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 Hydrohalogenation

Organic Chemistry

10. Addition Reactions

Alkyne Hydrohalogenation

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5:30 minutes

Problem 44c(iv)

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (iv) HBr (1 equiv.). If you expect two products, show both.
(c)

Verified step by step guidance

Identify the structure of the alkyne provided in part (c). Determine whether it is a terminal alkyne (with a hydrogen at one end) or an internal alkyne (with substituents on both sides of the triple bond). This will influence the regioselectivity of the reaction.

Recall the reaction mechanism for the addition of HBr to an alkyne. The reaction proceeds via Markovnikov addition, where the hydrogen atom from HBr adds to the carbon of the triple bond that already has more hydrogens, and the bromine atom adds to the other carbon.

Since only 1 equivalent of HBr is used, the reaction will stop at the alkene stage, forming a bromoalkene. The triple bond will be converted into a double bond, and the bromine will be added to the more substituted carbon (Markovnikov's rule).

If the alkyne is unsymmetrical, consider the possibility of regioisomers. Draw both potential products if applicable, showing the placement of the bromine atom and the double bond in each case.

Verify the stereochemistry of the product(s). For alkynes, the addition of HBr does not typically lead to stereoisomers because the reaction proceeds through a planar carbocation intermediate. Ensure the final product(s) are drawn correctly with the double bond and bromine in the appropriate positions.

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Key Concepts

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

Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. They are unsaturated compounds and are generally more reactive than alkenes and alkanes due to the presence of the triple bond. The reactivity of alkynes allows them to undergo various addition reactions, which is crucial for predicting the products formed when they react with reagents like HBr.

Electrophilic Addition

Electrophilic addition is a reaction mechanism where an electrophile reacts with a nucleophile, leading to the formation of a more saturated product. In the case of alkynes reacting with HBr, the triple bond acts as a nucleophile, attacking the electrophilic hydrogen atom of HBr. This reaction can lead to the formation of either a vinyl bromide or a bromoalkane, depending on the reaction conditions and the regioselectivity of the addition.

Markovnikov's Rule

Markovnikov's Rule states that when HX (where X is a halogen) adds to an alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This rule helps predict the major product in reactions involving unsymmetrical alkynes. In the case of HBr addition to an alkyne, this rule can lead to the formation of two different products, depending on the regioselectivity of the addition.

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Related Practice

Textbook Question

Predict the products of reaction of pent-1-yne with the following reagents.

a. 1 equivalent of HCl

b. 2 equivalents of HCl

c. excess H₂, Ni

Multiple Choice

Predict the major, organic product of the following reaction.

Textbook Question

Predict the major product(s) of the following reactions:

a. phenylacetylene + 2 HBr

b. hex-1-yne + 2 HCl

Textbook Question

Propose a mechanism for the entire reaction of pent-1-yne with 2 moles of HBr. Show why Markovnikov's rule should be observed in both the first and second additions of HBr.

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (v) HCl (2 equiv.). If you expect two products, show both.

(c)

Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(a)

Textbook Question

HBr and peroxides are also used to generate 1-bromo-1-alkenes. Suggest a starting reactant that would successfully undergo this reaction.

Textbook Question

Predict the major products resulting from the addition of one equivalent of HX to the following alkynes.

(b)

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For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (iv) HBr (1 equiv.). If you expect two products, show both. (c)

Key Concepts

Alkynes

Electrophilic Addition

Markovnikov's Rule

Watch next

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (iv) HBr (1 equiv.). If you expect two products, show both.
(c)