Question

The first widely used herbicide for the control of weeds was 2,4 -dichlorophenoxyacetic acid \((2,4\)-D). Show how this compound might be synthesized from phenol and chloroacetic acid by way of the given chlorinated phenol intermediate.

Short answer

Question: Describe the synthesis of 2,4-dichlorophenoxyacetic acid (2,4-D) from phenol and chloroacetic acid. Answer: The synthesis of 2,4-dichlorophenoxyacetic acid (2,4-D) from phenol and chloroacetic acid involves two steps: 1) Chlorination of phenol to form 2,4-dichlorophenol using Cl₂ and a catalyst like AlCl₃ or FeCl₃, and 2) Esterification of the chlorinated phenol with chloroacetic acid in the presence of an acid catalyst (e.g., H₂SO₄) to form the ester 2,4-D and water.

Step-by-step solution

Chlorination of Phenol

Firstly, we need to chlorinate phenol in order to obtain a chlorinated phenol intermediate. This can be achieved using electrophilic aromatic substitution. The chlorination process uses Cl₂ and a catalyst such as AlCl₃ or FeCl₃, which generates electrophilic Cl⁺ species. This electrophilic species then attacks the electron-rich aromatic ring of phenol. Since phenol is an activating group, its aromatic ring is ortho- and para-directing. Therefore, the chlorine substitution will occur at the 2 or 4 positions of the phenol ring, producing 2,4-dichlorophenol, which serves as the intermediate in this synthesis. Phenol + Cl₂ → 2,4-dichlorophenol

Esterification of Chlorinated Phenol

The Esterification process involves the reaction of the chlorinated phenol intermediate with chloroacetic acid to form the ester 2,4-dichlorophenoxyacetic acid (2,4-D). In this step, the hydroxyl group \((\)OH\()\) in the chlorinated phenol reacts with the carboxylic acid group \((-COOH)\) in chloroacetic acid, resulting in the formation of ester and water molecules. This is a typical esterification reaction and can be performed under the presence of an acid catalyst, such as sulfuric acid \((\)H₂SO₄\()\). 2,4-dichlorophenol + Chloroacetic acid → 2,4-dichlorophenoxyacetic acid (2,4-D) + H₂O To summarize, 2,4-dichlorophenoxyacetic acid (2,4-D) can be synthesized from phenol and chloroacetic acid, using a two-step procedure that involves the chlorination of phenol and the subsequent esterification of the intermediate chlorinated phenol.