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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6:15 minutes

Problem 12a

Textbook Question

Propose a mechanism for the reaction of
(a) 1-methylcyclohexanol with HBr to form 1-bromo-1-methylcyclohexane.

Verified step by step guidance

Step 1: Protonation of the alcohol group - The hydroxyl group (-OH) in 1-methylcyclohexanol is a poor leaving group. To make it a better leaving group, it is protonated by HBr, forming water (H2O) as a better leaving group. This step involves the transfer of a proton (H⁺) from HBr to the oxygen atom of the hydroxyl group.

Step 2: Formation of a carbocation intermediate - After protonation, the water molecule leaves, resulting in the formation of a carbocation at the carbon atom that was originally bonded to the hydroxyl group. Since the hydroxyl group is attached to a tertiary carbon (1-methylcyclohexanol is a tertiary alcohol), the resulting carbocation is a tertiary carbocation, which is relatively stable due to hyperconjugation and inductive effects.

Step 3: Nucleophilic attack by bromide ion - The bromide ion (Br⁻), which is generated from the dissociation of HBr, acts as a nucleophile and attacks the positively charged carbocation. This step involves the formation of a new C-Br bond, resulting in the product 1-bromo-1-methylcyclohexane.

Step 4: Verify the stereochemistry - Since the reaction proceeds through a carbocation intermediate, the product may exhibit racemization if the carbon center is chiral. However, in this case, the product is not chiral, so no stereochemical considerations are necessary.

Step 5: Summarize the mechanism - The overall reaction involves three key steps: protonation of the alcohol group, formation of a carbocation intermediate, and nucleophilic attack by the bromide ion. The final product is 1-bromo-1-methylcyclohexane.

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Key Concepts

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

Alcohol Reactivity

Alcohols, such as 1-methylcyclohexanol, can undergo nucleophilic substitution reactions. In the presence of strong acids like HBr, the hydroxyl (-OH) group is protonated, converting it into a better leaving group (water). This transformation is crucial for facilitating the subsequent substitution reaction where the bromide ion replaces the -OH group.

Nucleophilic Substitution Mechanism

The reaction mechanism for converting alcohols to alkyl halides typically follows an SN1 or SN2 pathway. In this case, the reaction likely proceeds via an SN1 mechanism due to the stability of the carbocation formed after the leaving group departs. The formation of a stable tertiary or secondary carbocation allows for the rapid attack by the bromide ion, leading to the final product.

Carbocation Stability

Carbocation stability is a key factor in determining the pathway of nucleophilic substitution reactions. Tertiary carbocations are more stable than secondary or primary ones due to hyperconjugation and inductive effects from surrounding alkyl groups. In the case of 1-methylcyclohexanol, the formation of a stable carbocation intermediate is essential for the efficient conversion to 1-bromo-1-methylcyclohexane.

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Related Practice

Textbook Question

Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vii) HCl; (viii) HBr. If no reaction occurs, write 'no reaction.'

(a)

Textbook Question

Predict the product of the following reactions.

(b)

Textbook Question

Suggest the appropriate reagents to carry out the following transformations.

(d)

Textbook Question

Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.

(a)

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

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

(a)

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