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Ch.9 - Alkynes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.9 - AlkynesProblem 7a,b,c
Chapter 9, Problem 7a,b,c

Show how you might synthesize the following compounds, using acetylene and any ­suitable alkyl halides as your starting materials. If the compound given cannot be ­synthesized by this method, explain why.
a. hex-1-yne
b. hex-2-yne
c. hex-3-yne

Verified step by step guidance
1
Step 1: Understand the problem. The goal is to synthesize the given alkynes (hex-1-yne, hex-2-yne, and hex-3-yne) using acetylene (C₂H₂) and suitable alkyl halides. Acetylene is a terminal alkyne, and its reactivity allows for chain elongation through alkylation reactions. Consider the structure of each compound and the position of the triple bond.
Step 2: For hex-1-yne: Begin by deprotonating acetylene using a strong base such as sodium amide (NaNH₂) to form the acetylide ion (C≡C⁻). Then, perform an alkylation reaction by reacting the acetylide ion with an alkyl halide, such as 1-bromobutane (CH₃CH₂CH₂CH₂Br). This will result in hex-1-yne (CH≡C-CH₂CH₂CH₂CH₃).
Step 3: For hex-2-yne: First, synthesize a terminal alkyne with a longer chain, such as pent-1-yne, using the method described in Step 2. Then, deprotonate pent-1-yne with a strong base to form the pentynyl anion. React this anion with methyl bromide (CH₃Br) to introduce a methyl group at the second carbon, forming hex-2-yne (CH₃C≡C-CH₂CH₂CH₃).
Step 4: For hex-3-yne: This compound cannot be synthesized directly using acetylene and alkyl halides because the triple bond is in an internal position that cannot be achieved through simple alkylation reactions. Hex-3-yne requires a different synthetic approach, such as starting with an appropriate internal alkyne precursor or using transition-metal-catalyzed coupling reactions.
Step 5: Summarize the synthesis strategies. Hex-1-yne and hex-2-yne can be synthesized using acetylene and alkyl halides through sequential deprotonation and alkylation steps. Hex-3-yne, however, cannot be synthesized using this method due to the limitations of acetylene-based alkylation reactions for internal alkynes.

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Key Concepts

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

Alkyne Synthesis

Alkynes can be synthesized through various methods, including the reaction of acetylene with alkyl halides via a process called nucleophilic substitution. This involves the formation of a carbon-carbon bond, where acetylene acts as a nucleophile, attacking the electrophilic carbon in the alkyl halide. The position of the triple bond in the resulting alkyne depends on the structure of the alkyl halide used.
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Markovnikov's Rule

Markovnikov's Rule is a principle that predicts the regioselectivity of electrophilic addition reactions to alkenes and alkynes. It states that when HX is added to an asymmetric alkene, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This concept is crucial when determining the possible products of reactions involving alkyl halides and acetylene, especially for synthesizing specific isomers.
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Rearrangement and Isomerization

Rearrangement and isomerization refer to the processes where the structure of a molecule changes to form a different isomer. In the context of synthesizing hex-1-yne, hex-2-yne, and hex-3-yne, understanding these concepts is essential, as certain alkyl halides may lead to the formation of undesired isomers or may not yield the target compound due to steric hindrance or stability issues in the reaction pathway.
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Related Practice
Textbook Question

The boiling points of hex-1-ene (64 °C) and hex-1-yne (71 °C) are sufficiently close that it is difficult to achieve a clean separation by distillation. Show how you might use the acidity of hex-1-yne to remove the last trace of it from a sample of hex-1-ene.

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Textbook Question

Solved Problem 9-1 showed the synthesis of dec-3-yne by adding the hexyl group first, then the ethyl group. Show the reagents and intermediates involved in the other order of synthesis of dec-3-yne, by adding the ethyl group first and the hexyl group last.

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Textbook Question

Show how you might synthesize the following compounds, using acetylene and any ­suitable alkyl halides as your starting materials. If the compound given cannot be ­synthesized by this method, explain why.

d. 4-methylhex-2-yne

e. 5-methylhex-2-yne

f. cyclodecyne

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Textbook Question

Show how you would synthesize each compound, beginning with acetylene and any necessary additional reagents

(a)

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Textbook Question

Show how you would synthesize each compound, beginning with acetylene and any necessary additional reagents

(b)

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Textbook Question

Predict the products of the following acid–base reactions, or indicate if no significant reaction would take place.

a. H—C≡C—H + NaNH2

b. H—C≡C—H + CH3Li

c. H—C≡C—H + NaOCH3

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