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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 7d,e,f
Chapter 9, Problem 7d,e,f

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

Verified step by step guidance
1
Step 1: Begin by understanding the problem. You are tasked with synthesizing the given compounds using acetylene (C₂H₂) and suitable alkyl halides. Acetylene is a versatile starting material for alkyne synthesis due to its ability to undergo nucleophilic substitution reactions after deprotonation.
Step 2: Deprotonate acetylene using a strong base, such as sodium amide (NaNH₂), to generate the acetylide ion (C≡C⁻). This ion is highly nucleophilic and can react with alkyl halides in an SN2 reaction to form longer carbon chains.
Step 3: For compound d (4-methylhex-2-yne), identify the two alkyl halides needed to construct the molecule. First, react acetylide ion with 1-bromopropane to form a propyl-substituted alkyne. Then, deprotonate the terminal alkyne again and react it with 2-bromo-2-methylpropane to introduce the 4-methyl group.
Step 4: For compound e (5-methylhex-2-yne), follow a similar approach. React acetylide ion with 1-bromobutane to form a butyl-substituted alkyne. Deprotonate the terminal alkyne again and react it with 2-bromo-2-methylpropane to introduce the 5-methyl group.
Step 5: For compound f (cyclodecyne), this cannot be synthesized using acetylene and alkyl halides alone. Cyclodecyne requires a ring-closing reaction, which typically involves specialized methods such as intramolecular coupling reactions or ring-closing metathesis. Acetylene and alkyl halides are not suitable for this synthesis due to the need for forming a cyclic structure.

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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 alkylation. This involves the nucleophilic attack of acetylide ions, generated from acetylene, on primary alkyl halides. Understanding the reactivity and selectivity of alkyl halides is crucial for determining the feasibility of synthesizing specific alkynes.
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Regioselectivity in Reactions

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the context of synthesizing alkynes, the position of the triple bond and the branching of the carbon chain can significantly affect the outcome. Recognizing how the structure of the starting materials influences the regioselectivity is essential for predicting the products of the synthesis.
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Cycloalkyne Formation

Cycloalkynes are cyclic compounds containing a carbon-carbon triple bond. Their synthesis often requires specific conditions due to ring strain and stability issues. Understanding the limitations and conditions necessary for forming cycloalkynes, such as cyclodecyne, is important for evaluating whether a compound can be synthesized from acetylene and alkyl halides.
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Related Practice
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 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

(c)

(d)

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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.

a. hex-1-yne

b. hex-2-yne

c. hex-3-yne

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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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