MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 5c Chiral Separation Enantiomer Resolution

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This deck focuses on 5c Chiral Separation Enantiomer Resolution, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Flashcard 1: What is the relationship between observed and pure optical rotation for a mixture?

Answer: $\alpha_{obs}=ee\times \alpha_{pure}$ (with $ee$ as a fraction). Observed rotation scales with the enantiomeric purity relative to the pure enantiomer's rotation.

Flashcard 2: What is a chiral resolving agent in enantiomer resolution?

Answer: A chiral reagent used to form separable diastereomers. The agent converts enantiomers into diastereomers with differing properties for separation.

Flashcard 3: What is a meso compound?

Answer: Achiral molecule with stereocenters and an internal plane of symmetry. The internal symmetry makes the molecule achiral despite having chiral centers.

Flashcard 4: Why can diastereomers be separated by ordinary methods but enantiomers cannot (in achiral media)?

Answer: Diastereomers have different physical properties; enantiomers do not. Diastereomers differ in energy and properties, while enantiomers are identical in achiral environments.

Flashcard 5: Which technique commonly resolves enantiomeric acids by forming diastereomeric salts?

Answer: Salt formation with a single-enantiomer chiral base. The chiral base forms diastereomeric salts with differing solubilities for resolution of acids.

Flashcard 6: What is the key idea behind resolving enantiomers by converting them to diastereomers?

Answer: Make diastereomeric derivatives, separate them, then regenerate enantiomers. Conversion exploits diastereomers' distinct physical properties for separation before reversal.

Flashcard 7: What is kinetic resolution in the context of enantiomer separation?

Answer: A chiral reagent/catalyst reacts faster with one enantiomer than the other. Selective reactivity leaves one enantiomer unreacted, achieving partial purification.

Flashcard 8: Identify the correct formula for specific rotation using observed rotation, path length, and concentration.

Answer: $[\alpha]=\frac{\alpha_{obs}}{l\,c}$. Specific rotation normalizes observed rotation by sample concentration and path length.

Flashcard 9: What does it mean for a compound to be chiral?

Answer: It is not superimposable on its mirror image. Chirality implies the molecule lacks a plane of symmetry, preventing overlap with its mirror image.

Flashcard 10: What is the relationship between enantiomers and plane-polarized light rotation direction?

Answer: They rotate equal magnitudes in opposite directions. Enantiomers have opposite configurations, causing equal but opposite rotations of polarized light.

Flashcard 11: What is the definition of enantiomeric excess (ee) in terms of enantiomer fractions?

Answer: $ee=|f_R-f_S|\times 100\%$. Enantiomeric excess quantifies the purity of one enantiomer over the other in a mixture.

Flashcard 12: What is the most common structural feature that makes a carbon center chiral?

Answer: An $sp^3$ carbon with four different substituents. Four distinct groups on an $sp^3$ carbon create a stereocenter without symmetry, enabling chirality.

Flashcard 13: Which separation method most directly resolves enantiomers using a chiral stationary phase?

Answer: Chiral chromatography (chiral HPLC or chiral GC). A chiral stationary phase discriminates enantiomers directly without forming derivatives.

Flashcard 14: Which technique commonly resolves enantiomeric amines by forming diastereomeric salts?

Answer: Salt formation with a single-enantiomer chiral acid. The chiral acid creates diastereomeric salts with amines, enabling separation by physical differences.

Flashcard 15: Identify the mixture composition (major enantiomer) if $ee=20\%$ in favor of $R$.

Answer: $60\%\,R$ and $40\%\,S$. A 20% excess means the major enantiomer is 10% above 50%, with the minor 10% below.

Flashcard 16: Calculate $\alpha_{obs}$ if $\alpha_{pure}=+20^\circ$ and $ee=0.60$ (fraction).

Answer: $+12^\circ$. Observed rotation is the product of purity fraction and pure enantiomer's rotation value.

Flashcard 17: Find $ee$ if $\alpha_{obs}=-5^\circ$ and $\alpha_{pure}=-10^\circ$ for the pure enantiomer.

Answer: $50\%$. Enantiomeric excess is the ratio of observed to pure rotation, expressed as a percentage.

Flashcard 18: What property difference is most often exploited to separate diastereomeric salts in resolution?

Answer: Different solubilities (fractional crystallization). Diastereomeric salts exhibit solubility differences, allowing purification via recrystallization.

Flashcard 19: What is the principle of chiral chromatography that allows enantiomer separation?

Answer: Different interactions with a chiral phase give different retention times. Enantiomers form transient diastereomeric complexes with the chiral phase, leading to differential elution.

Flashcard 20: Which statement is true about optical rotation of a racemic mixture?

Answer: It has $\alpha_{obs}=0$ (optically inactive). Equal amounts of enantiomers cancel each other's optical rotations, yielding zero net rotation.

Flashcard 21: Calculate $ee$ for a mixture that is $70\%$ $R$ and $30\%$ $S$.

Answer: $40\%$. The difference in enantiomer percentages gives the excess of the major form.

Flashcard 22: What is the maximum theoretical yield of a single enantiomer from simple kinetic resolution?

Answer: $50\%$ (at $100\%$ conversion to the favored product). From a racemate, complete conversion of one enantiomer yields at most half as pure product.