7. Addition to the double bonds and chirality | Quizlet

What are enantiomers?

Non-superimposable mirror images of a chiral molecule.

Define chemoselectivity in reactions.

Chemoselectivity occurs when a reaction prefers one functional group over others in a molecule.

Define a prochiral center.

It has two enantiotopic groups and can become a chiral center through a chemical reaction.

What is the difference between pro-R and pro-S groups?

They distinguish enantiotopic groups on a prochiral center; transforming one creates an R or S configuration.

What are constitutional isomers?

Compounds with the same molecular formula but different connectivity of atoms.

What is a stereoselective reaction?

A reaction where one stereoisomer is formed preferentially over others.

How does enantioselectivity differ from diastereoselectivity?

Enantioselectivity refers to the preferential formation of one enantiomer; diastereoselectivity involves the preference for one diastereomer.

What is Markovnikov's rule?

In electrophilic additions, the hydrogen atom bonds to the carbon with the most hydrogens already attached.

What is chirality in organic molecules?

It refers to molecules that are non-superimposable on their mirror images, often lacking a plane of symmetry.

What is the difference between threo and erythro nomenclature?

Threo refers to opposite side substituents in Fischer projections.
Erythro refers to same-side substituents.

Give an example of a reaction that follows anti-Markovnikov addition.

Hydroboration-oxidation of alkenes.

Why is 9-BBN more regioselective in hydroboration compared to B2H6?

Its bulkiness enhances selectivity for the alpha-position.

What is enantiomeric excess (e.e.)?

The percentage difference between the major and minor enantiomer in a mixture.

How do enzymes distinguish between prochiral faces?

Enzymes can recognize and react with pro-R or pro-S faces due to their specific stereochemistry.

What is the significance of diastereomeric transition states in stereochemical induction?

They determine the preferred stereoisomeric product due to energy differences.

How is chirality utilized in pharmaceuticals?

Chirality affects drug interactions, with specific enantiomers often being biologically active.

Name an industrial application of enantioselective catalysis.

Katsuki-Sharpless epoxidation, used in synthesizing chiral epoxides for pharmaceuticals.

Why is citric acid an example of prochirality?

It contains two prochiral centers that can be converted into distinct chiral centers.

What is the relationship between symmetry planes and chirality?

Chiral molecules lack symmetry planes (e.g., σv\sigma_vσv​ or σh\sigma_hσh​). Symmetry elements such as reflection planes result in achirality.

What are atropisomers, and why are they significant?

Atropisomers are stereoisomers resulting from hindered rotation around a single bond, often exhibiting chirality without a chiral center.

How are prochiral faces labeled?

Using Re and Si nomenclature; Re-face refers to the clockwise configuration when viewed from above the plane, and Si-face refers to the counterclockwise configuration.

Provide an example of an enzyme that distinguishes prochiral faces.

Hydroxynitrile lyase (HNL) from almonds catalyzes reactions where the cyanide ion (CN⁻) attacks the Si-face of benzaldehyde.

What determines the major product in a stereoselective E2 elimination reaction?

Anti-elimination mechanism, steric hindrance, and the stability of the resulting alkene (Zaitsev's rule).

What makes bromination of Z-2-butene highly diastereoselective?

The syn-addition mechanism ensures nearly exclusive formation of one diastereomer due to steric and electronic factors.

Explain why hydroboration is a syn-addition process.

The boron atom and hydrogen atom add simultaneously to the same side of the double bond, maintaining syn-stereochemistry.

How does hydroboration differ in regioselectivity compared to acid-catalyzed hydration?

In hydroboration, the boron attaches to the less substituted carbon due to the opposite polarization of the H-B bond compared to H-OH in acid hydration.

Why is hydroboration of chiral alkenes stereoselective?

Pre-existing chiral centers in the alkene influence the stereochemical outcome of the new chiral center, leading to preferred diastereomers.

What is the Katsuki-Sharpless epoxidation?

An enantioselective reaction that uses chiral tartrate ligands to produce optically active epoxides.

How does the choice of ligand in Katsuki-Sharpless epoxidation affect the product?

L-(+)-diethyl tartrate leads to epoxidation from the bottom side, while D-(-)-diethyl tartrate leads to epoxidation from the top side.

Why does the addition of bromine to 1-phenylpropene exhibit a diastereomeric ratio of 78:22?

The bulky phenyl group creates steric hindrance, reducing selectivity compared to smaller substituents.

What governs the diastereoselectivity in hydroboration of chiral alkenes?

The steric and electronic effects of substituents influence the diastereomeric transition states, favoring one product.

Why is Markovnikov's rule followed in most electrophilic additions to alkenes?

The carbocation intermediate is stabilized when the positive charge is on the more substituted carbon atom.

Why do anti-Markovnikov products form during hydroboration?

The nucleophilic boron preferentially adds to the less substituted carbon due to electronic factors.

What is helicity, and how does it relate to chirality?

Helicity describes the handedness (P for right-handed, M for left-handed) of chiral helical systems like corkscrews and helicenes.

How is chirality evident in citric acid?

Citric acid has two prochiral centers that become chiral upon reaction, producing different stereoisomers.

Why is stereoselectivity critical in enzymatic reactions?

Enzymes are stereospecific, often catalyzing reactions that produce a single enantiomer or diastereomer.