Ch.9 - Alkynes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.9 - Alkynes

Problem 33ab,c

Chapter 9, Problem 33ab,c

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

Using cyclooctyne 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.)

(i)

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Textbook Question

The application box in the margin of states, “The addition of an acetylide ion to a carbonyl group is used in the synthesis of ethchlorvynol, a drug used to cause drowsiness and induce sleep.” Show how you would accomplish this synthesis from acetylene and a carbonyl compound.

1013

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Textbook Question

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

g. cold, dilute KMnO₄

h. warm, concd. KMnO₄, NaOH

i. Na, liquid ammonia

1453

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Textbook Question

Muscalure, the sex attractant of the common housefly, is cis-tricos-9-ene. Most syntheses of alkenes give the more stable trans isomer as the major product. Devise a synthesis of muscalure from acetylene and other compounds of your choice. Your synthesis must give specifically the cis isomer of muscalure.

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Textbook Question

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

d. H₂, Pd/BaSO₄, quinoline

e. 1 equivalent of Br₂

f. 2 equivalents of Br₂

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