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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 37g,h,i
Chapter 9, Problem 37g,h,i

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

Verified step by step guidance
1
Step 1: For pentanal (CH3CH2CH2CH2CHO), start with acetylene (C2H2). Perform a hydroboration-oxidation reaction to convert acetylene into an aldehyde. Use reagents such as BH3 (borane) followed by H2O2/NaOH to form an aldehyde with two carbons (ethanal, CH3CHO).
Step 2: Extend the carbon chain of ethanal to five carbons. Use a Grignard reaction by reacting ethanal with a Grignard reagent (e.g., CH3CH2CH2MgBr) followed by acidic workup to form a secondary alcohol. Then, oxidize the alcohol to the aldehyde (pentanal) using PCC (pyridinium chlorochromate).
Step 3: For pentan-2-one (CH3-CO-CH2CH2CH3), start with acetylene and perform a hydration reaction using H2SO4 and HgSO4 to form acetone (CH3-CO-CH3).
Step 4: Extend the carbon chain of acetone to five carbons. Use a Grignard reaction by reacting acetone with CH3CH2MgBr followed by acidic workup to form a tertiary alcohol. Oxidize the tertiary alcohol to the ketone (pentan-2-one) using PCC or another mild oxidizing agent.
Step 5: For (±) 3,4-dibromohexane, start with acetylene and perform a series of alkylation reactions using alkyl halides (e.g., CH3CH2Br) to extend the carbon chain to six carbons. Then, perform a bromination reaction using Br2 in CCl4 to add bromine atoms to the 3 and 4 positions of the hexane chain, ensuring the formation of the racemic mixture (±).

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Key Concepts

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

Acetylene as a Building Block

Acetylene (C2H2) is a simple alkyne that serves as a versatile building block in organic synthesis. It can undergo various reactions, such as addition reactions with electrophiles, to form longer carbon chains. Understanding how to manipulate acetylene through reactions like hydrohalogenation or alkylation is crucial for synthesizing more complex molecules, including those with longer carbon chains.
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Carbon Chain Elongation

Carbon chain elongation involves increasing the number of carbon atoms in a molecule through reactions such as nucleophilic substitution or coupling reactions. This concept is essential for synthesizing compounds like pentanal and pentan-2-one, where specific carbon skeletons must be constructed. Mastery of techniques like the Grignard reaction or the use of organolithium reagents can facilitate this process.
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Functional Group Transformation

Functional group transformation refers to the process of converting one functional group into another, which is vital for achieving the desired properties in organic compounds. For example, converting an alcohol to a carbonyl group or introducing halogens into a hydrocarbon chain is often necessary in synthesis. Understanding the reactivity and compatibility of different functional groups allows chemists to design effective synthetic routes for compounds like 3,4-dibromohexane.
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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

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 convert the following starting materials into the target compound. You may use any additional reagents you need.

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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.

a. hex-1-yne

b. hex-2-yne

c. cis-hex-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

When compound Z is treated with ozone, followed by dimethyl sulfide and washing with water, the products are formic acid, 3-oxobutanoic acid, and hexanal.

Propose a structure for compound Z. What uncertainty is there in the structure you have proposed?