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

12. Alcohols, Ethers, Epoxides and Thiols

Leaving Group Conversions - Using HX

Organic Chemistry

12. Alcohols, Ethers, Epoxides and Thiols

Leaving Group Conversions - Using HX

Previous problemNext problem

7:40 minutes

Problem 62b

Textbook Question

(b) Under the same conditions, an optically active sample of trans-2-bromocyclopentanol reacts with concentrated aqueous HBr to give an optically inactive product, (racemic) trans-1,2-dibromocyclopentane. Propose a mechanism to show how this reaction goes with apparently complete retention of configuration, yet with racemization. (Hint: Draw out the mechanism of the reaction of cyclopentene with Br₂ in water to give the starting material, trans-2- bromocyclopentanol. Consider how parts of this mechanism might be involved in the reaction with HBr.)

Verified step by step guidance

Step 1: Begin by analyzing the reaction conditions. The reaction involves trans-2-bromocyclopentanol reacting with concentrated aqueous HBr to form trans-1,2-dibromocyclopentane. The key observation is that the product is racemic, meaning it contains equal amounts of both enantiomers, despite the starting material being optically active.

Step 2: Consider the mechanism of the reaction. The presence of HBr suggests that the reaction proceeds via an acid-catalyzed pathway. Protonation of the hydroxyl group (-OH) in trans-2-bromocyclopentanol by HBr will convert it into a better leaving group, forming water (H2O). This step generates a carbocation intermediate.

Step 3: Examine the carbocation intermediate. The carbocation formed is planar and sp2 hybridized, which allows for attack by the bromide ion (Br⁻) from either side of the plane. This is the key step that leads to racemization, as the bromide ion can attack equally from the top or bottom face of the carbocation, producing both enantiomers of trans-1,2-dibromocyclopentane.

Step 4: Consider the retention of configuration. The reaction appears to retain the trans configuration because the bromide ion attacks the carbocation at the same position where the original bromine atom was located. This ensures that the relative stereochemistry of the two bromine atoms in the product remains trans.

Step 5: Reflect on the hint provided. The mechanism of cyclopentene reacting with Br2 in water to form trans-2-bromocyclopentanol involves anti-addition, which establishes the trans configuration. Similarly, in the reaction with HBr, the planar carbocation intermediate allows for racemization while maintaining the trans stereochemistry due to the symmetrical attack of Br⁻. This explains the formation of a racemic mixture with retention of configuration.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Video duration:

Play a video:

Was this helpful?

Key Concepts

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

Optical Activity and Racemization

Optical activity refers to the ability of a chiral compound to rotate plane-polarized light, which is a characteristic of enantiomers. Racemization is the process by which an optically active compound converts into a mixture of its enantiomers, resulting in a loss of optical activity. In the context of the reaction, the formation of racemic trans-1,2-dibromocyclopentane indicates that the reaction leads to equal amounts of both enantiomers, thus becoming optically inactive.

Mechanism of Electrophilic Addition

The mechanism of electrophilic addition involves the attack of an electrophile on a nucleophile, leading to the formation of a more complex product. In this case, the reaction of cyclopentene with Br2 in water illustrates how bromonium ion intermediates can form, allowing for the addition of bromine across the double bond. Understanding this mechanism is crucial for explaining how the initial trans-2-bromocyclopentanol is formed and how it retains configuration during the subsequent reaction with HBr.

Retention of Configuration

Retention of configuration refers to the preservation of the spatial arrangement of substituents around a chiral center during a chemical reaction. In the context of the reaction with HBr, despite the apparent retention of configuration, the formation of a racemic mixture suggests that the reaction proceeds through a mechanism that allows for the inversion of configuration at the chiral center. This duality is key to understanding how the product can be optically inactive while originating from an optically active starting material.

Watch next

Master Why do we need to convert Alcohol into a good leaving group? with a bite sized video explanation from Johnny

Start learning