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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9:13 minutes

Problem 35b

Textbook Question

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.
(b)

Verified step by step guidance

Step 1: Recognize that the reaction involves the dehydration of an alcohol. Dehydration typically occurs under acidic conditions (e.g., sulfuric or phosphoric acid) and heat, leading to the formation of an alkene.

Step 2: Protonation of the alcohol group occurs first. The hydroxyl group (-OH) is protonated by the acid, converting it into a better leaving group (water, H₂O). This step increases the likelihood of the alcohol undergoing elimination.

Step 3: Formation of a carbocation intermediate. After the protonation, the water molecule leaves, generating a carbocation at the carbon where the hydroxyl group was originally attached. In this case, the carbocation forms at the secondary carbon.

Step 4: Consider carbocation rearrangement. Secondary carbocations can rearrange to form more stable tertiary carbocations if possible. In this molecule, the carbocation is already at a secondary position, and no rearrangement to a tertiary carbocation is possible.

Step 5: Elimination of a proton from a β-carbon occurs, forming the double bond. A base (often the conjugate base of the acid used) removes a proton from a β-carbon adjacent to the carbocation, resulting in the formation of the alkene. The major product will follow Zaitsev's rule, favoring the more substituted alkene.

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Key Concepts

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

Dehydration of Alcohols

Dehydration of alcohols involves the elimination of a water molecule from the alcohol, typically in the presence of an acid catalyst like sulfuric or phosphoric acid. This process leads to the formation of alkenes. The reaction can proceed via either an E1 or E2 mechanism, depending on the structure of the alcohol and the reaction conditions.

E1 and E2 Mechanisms

The E1 mechanism is a two-step process where the alcohol first loses a proton to form a carbocation intermediate, followed by the elimination of a leaving group to form the alkene. In contrast, the E2 mechanism is a one-step process where the base abstracts a proton while the leaving group departs simultaneously, leading to the formation of the alkene. The choice between these mechanisms depends on the substrate and conditions.

Regioselectivity and Zaitsev's Rule

Regioselectivity refers to the preference for the formation of one constitutional isomer over others in a chemical reaction. Zaitsev's Rule states that in elimination reactions, the more substituted alkene is typically favored as the major product. This principle is crucial when predicting the products of dehydration reactions, especially when multiple alkene products are possible.

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