Beginning with acetylene and benzyl bromide and using any other inorganic reagents, propose a synthesis of the alkene shown here.

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Step 1: Begin by deprotonating acetylene (C₂H₂) using a strong base such as sodium amide (NaNH₂). This will generate the acetylide anion (C≡C⁻), which is a nucleophile.

Step 2: Perform an SN2 reaction between the acetylide anion and benzyl bromide (C₆H₅CH₂Br). The acetylide anion will attack the benzyl bromide, displacing the bromide ion and forming a new carbon-carbon bond, resulting in phenylpropyne (C₆H₅CH₂C≡CH).

Step 3: Hydrogenate phenylpropyne selectively to form the desired alkene. Use a Lindlar catalyst (Pd/CaCO₃ poisoned with Pb(OAc)₂ and quinoline) to perform partial hydrogenation, converting the triple bond (C≡C) into a cis double bond (C=C). This ensures the formation of the cis-alkene.

Step 4: Verify the stereochemistry of the product to ensure the alkene formed is the desired one. The Lindlar catalyst ensures cis-selectivity during hydrogenation.

Step 5: Purify the product using techniques such as distillation or chromatography to isolate the desired alkene and remove any impurities or side products.

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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 simple alkyne that can undergo various reactions due to its triple bond. It can participate in nucleophilic addition reactions, where it acts as a nucleophile, and can also be converted into more complex structures through coupling reactions. Understanding its reactivity is crucial for designing synthetic pathways involving acetylene.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile replaces a leaving group in a molecule. In the context of benzyl bromide, the bromine atom serves as a leaving group, allowing nucleophiles to attack the carbon atom bonded to it. This concept is essential for understanding how to manipulate benzyl bromide in the synthesis process.

Alkene Formation

Alkene formation is a key transformation in organic synthesis, often achieved through elimination reactions or coupling reactions. In this case, the goal is to form a double bond between carbon atoms, which can be accomplished by removing elements from adjacent carbon atoms. Recognizing the mechanisms and conditions that favor alkene formation is vital for successfully proposing a synthetic route.