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Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
Chapter 8, Problem 28c

How could the following compounds be synthesized from acetylene?
c. CH3CH═CH2

Verified step by step guidance
1
Step 1: Begin by understanding the structure of acetylene (C≡CH), which is a simple alkyne. The goal is to transform this into CH3CH=CH2, a propene molecule with a double bond.
Step 2: Perform a hydrohalogenation reaction on acetylene. React acetylene with HBr to form bromoethene (CH2=CHBr). This step introduces a halogen atom to the molecule while converting the triple bond to a double bond.
Step 3: Carry out a Grignard reaction or an alkylation step. React bromoethene with a methyl Grignard reagent (CH3MgBr) or a methyl halide (CH3X) in the presence of a strong base to replace the bromine atom with a methyl group, forming CH3CH=CH2.
Step 4: Ensure stereochemistry and regioselectivity are correct. Verify that the addition of the methyl group occurs at the correct position to yield the desired product, propene.
Step 5: Purify the product using distillation or another appropriate method to isolate CH3CH=CH2 and confirm its structure using spectroscopic techniques such as NMR or IR.

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Key Concepts

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

Acetylene Reactivity

Acetylene (C2H2) is a highly reactive alkyne that can undergo various reactions, including addition and substitution. Understanding its reactivity is crucial for synthesizing other compounds, as it can react with electrophiles or undergo polymerization. The ability to manipulate its triple bond allows for the formation of different functional groups and carbon skeletons.
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Alkene Synthesis

The target compound, CH3CH═CH2, is an alkene known as propene. Synthesizing alkenes from alkynes often involves a process called elimination, where hydrogen atoms are removed from adjacent carbon atoms. This transformation can be achieved through dehydrohalogenation or dehydrogenation reactions, which are essential for converting acetylene into more complex alkenes.
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Reagents and Reaction Conditions

The choice of reagents and reaction conditions is critical in organic synthesis. For the conversion of acetylene to propene, specific catalysts or bases may be required to facilitate the elimination reaction. Understanding how different reagents interact with acetylene and the conditions that favor the desired reaction pathway is vital for successful synthesis.
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