Question

The Wittig reaction can be used for the synthesis of conjugated dienes, as, for example, 1-phenyl- 1,3 -pentadiene.

Short answer

Answer: The two crucial steps in synthesizing 1-phenyl-1,3-pentadiene through the Wittig reaction are (1) the formation of the Wittig reagent (ylide) involving the reaction between a phosphonium salt and a strong base, and (2) the actual Wittig reaction, where the ylide reacts with an aldehyde or ketone, in this case, pentanal, to form the desired product, 1-phenyl-1,3-pentadiene, and triphenylphosphine oxide as a byproduct.

Step-by-step solution

Formation of the Wittig Reagent (Ylide)

The Wittig reagent (ylide) is an important reactive intermediate used in the Wittig reaction. It is formed through the reaction between an organophosphorus compound (a phosphonium salt) and a strong base. In this case, we need to synthesize 1,3-pentadiene with a phenyl group attached at the first carbon. Therefore, the Wittig reagent needed is (methylthio)methylene triphenylphosphorane. The phosphonium salt can be synthesized from the corresponding alkyl halide (methyl phenyl sulfide) and triphenylphosphine. You can represent this reactant system as: Methyl phenyl sulfide + Triphenylphosphine → Methyl phenyl sulfide triphenylphosphonium salt. Next, this phosphonium salt is treated with a strong base (such as n-butyllithium) to form the ylide: Phosphonium salt + n-BuLi → (methylthio)methylene triphenylphosphorane.

Wittig Reaction with Aldehyde or Ketone

After forming the Wittig reagent, the next step is the actual Wittig reaction, where the ylide reacts with an aldehyde or ketone. In this case, we need to react the ylide with pentanal (an aldehyde) to form 1-phenyl-1,3-pentadiene. The reaction proceeds through the formation of a betaine intermediate, which undergoes a [2+2] cycloaddition and subsequent ring-opening to form the desired product and triphenylphosphine oxide. The overall reaction can be represented as: (methylthio)methylene triphenylphosphorane + Pentanal → 1-phenyl-1,3-pentadiene + Triphenylphosphine oxide. By following these steps, the Wittig reaction successfully synthesizes the conjugated diene, 1-phenyl-1,3-pentadiene.