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

POCl3 Dehydration

Organic Chemistry

9. Alkenes and Alkynes

POCl3 Dehydration

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5:28 minutes

Problem 23

Textbook Question

Some alcohols undergo rearrangement or other unwanted side reactions when they dehydrate in acid. Alcohols may be dehydrated under mildly basic conditions using phosphorus oxy-chloride (POCl₃) in pyridine. The alcohol reacts with phosphorus oxychloride much like it reacts with tosyl chloride (Section 11-5), displacing a chloride ion from phosphorus to give an alkyl dichlorophosphate ester. The dichlorophosphate group is an outstanding leaving group. Pyridine reacts as a base with the dichlorophosphate ester to give an E2 elimination. Propose a mechanism for the dehydration of cyclohexanol by POCl₃ in pyridine.

Verified step by step guidance

Step 1: Activation of cyclohexanol - Cyclohexanol reacts with phosphorus oxychloride (POCl3). The hydroxyl group of cyclohexanol acts as a nucleophile and attacks the phosphorus atom of POCl3, displacing one of the chloride ions. This forms an alkyl dichlorophosphate ester intermediate.

Step 2: Formation of the leaving group - The dichlorophosphate group attached to cyclohexanol is an excellent leaving group. Pyridine, acting as a base, facilitates the next step by abstracting a proton from the cyclohexanol molecule.

Step 3: E2 elimination mechanism - Pyridine abstracts a proton from the β-carbon of the cyclohexanol molecule. Simultaneously, the dichlorophosphate group leaves, resulting in the formation of a double bond between the α and β carbons. This is an E2 elimination reaction.

Step 4: Formation of cyclohexene - The result of the E2 elimination is the formation of cyclohexene, a dehydrated product of cyclohexanol.

Step 5: Regeneration of pyridine - Pyridine is regenerated in the reaction and can continue to act as a base in subsequent reactions.

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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 removal of a water molecule to form an alkene. This process can occur through different mechanisms, including E1 and E2 eliminations. In acidic conditions, alcohols can undergo rearrangements, leading to unwanted side products. Understanding the conditions and mechanisms is crucial for predicting the outcome of dehydration reactions.

Phosphorus Oxychloride (POCl3)

Phosphorus oxychloride (POCl3) is a reagent used for the conversion of alcohols to alkyl chlorides or for dehydration under mild conditions. It reacts with alcohols to form alkyl dichlorophosphate esters, which are more stable and can facilitate elimination reactions. The presence of good leaving groups, such as the dichlorophosphate group, enhances the efficiency of the dehydration process.

Pyridine as a Base

Pyridine is a basic heterocyclic compound that can act as a nucleophile in organic reactions. In the context of dehydration, pyridine reacts with the alkyl dichlorophosphate ester formed from POCl3, facilitating an E2 elimination mechanism. This reaction pathway is favored under mild conditions, allowing for the formation of alkenes without the complications of rearrangements often seen in acid-catalyzed dehydration.

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