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

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7:22 minutes

Problem 64a

Textbook Question

Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.
(a)

Verified step by step guidance

Step 1: Protonation of the alcohol group: The hydroxyl group (-OH) in the alcohol is a poor leaving group. In the presence of HBr, the oxygen atom of the hydroxyl group is protonated, forming a positively charged oxonium ion (R-OH2+), which is a much better leaving group.

Step 2: Determine the reaction mechanism (SN1 or SN2): The reaction mechanism depends on the structure of the alcohol. In this case, the alcohol is a secondary alcohol (cyclohexanol). Secondary alcohols can undergo either SN1 or SN2 mechanisms, but the reaction conditions (heat and the presence of a strong acid like HBr) favor the SN1 mechanism.

Step 3: Formation of the carbocation intermediate: In the SN1 mechanism, the protonated hydroxyl group (R-OH2+) leaves, forming a carbocation intermediate (cyclohexyl carbocation). This step is the rate-determining step of the reaction.

Step 4: Nucleophilic attack by bromide ion: The bromide ion (Br⁻), which is generated from the dissociation of HBr, acts as a nucleophile and attacks the carbocation intermediate. This results in the formation of the final product, cyclohexyl bromide.

Step 5: Formation of water as a byproduct: During the reaction, the leaving group (H2O) is formed as a byproduct when the protonated hydroxyl group departs. The overall reaction converts cyclohexanol to cyclohexyl bromide with water as a byproduct.

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Key Concepts

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

Protonation of Alcohols

Protonation is the addition of a proton (H+) to a molecule, which in the case of alcohols, converts the hydroxy (-OH) group into a better leaving group, water (H2O). This process enhances the electrophilicity of the carbon atom bonded to the hydroxy group, facilitating nucleophilic substitution reactions.

Nucleophilic Substitution Mechanisms (SN1 and SN2)

Nucleophilic substitution reactions can occur via two primary mechanisms: SN1 and SN2. SN1 is a two-step process involving the formation of a carbocation intermediate, while SN2 is a one-step process where the nucleophile attacks the substrate simultaneously as the leaving group departs. The choice between these mechanisms depends on factors like substrate structure and the nature of the nucleophile.

Role of HBr in Reaction

Hydrobromic acid (HBr) serves as both a proton source and a nucleophile in the reaction. It protonates the alcohol, converting it into a better leaving group, and the bromide ion (Br-) acts as a strong nucleophile that can attack the electrophilic carbon, leading to the substitution product. The heat applied can also facilitate the reaction by increasing the kinetic energy of the molecules involved.

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Related Practice

Textbook Question

Propose a mechanism for the reaction of

(a) 1-methylcyclohexanol with HBr to form 1-bromo-1-methylcyclohexane.

Textbook Question

When cis-2-methylcyclohexanol reacts with the Lucas reagent, the major product is 1-chloro-1-methylcyclohexane. Propose a mechanism to explain the formation of this product.

Textbook Question

(a) The reaction of butan-2-ol with concentrated aqueous HBr goes with partial racemization, giving more inversion than retention of configuration. Propose a mechanism that accounts for racemization with excess inversion.

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.)

Textbook Question

What product would be formed if the four-membered ring alcohol in Problem 70 were heated with an equivalent amount of HBr rather than with a catalytic amount of H₂SO₄?

Textbook Question

Propose a mechanism for each of the following reactions:

Textbook Question

When 3-methyl-2-butanol is heated with concentrated HBr, a rearranged product is obtained. When 2-methyl-1-propanol reacts under the same conditions, a rearranged product is not obtained. Explain.

Textbook Question

Explain the products observed in the following reaction of an alcohol with the Lucas reagent.

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Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.(a)

Key Concepts

Protonation of Alcohols

Nucleophilic Substitution Mechanisms (SN1 and SN2)

Role of HBr in Reaction

Watch next

Protonation converts the hydroxy group of an alcohol to a good leaving group. Suggest a mechanism for each reaction.
(a)