Question

The insecticide DDT is prepared by the following route. Suggest a mechanism for this reaction. The abbreviation DDT is derived from the common name dichlorodiphenyltrichloroethane.

Short answer

Answer: The reaction mechanism for the synthesis of DDT involves a series of electrophilic aromatic substitution reactions. It starts with the protonation of trichloroacetic acid by sulfuric acid, followed by the formation of an acylium ion. The acylium ion then reacts with chlorobenzene via electrophilic aromatic substitution, forming a chloronium ion intermediate. The chloronium ion loses a proton to restore aromaticity, resulting in an intermediate product. This process is repeated with another chlorobenzene molecule, ultimately leading to the formation of DDT.

Step-by-step solution

Understand the structure of DDT

DDT has a trichloroethane group attached to two phenyl rings (benzene rings). The molecular formula of DDT is C14H9Cl5. To suggest a mechanism for the synthesis of DDT, we need to first determine the reactants that result in DDT formation.

Determine the reactants

The synthesis of DDT can be achieved by the reaction of trichloroacetic acid and chlorobenzene in the presence of a sulfuric acid catalyst.

Propose the reaction mechanism

The reaction mechanism for the synthesis of DDT can be described as follows: 1. Protonation of trichloroacetic acid: In this step, trichloroacetic acid reacts with sulfuric acid (a strong acid) to form a protonated trichloroacetic acid. 2. Formation of acylium ion: The protonated trichloroacetic acid eliminates a water molecule, forming an acylium ion (a highly reactive electrophile) and also regenerating sulfuric acid. 3. Electrophilic aromatic substitution on chlorobenzene: The acylium ion reacts with chlorobenzene, which acts as a nucleophile, via an electrophilic aromatic substitution reaction. The acylium ion attacks the benzene ring, and a chloronium ion is formed as an intermediate. 4. Aromaticity restoration and formation of intermediate: The chloronium ion loses a proton, leading to the formation of a new aromatic ring, and thus an intermediate product is formed. 5. Repetition of steps 1-4: The intermediate undergoes the same reaction steps as described earlier (protonation, formation of acylium ion, electrophilic aromatic substitution, and aromaticity restoration) with another molecule of chlorobenzene, leading to the formation of DDT. Overall, the reaction mechanism for the synthesis of DDT involves a series of electrophilic aromatic substitution reactions resulting in the formation of the desired product, DDT.