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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5:42 minutes

Problem 40a,b

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)

Verified step by step guidance

Step 1: Understand the structure of 2-butyne. It is an alkyne with the molecular formula C4H6, featuring a triple bond between the second and third carbon atoms. Its structure can be represented as CH3-C≡C-CH3.

Step 2: For part (a), when 2-butyne reacts with 1 mol of HBr, the reaction proceeds via Markovnikov's rule. The hydrogen atom from HBr will add to the carbon of the triple bond with more hydrogens (the terminal carbon), and the bromine atom will add to the other carbon of the triple bond. This results in the formation of a bromoalkene. The intermediate product will be CH3-CH(Br)-C≡CH.

Step 3: For part (b), when 2-butyne reacts with 2 mol of HBr, the reaction proceeds further. The second equivalent of HBr will add to the double bond of the intermediate bromoalkene formed in part (a). Again, following Markovnikov's rule, the hydrogen atom will add to the carbon already bearing the hydrogen, and the bromine atom will add to the carbon already bearing the bromine. This results in the formation of a geminal dibromide (a compound with two bromine atoms on the same carbon). The final product will be CH3-C(Br)2-CH3.

Step 4: Summarize the reaction mechanism for both parts. In part (a), the addition of HBr to the triple bond occurs in one step, forming a bromoalkene. In part (b), the addition of the second mol of HBr to the double bond occurs in a second step, forming a geminal dibromide. Both steps follow Markovnikov's rule.

Step 5: To confirm the products, consider the regioselectivity and stereochemistry of the reaction. Since the reaction follows Markovnikov's rule, the bromine atoms will add to the more substituted carbon atoms, and the products will be consistent with this rule. No stereoisomers are formed in this case because the products are symmetrical.

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Key Concepts

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

Alkyne Reactivity

Alkynes, such as 2-butyne, are hydrocarbons with a carbon-carbon triple bond. This triple bond makes them more reactive than alkenes and alkanes, allowing them to undergo various addition reactions. Understanding the reactivity of alkynes is crucial for predicting the products formed when they react with reagents like HBr.

Electrophilic Addition

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 reacting with HBr, the triple bond acts as a nucleophile, attacking the electrophilic hydrogen, resulting in the formation of a bromoalkene or bromoalkane depending on the amount of HBr used.

Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX to an alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This principle helps predict the major product of the reaction when 2-butyne reacts with HBr, guiding the understanding of regioselectivity in the formation of bromoalkenes or bromoalkanes.

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Related Practice

Textbook Question

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

(b)

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

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

c. cyclooctyne + 2 HBr

d. *hex-2-yne + 2 HCl

Textbook Question

Show how hex-1-yne might be converted to

f. 2,2-dibromohexane.

Textbook Question

Using hex-1-ene as your starting material, show how you would synthesize the following compounds. (Once you have shown how to synthesize a compound, you may use it as the starting material in any later parts of this problem.)

a. 1,2-dibromohexane

b. hex-1-yne

c. 2,2-dibromohexane

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

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