Step 1: Benzene reacts with propionyl chloride (catalyst: anhydrous aluminum trichloride) to generate 1- phenyl-1- acetone.

C6H6 +CH3CH2COCl===C6H5COCH2CH3

Step 2: 1- phenyl-1- acetone reacts with ethyl magnesium chloride, and after hydrolysis, 3- phenyl -3- pentanol is generated.

Step 3: 3- phenyl -3- pentanol reacts with sodium to generate sodium alkoxide, and sodium alkoxide reacts with chloroethane (nucleophilic substitution) to generate the target product.

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Step 1: Benzene reacts with methyl chloride to produce toluene. (Catalyst: anhydrous aluminum trichloride)

Step 2: Toluene and chlorine are halogenated under illumination to generate benzyl chloride.

Step 3: Chlorobenzyl reacts with sodium acetylene to generate 3- phenylpropyne.

Step 4: Hydrogenate 3- phenylpropyne with Lindera catalyst to generate 3- phenylpropylene.

Step 5: 3- phenylpropene and HBr carry out anti-Markov addition in the presence of peroxide to obtain γ-bromopropylbenzene.

Step 6: γ-bromopropylbenzene reacts with sodium cyanide in ethanol solution to generate 4- phenylbutyronitrile, and then it is hydrolyzed to obtain 4- phenylbutyric acid (product).