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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Problem 33b

Textbook Question

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

Verified step by step guidance

Step 1: Identify the alkyne functional group in the given molecule. The molecule contains a triple bond between two carbon atoms, which is the site of reactivity for the addition of HCl.

Step 2: Recall the mechanism of electrophilic addition of HX (hydrogen halide) to alkynes. The reaction proceeds via Markovnikov's rule, where the hydrogen atom from HX adds to the carbon of the triple bond with more hydrogens, and the halide (Cl⁻) adds to the carbon with fewer hydrogens.

Step 3: Analyze the structure of the alkyne. The triple bond is located between two carbons, one of which is more substituted (attached to more carbon groups) than the other. According to Markovnikov's rule, the chlorine atom will add to the more substituted carbon.

Step 4: Consider the stereochemistry and regioselectivity of the product. Since only one equivalent of HCl is added, the reaction stops at the alkene stage, forming a double bond. The major product will be a vinyl halide (an alkene with a halogen attached).

Step 5: Draw the structure of the major product based on the addition pattern. The double bond will remain between the two carbons, and the chlorine atom will be attached to the more substituted carbon. Ensure the product reflects Markovnikov's addition and retains the original substituents on the molecule.

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Key Concepts

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

Alkyne Reactivity

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. They are more reactive than alkenes due to the high electron density of the triple bond, which can undergo addition reactions. When treated with HX (where X is a halogen), alkynes can react to form haloalkenes, with the addition occurring across the triple bond.

Markovnikov's Rule

Markovnikov's Rule states that when HX is added to an unsymmetrical alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This results in the more stable carbocation intermediate and leads to the formation of the major product. Understanding this rule is crucial for predicting the outcome of the addition reaction.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the context of alkyne addition reactions, regioselectivity is influenced by the stability of the intermediates formed during the reaction, which is guided by factors such as sterics and electronic effects. This concept is essential for predicting the major product in the reaction with HX.

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Related Practice

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)

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

(b)

Textbook Question

The addition of H―X to alkynes has been shown to occur predominately via anti addition:

Two chemists disagreed on whether or not anti addition would happen on terminal alkynes as well. Suggest an experiment through which you could resolve this dispute.

Textbook Question

What is the major product obtained from the reaction of each of the following compounds with excess HCl?

c. CH₃CH₂C☰CCH₂CH₂CH₃

Textbook Question

Answer Problem 39, parts a–h, using 2-butyne as the starting material instead of propyne.

a. HBr (1 mol)

b. HBr (2 mol)

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

Key Concepts

Alkyne Reactivity

Markovnikov's Rule

Regioselectivity

Watch next

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