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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5:50 minutes

Problem 33ab,c

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

Verified step by step guidance

Step 1: Analyze the structure of pent-1-yne. It is a terminal alkyne with the triple bond located between the first and second carbons in the chain. Terminal alkynes are reactive and undergo addition reactions with electrophiles like HCl and hydrogen gas.

Step 2: For part (a), when 1 equivalent of HCl is added to pent-1-yne, the reaction proceeds via Markovnikov addition. The hydrogen atom from HCl will add to the terminal carbon (C1), and the chlorine atom will add to the adjacent carbon (C2). This results in the formation of a vinyl halide (chloroalkene). The product will have a double bond between C1 and C2.

Step 3: For part (b), when 2 equivalents of HCl are added, the reaction proceeds in two steps. First, the initial addition of HCl forms the vinyl halide as described in part (a). Then, the second equivalent of HCl adds to the double bond, again following Markovnikov's rule. The hydrogen adds to C1, and the chlorine adds to C2, resulting in a geminal dihalide (both chlorine atoms attached to C2).

Step 4: For part (c), when pent-1-yne is treated with excess H2 in the presence of a nickel (Ni) catalyst, the alkyne undergoes complete hydrogenation. The triple bond is reduced to a single bond, and the product is pentane, a fully saturated alkane.

Step 5: Summarize the products: (a) vinyl chloride (C1=C2 with Cl on C2), (b) geminal dichloride (C1-C2 with two Cl atoms on C2), and (c) pentane (fully saturated alkane). These products are formed based on the reagents and reaction conditions provided.

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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 pent-1-yne, are hydrocarbons containing a carbon-carbon triple bond. This triple bond makes them more reactive than alkenes and alkanes, allowing them to undergo various addition reactions. Understanding how alkynes react with electrophiles, such as HCl, is crucial for predicting the products of these reactions.

Electrophilic Addition

Electrophilic addition is a key 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 HCl, the triple bond acts as a nucleophile, attacking the electrophilic hydrogen, resulting in the formation of a vinyl halide or a geminal dihalide, depending on the amount of HCl used.

Hydrogenation

Hydrogenation is the process of adding hydrogen (H2) to unsaturated hydrocarbons, such as alkynes, to convert them into saturated hydrocarbons. When pent-1-yne is treated with excess H2 in the presence of a catalyst like nickel (Ni), the triple bond is fully reduced to a single bond, resulting in the formation of pentane. This concept is essential for understanding how to manipulate the saturation of organic compounds.

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Related Practice

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)

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

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: (iv) HBr (1 equiv.). If you expect two products, show both.

(c)

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Predict the products of reaction of pent-1-yne with the following reagents. a. 1 equivalent of HClb. 2 equivalents of HCl c. excess H2, Ni

Key Concepts

Alkyne Reactivity

Electrophilic Addition

Hydrogenation

Watch next

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