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

Double Elimination

Organic Chemistry

9. Alkenes and Alkynes

Double Elimination

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1:56 minutes

Problem 61

Textbook Question

An alternate method for the synthesis of alkynes relies on the double elimination of H―Br from a dihaloalkane under basic conditions. Suggest a mechanism for this reaction that we discuss in Chapter 12.

Verified step by step guidance

Step 1: Begin by identifying the starting material, which is a dihaloalkane. A dihaloalkane contains two halogen atoms (e.g., bromine) attached to adjacent carbon atoms in the molecule.

Step 2: Recognize that the reaction occurs under basic conditions. A strong base, such as NaNH₂ or KOH, is typically used to facilitate the elimination process. The base will abstract a proton (H⁺) from the molecule.

Step 3: In the first elimination step, the base abstracts a proton from a carbon atom adjacent to one of the halogen atoms. This leads to the formation of a carbanion intermediate. The halogen atom (e.g., Br) leaves as a halide ion (Br⁻), resulting in the formation of a double bond (alkene).

Step 4: In the second elimination step, the base abstracts another proton from the carbon atom adjacent to the remaining halogen atom. This leads to the formation of another carbanion intermediate. The second halogen atom leaves as a halide ion, resulting in the formation of a triple bond (alkyne).

Step 5: The final product is an alkyne, formed after the double elimination of H―Br from the dihaloalkane. The reaction mechanism involves two consecutive E2 elimination steps, where the base facilitates the removal of protons and halide ions to form the alkyne.

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Key Concepts

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

Alkynes and Their Synthesis

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. They can be synthesized through various methods, including the double elimination of hydrogen halides from dihaloalkanes. Understanding the structure and reactivity of alkynes is crucial for predicting the outcomes of synthetic reactions.

Dihaloalkanes

Dihaloalkanes are organic compounds that contain two halogen atoms attached to a carbon chain. Their reactivity is influenced by the nature of the halogens and the surrounding conditions. In the context of alkyne synthesis, the elimination of H-Br from a dihaloalkane is a key step that requires a strong base to facilitate the reaction.

Elimination Reactions

Elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of a double or triple bond. In this case, the double elimination of H-Br from a dihaloalkane leads to the formation of an alkyne. Understanding the mechanism of elimination reactions, including the role of bases and the formation of intermediates, is essential for predicting reaction pathways.

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Related Practice

Textbook Question

Show how you would accomplish the following synthetic transformations. Show all intermediates.

a. 2,2-dibromobutane → but-1-yne

Textbook Question

Show how you would accomplish the following synthetic transformations. Show all intermediates.

b. 2,2-dibromobutane → but-2-yne

Textbook Question

When 2,2-dibromo-1-phenylpropane is heated overnight in fused KOH at 200 C, the major product is a foul-smelling compound of formula C₉H₈. Propose a structure for this product, and give a mechanism to account for its formation.

Problem-Solving Hint:

Textbook Question

When 2,2-dibromo-1-phenylpropane is heated overnight with sodium amide at 150°C, the major product (after addition of water) is a different foul-smelling compound of formula C₉H₈. Propose a structure for this product, and give a mechanism to account for its formation.

Textbook Question

Show how you would accomplish the following synthetic transformations. Show all intermediates.

a. 2,2-dibromobutane → but-1-yne

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