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

Enantiomeric Excess

Organic Chemistry

5. Chirality

Enantiomeric Excess

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5:41 minutes

Problem 13

Textbook Question

A chemist finds that the addition of (+)-epinephrine to the catalytic reduction of butan-2-one (Figure 5-17 ) gives a product that is slightly optically active, with a specific rotation of +0.45°. Calculate the percentages of (+)-butan-2-ol and (−)-butan-2-ol formed in this reaction.

Verified step by step guidance

Understand the problem: The reaction involves the catalytic reduction of butan-2-one to butan-2-ol in the presence of (+)-epinephrine, which results in a product that is optically active. This indicates that the reaction produces an unequal mixture of the enantiomers (+)-butan-2-ol and (−)-butan-2-ol. The specific rotation of the product is given as +0.45°.

Recall the formula for specific rotation: The specific rotation of a mixture of enantiomers is given by the equation:

[α]

_mixture = (f₊ × [α]₊) + (f_- × [α]_-) , where

f

₊ and

f

_- are the fractions of the (+) and (−) enantiomers, respectively, and

[α]

₊ and

[α]

_- are their specific rotations. Note that

[α]

_- = -[α]₊ for enantiomers.

Set up the equation: The specific rotation of pure (+)-butan-2-ol is a known value (look up this value in a reference, typically around +13.5°). Let

f

₊ represent the fraction of (+)-butan-2-ol and

f

_- represent the fraction of (−)-butan-2-ol. Since the mixture is optically active with a specific rotation of +0.45°, substitute the known values into the equation:

0.45 = (f

₊ × 13.5) + (f_- × -13.5).

Use the relationship between fractions: The total fraction of the two enantiomers must equal 1, so

f

₊ + f_- = 1. Solve for

f

_- in terms of

f

₊:

f

_- = 1 - f₊. Substitute this into the specific rotation equation.

Solve for the fractions: Substitute

f

_- = 1 - f₊ into the equation

0.45 = (f

₊ × 13.5) + ((1 - f₊) × -13.5). Simplify and solve for

f

₊. Once

f

₊ is determined, calculate

f

_- using

f

_- = 1 - f₊. Convert these fractions to percentages by multiplying by 100.

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Key Concepts

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

Optical Activity

Optical activity refers to the ability of chiral compounds to rotate plane-polarized light. This property is crucial in organic chemistry for distinguishing between enantiomers, which are non-superimposable mirror images of each other. The specific rotation, measured in degrees, quantifies this rotation and helps determine the composition of a mixture of enantiomers.

Chirality and Enantiomers

Chirality is a property of a molecule that makes it non-superimposable on its mirror image, often due to the presence of a chiral center, typically a carbon atom bonded to four different groups. Enantiomers are pairs of chiral molecules that are mirror images of each other, and they can exhibit different biological activities and properties, making their separation and quantification important in synthesis and analysis.

Catalytic Reduction

Catalytic reduction is a chemical reaction that involves the addition of hydrogen to a compound, often facilitated by a catalyst, to reduce functional groups such as ketones to alcohols. In the context of butan-2-one, the reaction produces butan-2-ol, and the presence of a chiral catalyst like (+)-epinephrine can influence the stereochemical outcome, leading to the formation of enantiomers in varying proportions.

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