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

10. Addition Reactions

Alkyne Halogenation

Organic Chemistry

10. Addition Reactions

Alkyne Halogenation

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4:51 minutes

Problem 56

Textbook Question

When alkynes are treated with water and bromine a bromoketone is produced. Provide a plausible arrow-pushing mechanism that accounts for the formation of this product.

Verified step by step guidance

Identify the starting material (alkyne) and the reagents (water and bromine). Recognize that the reaction involves electrophilic addition followed by nucleophilic attack and tautomerization.

In the first step, bromine (Br₂) acts as an electrophile. The π-electrons of the alkyne attack one bromine atom, forming a bromonium ion intermediate. This step involves the breaking of the Br-Br bond, leaving a bromide ion (Br⁻) as a byproduct.

Next, water (H₂O) acts as a nucleophile and attacks the more substituted carbon of the bromonium ion, opening the three-membered ring. This step results in the formation of a bromohydrin intermediate.

The bromohydrin undergoes tautomerization. The hydroxyl group (-OH) is protonated by a proton donor (e.g., H⁺), forming a good leaving group (H₂O). The adjacent carbon-carbon bond electrons shift to form a double bond, and the bromide ion (Br⁻) abstracts a proton from the neighboring carbon, resulting in the formation of a bromoketone.

Finally, confirm the product structure as a bromoketone, where the carbonyl group (C=O) is adjacent to a bromine atom. This product is stabilized due to resonance and the electron-withdrawing effect of bromine.

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Key Concepts

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

Alkynes and Their Reactivity

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. Their reactivity is significantly influenced by this triple bond, which can undergo various reactions, including electrophilic addition. Understanding the nature of alkynes is crucial for predicting how they will interact with reagents like water and bromine.

Electrophilic Addition Mechanism

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, leading to the formation of a more stable product. In the case of alkynes, the triple bond acts as a nucleophile, attacking the electrophilic bromine, which initiates the mechanism that ultimately leads to the formation of a bromoketone.

Arrow-Pushing Notation

Arrow-pushing notation is a visual representation used to depict the movement of electrons during chemical reactions. It illustrates how electrons are transferred between atoms and bonds, helping to clarify the steps in a reaction mechanism. Mastery of this notation is essential for accurately conveying the sequence of events in the formation of products like bromoketones from alkynes.

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