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Chapter 16: Problem 25

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

Short Answer

Expert verified
Question: Explain the synthesis of 1-phenyl-1,3-pentadiene using the Wittig reaction. Answer: The synthesis of 1-phenyl-1,3-pentadiene using the Wittig reaction involves the reaction of benzaldehyde with 3-pentenyltriphenylphosphonium ylide. The carbanion in the ylide attacks the carbonyl carbon in benzaldehyde, forming a four-membered ring oxaphosphetane intermediate. This intermediate undergoes a concerted decomposition to form the desired product, 1-phenyl-1,3-pentadiene, and triphenylphosphine oxide as a byproduct.

Step by step solution

01

Understanding the Wittig Reaction

The Wittig reaction involves the reaction of an aldehyde or ketone with a phosphonium ylide, which is also known as a Wittig reagent. The product of this reaction is an alkene. The general reaction is as follows: R_1-CHO + R_2_2P=CR_3 ---> R_1-CH=CR_3 + R_2_2PO
02

Identifying the reactants for the synthesis of 1-phenyl-1,3-pentadiene

In order to synthesize 1-phenyl-1,3-pentadiene, we need to identify the appropriate reactants, an aldehyde or ketone and a Wittig reagent. For our desired product: Aldehyde: Benzaldehyde (Ph-CHO) Wittig reagent: 3-pentenyltriphenylphosphonium ylide ((CH_3)_2-CH-PPh_3) Here, Ph stands for the phenyl group (C_6H_5).
03

Reaction of benzaldehyde with Wittig reagent

The benzaldehyde reacts with the Wittig reagent, 3-pentenyltriphenylphosphonium ylide, through a nucleophilic addition process. The carbanion in the ylide attacks the carbonyl carbon in the benzaldehyde molecule, forming a four-membered ring intermediate with a negatively charged oxygen known as an oxaphosphetane. Ph-CHO + (CH_3)_2-CH-PPh_3 -------> Ph-CH(-O^-)-(CH_3)_2-CH-PPh_3^+
04

Formation of the product and byproduct

The oxaphosphetane intermediate undergoes a concerted decomposition, involving a [2+2]-cycloreversion, to form the desired alkene and triphenylphosphine oxide as a byproduct. Ph-CH(-O^-)-(CH_3)_2-CH-PPh_3^+ ------> Ph-CH=CH(CH_3)_2 + Ph_3P=O
05

Identifying the final product

The resulting alkene is the desired product, 1-phenyl-1,3-pentadiene, with the structure: Ph-CH=CH(CH_3)_2 In summary, the Wittig reaction allows the formation of 1-phenyl-1,3-pentadiene by reacting benzaldehyde with 3-pentenyltriphenylphosphonium ylide, followed by the decomposition of the oxaphosphetane intermediate to yield the desired product and triphenylphosphine oxide as a byproduct.

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