to restricted rotation around carbon-carbon single bonds?

Geometric isomers

Atropisomers (correct)

Optical isomers

Structural isomers

For biphenyl compounds to display enantiomerism, which condition must be met?

Each ring must be unsymmetrically substituted Signup and view all the answers

What occurs when double bonds in allene are replaced with a cycloalkane ring?

Creation of a stable alkylidenecycloalkane Signup and view all the answers

Which statement about stereochemistry in alkylidenecycloalkanes is true?

Their chirality arises from the presence of a chiral axis. Signup and view all the answers

What geometric characteristic do spiranes possess?

Two rings are perpendicular to each other Signup and view all the answers

What term describes the phenomenon observed in biphenyls due to steric hindrance at the ortho-position?

Atropisomerism Signup and view all the answers

What is a plane of symmetry in reference to a molecule?

A surface that reflects the entire molecule creating a mirror image Signup and view all the answers

What defines a center of symmetry in a molecule?

A situation where all identical atoms are positioned diagonally Signup and view all the answers

In the D, L system of nomenclature, what indicates a sugar belongs to the D-series?

The OH group is on the right side of the chiral carbon Signup and view all the answers

What is necessary for a molecule to have an alternate axis of symmetry?

A reflection plane must be perpendicular to the rotation axis Signup and view all the answers

Which of the following describes a molecule with a 2-fold alternate axis of symmetry?

It can be rotated 180 degrees and appear unchanged Signup and view all the answers

Which factor indicates that a molecule does NOT have a plane of symmetry?

When one side of the molecule is different from the other Signup and view all the answers

What is the primary application of the D, L system of nomenclature?

To describe the configuration of optically active sugars Signup and view all the answers

Which characteristic is NOT true about a molecule with a center of symmetry?

It requires an even number of atoms to show symmetry Signup and view all the answers

What is the term used for two atoms that can be interchanged by a mirror reflection within a molecule?

Enantiotopic Signup and view all the answers

Which of the following best describes homotopic protons in n-propane?

They are equivalent in environment. Signup and view all the answers

In the context of propionic acid, what type of ligands are Ha and Hb considered?

Enantiotopic ligands Signup and view all the answers

What defines a prochiral center in a molecule?

A tetrahedrally bonded atom with two identical and two different groups. Signup and view all the answers

Which statement accurately describes a prochiral molecule?

It can become asymmetric with a single substitution. Signup and view all the answers

In the example of propionic acid, what happens to the optical activity upon replacing one of its hydrogen atoms?

The compound gives optically active results. Signup and view all the answers

Which term describes protons placed in diastereomeric positions by a mirror reflection?

Diastereotopic Signup and view all the answers

Which of the following statements is true regarding chiral and achiral molecules?

Achiral molecules can have prochiral centers with specific substitutions. Signup and view all the answers

What designation is given to the ligand Ha in ethanol when it has higher priority over Hb?

pro-R Signup and view all the answers

If Hb is given higher priority over Ha in ethanol, what will be the hypothetical configuration of the stereocenter?

S Signup and view all the answers

In the designation of enantiotopic faces, what path indicates a Re face?

Clockwise from highest to lowest priority Signup and view all the answers

What type of ligands or atoms generate diastereomers when substituted?

Diastereotopic ligands Signup and view all the answers

In the context of propene, what results from replacing one homomorphic hydrogen atom with chlorine?

Leads to diastereomers Signup and view all the answers

What is the priority order for the groups attached to central carbon in ethanol if Ha is preferred?

OH > CH3 > Ha > Hb Signup and view all the answers

What is the E-alkene produced from propene when the opposite homomorphic hydrogen is replaced with Cl?

(E)-1-chloroprop-1-ene Signup and view all the answers

What configuration is assigned to Ha when it is replaced with deuterium 'D' in (R)-ethanol-1-D?

pro-R Signup and view all the answers

What type of products are formed when hydride is added to 4-tert-butylcyclohexanone?

Diastereomers Signup and view all the answers

Why is the carbonyl carbon in 4-tert-butylcyclohexanone considered a prostereo center?

It can lead to the formation of diastereomers. Signup and view all the answers

What is the significance of asymmetric induction in chemical reactions?

It leads to preferential formation of one enantiomer or diastereomer. Signup and view all the answers

What role does a chiral agent play in asymmetric induction?

It assists in the formation of a transition state. Signup and view all the answers

What distinguishes a prochiral center from a prostereo center?

Prostereo centers can result in diastereomers through specific reactions. Signup and view all the answers

Before 1940, how were optically active compounds typically obtained?

By isolating racemic mixtures from natural sources. Signup and view all the answers

What happens to the reactivity of enantiomers in a biological system?

Their activities vary based on the chiral environment. Signup and view all the answers

What term describes the stereo-chemical transformation that favors the formation of one stereoisomer over another in the presence of a chiral factor?

Asymmetric synthesis Signup and view all the answers

Study Notes

Symmetry Elements

A plane of symmetry divides a molecule into two identical halves.
A centre of symmetry exists when for every atom in a molecule, an identical atom exists diagonally opposite at an equal distance.
An alternating axis of symmetry (Sn) exists when a molecule is rotated by 360°/n degrees and then reflected in a plane perpendicular to the axis, resulting in an identical molecule.

Nomenclature of Optical Isomers

D, L System of Nomenclature is used primarily for sugars and polyhydric carbonyl compounds. It's relative, meaning configurations are defined with respect to glyceraldehyde. Sugars with the OH group on the right side of the chiral atom in a Fischer projection belong to the D-series, and those with the OH on the left belong to the L-series.

Stereochemistry of Allenes

Allenes, with cumulative double bonds, are chiral despite lacking a chiral center.
Alkylidenecycloalkanes are formed by replacing a double bond in an allene with a cycloalkane ring. They can also exhibit enantiomerism due to a chiral axis.
Spiranes are formed when both double bonds in an allene are replaced with rings. They can be chiral due to a chiral axis, which arises because the two rings are perpendicular.

Stereochemistry of Biphenyls

Atropisomers are stereoisomers resulting from restricted rotation around a single bond. This phenomenon is called atropisomerism.
Suitably substituted biphenyls exhibit enantiomerism due to a chiral axis. This arises from steric hindrance between ortho-substituents on each ring which prevents free rotation.

Topicity

Homotopic atoms or groups are in equivalent environments.
Enantiotopic atoms or groups are related by mirror reflection.
Diastereotopic atoms or groups, when replaced by a different substituent, lead to diastereomeric products.

Prochirality

A prochiral center is an achiral carbon with two identical substituents. Replacing one of these identical groups with a different one would create a chiral center.
A prochiral molecule contains a prochiral center.

Nomenclature of Enantiotopic Ligands and Faces

Enantiotopic ligands and faces are assigned pro-R and pro-S designations based on the CIP sequence rule.
Two faces of a carbonyl carbon are often labeled Re and Si based on the CIP sequence rule and the direction of traversal from the highest priority group to the lowest priority group.

Diastereotopic Ligands and Faces

Diastereotopic ligands, when replaced with a different substituent, create diastereomers.

Asymmetric Induction

This refers to reactions favoring the formation of one enantiomer or diastereomer over the other in the presence of a chiral environment. This is also known as asymmetric synthesis.
Chiral agents play a crucial role in asymmetric induction by influencing the transition state.

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Description

Explore the fascinating concepts of symmetry elements, optical isomer nomenclature, and the stereochemistry of allenes. This quiz covers important principles such as planes of symmetry, chiral structures, and the D, L system used for naming sugars. Test your knowledge of these key topics in molecular chemistry.