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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 37a,b,c
Chapter 9, Problem 37a,b,c

Show how you would synthesize the following compounds, starting with acetylene and any compounds containing no more than four carbon atoms.
a. hex-1-yne
b. hex-2-yne
c. cis-hex-2-ene

Verified step by step guidance
1
Step 1: Begin with acetylene (C₂H₂) as the starting material. To extend the carbon chain, perform an alkylation reaction. First, deprotonate acetylene using a strong base like sodium amide (NaNH₂) to form the acetylide ion (C≡C⁻). This ion is highly nucleophilic and can react with alkyl halides to add carbon chains.
Step 2: For hex-1-yne (a), perform two successive alkylation reactions. First, react the acetylide ion with ethyl bromide (CH₃CH₂Br) to form butyne (C≡C-CH₂CH₃). Then, deprotonate the terminal alkyne again with NaNH₂ and react with another ethyl bromide to extend the chain to hex-1-yne (CH≡C-CH₂CH₂CH₂CH₃).
Step 3: For hex-2-yne (b), start with acetylene and perform one alkylation reaction with propyl bromide (CH₃CH₂CH₂Br) to form pent-1-yne (CH≡C-CH₂CH₂CH₃). Then, deprotonate the terminal alkyne with NaNH₂ and react with methyl bromide (CH₃Br) to form hex-2-yne (CH₃-C≡C-CH₂CH₂CH₃).
Step 4: For cis-hex-2-ene (c), begin with hex-2-yne synthesized in the previous step. Perform a partial hydrogenation using a Lindlar catalyst (Pd/CaCO₃, poisoned with quinoline). This catalyst selectively reduces alkynes to cis-alkenes, resulting in cis-hex-2-ene (CH₃-CH=CH-CH₂CH₂CH₃).
Step 5: Verify the stereochemistry and structure of the final products using spectroscopic techniques such as NMR and IR. Ensure that the cis-alkene configuration is achieved for part (c) and that the alkyne positions are correct for parts (a) and (b).

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Key Concepts

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

Alkyne Synthesis

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. To synthesize alkynes from smaller carbon compounds, one common method is to perform a series of reactions, including dehydrohalogenation of alkyl halides or coupling reactions. Starting with acetylene, which is a two-carbon alkyne, allows for the construction of longer carbon chains through these reactions.
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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 (where X is a halogen) is added to an unsymmetrical alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This concept is crucial for determining the correct product when synthesizing compounds like hex-1-yne and hex-2-yne.
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Geometric Isomerism

Geometric isomerism occurs in compounds with restricted rotation around a double bond, leading to different spatial arrangements of substituents. In the case of cis-hex-2-ene, the 'cis' configuration indicates that the two substituents on the double bond are on the same side. Understanding this concept is essential for correctly identifying and synthesizing geometric isomers during organic reactions.
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Related Practice
Textbook Question

When treated with hydrogen and a platinum catalyst, an unknown compound X absorbs 5 equivalents of hydrogen to give n-butylcyclohexane. Treatment of X with an excess of ozone, followed by dimethyl sulfide and water, gives the following products:

Propose a structure for the unknown compound X. Is there any uncertainty in your structure?

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Textbook Question

Predict the products formed when CH3CH2–C≡C:–Na+ reacts with the following compounds.

a. ethyl bromide

b. tert-butyl bromide

c. formaldehyde

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Textbook Question

Show how you would synthesize the following compounds, starting with acetylene and any compounds containing no more than four carbon atoms.

d. trans-hex-2-ene

e. 1,1-dibromohexane

f. 2,2-dibromohexane

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Textbook Question

Show how you would synthesize the following compounds from acetylene and any other needed reagents:

(a) 6-phenylhex-1-en-4-yne

(b) cis-1-phenylpent-2-ene

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Textbook Question

Predict the products formed when CH3CH2–C≡C:–Na+ reacts with the following compounds.

(d) cyclohexanone

(e) CH3CH2CH2CHO

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Textbook Question

Show how you would synthesize the following compounds, starting with acetylene and any compounds containing no more than four carbon atoms.

g. pentanal, CH3CH2CH2CH2CHO

h. pentan-2-one, CH3–CO–CH2CH2CH3

i. (±) 3,4-dibromohexane

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