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

7. Substitution Reactions

Substitution Comparison

Organic Chemistry

7. Substitution Reactions

Substitution Comparison

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3:55 minutes

Problem 39

Textbook Question

When (R)-(1-bromoethyl)benzene is treated with sodium cyanide, a single enantiomer is produced. However, upon treatment of the same molecule with water, a mixture of two enantiomers is obtained.
(a) Explain these results.
(b) Why is only partial racemization sometimes observed?

Verified step by step guidance

Step 1: Analyze the reaction with sodium cyanide (NaCN) in diethyl ether (Et2O). The reaction involves a nucleophilic substitution (SN2) mechanism. In an SN2 reaction, the nucleophile (CN⁻) attacks the carbon bonded to the bromine atom from the opposite side, leading to inversion of configuration. This results in the formation of a single enantiomer of the product.

Step 2: Examine the reaction with water (H2O). Water acts as a weak nucleophile and the reaction proceeds via an SN1 mechanism. In an SN1 reaction, the bromine atom leaves first, forming a carbocation intermediate. This intermediate is planar and can be attacked by water from either side, leading to the formation of both enantiomers.

Step 3: Explain the observation of partial racemization. Partial racemization occurs because the carbocation intermediate formed during the SN1 reaction is not perfectly symmetrical. Steric and electronic effects from the phenyl group and the ethyl group can slightly favor attack from one side over the other, resulting in an unequal mixture of enantiomers (70:30 ratio).

Step 4: Highlight the stereochemical outcome. In the SN2 reaction, the stereochemistry is inverted due to the backside attack of the nucleophile. In the SN1 reaction, the planar carbocation intermediate allows for attack from both sides, leading to a racemic mixture or partial racemization.

Step 5: Summarize the key difference between the two reactions. The SN2 mechanism produces a single enantiomer due to stereochemical inversion, while the SN1 mechanism produces a mixture of enantiomers due to the planar nature of the carbocation intermediate.

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Key Concepts

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

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile replaces a leaving group in a molecule. In the case of (R)-(1-bromoethyl)benzene reacting with sodium cyanide, the cyanide ion acts as a nucleophile, leading to the formation of a single enantiomer due to the stereochemistry of the reaction. This process typically follows either an SN1 or SN2 mechanism, influencing the stereochemical outcome.

Enantiomers and Racemization

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, often differing in their optical activity. When (R)-(1-bromoethyl)benzene is treated with water, the reaction can lead to racemization, where both enantiomers are formed due to the reversible nature of the reaction. This occurs because water can act as a nucleophile, allowing for the formation of both configurations at the chiral center.

Partial Racemization

Partial racemization refers to the situation where a reaction produces a mixture of enantiomers, but one enantiomer is favored over the other. This can occur due to the reaction conditions, such as temperature or solvent effects, which can stabilize one transition state over another. In the case of (R)-(1-bromoethyl)benzene with water, the presence of competing pathways and the stability of intermediates can lead to a predominance of one enantiomer while still allowing some formation of the other.

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