Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

5. Chirality

Calculations with Enantiomeric Percentages

Organic Chemistry

5. Chirality

Calculations with Enantiomeric Percentages

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5:07 minutes

Problem 28

Textbook Question

Under certain conditions, when (R)-2-bromobutane is heated with water, the S_N1 substitution proceeds twice as fast as the S_N2. Calculate the e.e. and the specific rotation expected for the product. The specific rotation of (R)-butan-2-ol is −13.5°. Assume that the S_N1 gives equal amounts of the two enantiomers.

Verified step by step guidance

Step 1: Understand the reaction mechanism. The problem involves two substitution reactions: SN1 and SN2. SN1 proceeds via a carbocation intermediate and typically results in a racemic mixture due to the planar nature of the carbocation. SN2 proceeds via a backside attack, leading to inversion of configuration.

Step 2: Analyze the given data. The SN1 reaction produces equal amounts of the two enantiomers, meaning the product from SN1 is racemic and has no optical activity. The SN2 reaction produces only the (S)-enantiomer due to inversion of configuration.

Step 3: Determine the enantiomeric excess (e.e.). Enantiomeric excess is calculated using the formula:

e e = \frac{% major - % minor}{%}

. Since SN1 proceeds twice as fast as SN2, the ratio of SN1 to SN2 products is 2:1.

Step 4: Calculate the specific rotation of the product. The specific rotation of a mixture is calculated using the formula:

[	heta] = \frac{% R - % S}{%} \times [	heta]

_pure. Here, the specific rotation of (R)-butan-2-ol is given as −13.5°.

Step 5: Combine the results. Use the ratio of SN1 and SN2 products, the enantiomeric excess formula, and the specific rotation formula to calculate the e.e. and the specific rotation of the final product. Ensure to account for the racemic nature of the SN1 product and the optical activity of the SN2 product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

SN1 and SN2 Mechanisms

SN1 and SN2 are two fundamental types of nucleophilic substitution reactions in organic chemistry. SN1 reactions involve a two-step mechanism where the leaving group departs first, forming a carbocation intermediate, followed by nucleophilic attack. In contrast, SN2 reactions occur in a single concerted step where the nucleophile attacks the substrate simultaneously as the leaving group departs. Understanding these mechanisms is crucial for predicting reaction rates and stereochemical outcomes.

Enantiomers and Optical Activity

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, often differing in their optical activity. Each enantiomer rotates plane-polarized light in opposite directions, measured as specific rotation. The concept of enantiomers is essential for calculating enantiomeric excess (e.e.), which quantifies the purity of one enantiomer over the other in a mixture, influencing the overall optical activity of the product.

Specific Rotation Calculation

Specific rotation is a property of chiral compounds that quantifies their ability to rotate plane-polarized light, expressed as [α] = α / (c × l), where α is the observed rotation, c is the concentration in g/mL, and l is the path length in decimeters. In this context, calculating the specific rotation of the product involves considering the contributions from both enantiomers produced in the reaction, particularly when the SN1 mechanism yields equal amounts of each enantiomer.

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Textbook Question

Imagine a sample that is enriched in the R enantiomer. If the % ee of the sample is 83%,

(a) what percent of the mixture is racemic?

(b) What is the ratio of R to S?

Textbook Question

(R)-Selegiline, a monoamine oxidase (MAO) inactivator, was approved by the FDA in 1989 for the treatment of Parkinson's disease. In pure form, it has a specific rotation, [α]²⁰_D = - 11.0°. What is the expected specific rotation of a mixture containing 64% S and 36% R?

Textbook Question

A chemist working on the synthesis of (+)-pilocarpine, an alkaloid used in the treatment of dry mouth and glaucoma, produced a mixture of enantiomers that gave a specific rotation [α]_D = +97°. Based on the specific rotation of the pure enantiomer, calculate the ratio of (+)- to (-)-pilocarpine produced by the chemist.

Textbook Question

The α- and β-anomers of glucose are shown here.

In solution, these two epimers can interconvert through a process called mutarotation. What is the ratio of anomers (α:β) when the specific rotation is +52.6°

Textbook Question

The specific rotation of (S)-2-iodobutane is +15.90°.

a. Draw the structure of (S)-2-iodobutane.

b. Predict the specific rotation of (R)-2-iodobutane.

c. Determine the percentage composition of a mixture of (R)- and (S)-2-iodobutane with a specific rotation of –7.95°.

Textbook Question

The specific rotation of (R)-(+)-glyceraldehyde is +8.7. If the observed specific rotation of a mixture of (R)-glyceraldehyde and (S)-glyceraldehyde is +1.4, what percent of glyceraldehyde is present as the R enantiomer?

Open Question

The [α] of pure S-epinephrine is +50°. Calculate the ee of a solution with an observed value of -40°. Calculate percent of each enantiomer. Then sketch the approximate mixture in our sample polarimeter tube.

Textbook Question

A sample of (S)-(+)-lactic acid was found to have an enantiomeric excess of 72%. How much R isomer is present in the sample?

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