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

9. Alkenes and Alkynes

Dehydration Reaction

Organic Chemistry

9. Alkenes and Alkynes

Dehydration Reaction

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8:35 minutes

Problem 52c,d

Textbook Question

Predict the major products of acid-catalyzed dehydration of the following alcohols.
(c)
(d)

Verified step by step guidance

Step 1: Understand the reaction mechanism. Acid-catalyzed dehydration of alcohols typically proceeds via an E1 or E2 mechanism, depending on the structure of the alcohol. The reaction involves the removal of a water molecule to form an alkene. The stability of the carbocation intermediate (if E1) or the transition state (if E2) determines the major product.

Step 2: Analyze the structure of 2-methylcyclohexanol (part c). Protonation of the hydroxyl group (-OH) by the acid converts it into a better leaving group (water). This is followed by the loss of water to form a carbocation. In this case, the carbocation forms at the carbon bearing the hydroxyl group. Consider possible rearrangements to form a more stable carbocation, such as a hydride or alkyl shift.

Step 3: Apply Zaitsev's rule to predict the major product for 2-methylcyclohexanol. Zaitsev's rule states that the more substituted alkene is generally the major product. After the carbocation forms, elimination of a proton (H⁺) from a β-carbon leads to the formation of the alkene. Identify all possible β-carbons and determine which elimination gives the more substituted alkene.

Step 4: Analyze the structure of 2,2-dimethylpropan-1-ol (part d). Protonation of the hydroxyl group by the acid converts it into a better leaving group (water). However, the primary carbocation that would form is highly unstable. Therefore, a rearrangement occurs to form a more stable tertiary carbocation. This involves a methyl shift from an adjacent carbon.

Step 5: Predict the major product for 2,2-dimethylpropan-1-ol. After the rearranged tertiary carbocation forms, elimination of a proton (H⁺) from a β-carbon leads to the formation of the alkene. Since there is only one β-carbon available, the product will be a single alkene. Ensure the structure of the product reflects the rearrangement and elimination steps.

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Key Concepts

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

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a reaction where an alcohol is converted into an alkene through the removal of a water molecule, facilitated by an acid. The acid protonates the hydroxyl group, making it a better leaving group, which then leads to the formation of a carbocation intermediate. This process is crucial for predicting the major products, as the stability of the carbocation influences the outcome.

Carbocation Stability

Carbocation stability is a key factor in determining the products of dehydration reactions. Carbocations can be classified as primary, secondary, or tertiary, with tertiary carbocations being the most stable due to hyperconjugation and inductive effects. The more stable the carbocation, the more likely it is to form during the reaction, guiding the formation of the major product.

Zaitsev's Rule

Zaitsev's Rule states that in elimination reactions, the more substituted alkene is typically the major product. This principle applies to dehydration reactions, where the elimination of water leads to the formation of alkenes. Understanding this rule helps predict which alkene will be favored based on the structure of the starting alcohol and the stability of the resulting alkenes.

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Textbook Question

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.

Textbook Question

What is the product of the following reaction?

Textbook Question

Which of the following alcohols dehydrates the fastest when heated with acid?

Textbook Question

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.

(a)

(Hint: Hydride shift)

Textbook Question

A vicinal diol has OH groups on adjacent carbons. The dehydration of a vicinal diol is accompanied by a rearrangement called the pinacol rearrangement. Propose a mechanism for this reaction.

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.

(c)

Multiple Choice

Which of the following is an example of a dehydration reaction?

Multiple Choice

Which of the following reactions is a dehydration reaction?

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