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Chapter 23: Problem 604

2, 4-Dichlorophenoxyacetic acid is the important weed-killer known as \(2,4-\mathrm{D}\). Outline the synthesis of this compound starting from benzene or toluene and acetic acid.

Short Answer

Expert verified
The synthesis of 2,4-dichlorophenoxyacetic acid (2,4-D) can be achieved starting with toluene as the base compound. First, chlorination of toluene is performed using chlorine gas and ferric chloride (FeCl3) as a catalyst to obtain 2,4-dichlorotoluene. Next, a nucleophilic aromatic substitution (S_NAr) reaction with sodium salt of phenoxyacetic acid in the presence of polar aprotic solvents, such as DMF or DMSO, is done to substitute one of the chlorine atoms with the phenoxyacetic group, yielding 2-(2,4-dichlorophenoxy)acetic acid as the final product, which is the important weed-killer 2,4-D.

Step by step solution

01

Choosing the starting material

We can start with either benzene or toluene as the starting material. However, if we choose benzene, we would need to first introduce a methyl group to convert it into toluene. To save this step, we will directly start with toluene as our starting material.
02

Introducing the chloro group

To introduce the chloro group at the 2 and 4 positions of the aromatic ring of toluene, we will perform a chlorination reaction using a mixture of chlorine gas and ferric chloride (FeCl3) as the catalyst. This will form 2,4-dichlorotoluene as the major product. \[ \text{Toluene} + 2Cl_2 \xrightarrow[]{FeCl_3} \text{2,4-Dichlorotoluene}\]
03

Introducing the phenoxyacetic group

The next step is to introduce the phenoxyacetic group to the 2,4-dichlorotoluene. We will perform a nucleophilic aromatic substitution (S_NAr) reaction by reacting 2,4-dichlorotoluene with sodium salt of phenoxyacetic acid in the presence of polar aprotic solvents like dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). This reaction will substitute one of the chlorine atoms on the aromatic ring with the phenoxyacetic group, forming 2-(2,4-dichlorophenoxy)acetic acid as the major product. \[ \text{2,4-Dichlorotoluene} + \text{NaOOCCH2Ph} \xrightarrow[]{DMF/DMSO} \text{2-(2,4-Dichlorophenoxy)acetic acid}\]
04

Final product

The final product obtained after the S_NAr reaction is the required 2,4-dichlorophenoxyacetic acid (2,4-D), an important weed-killer.

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