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 three main steps in the reaction mechanism for the synthesis of DDT are nucleophilic addition, elimination, and electrophilic aromatic substitution. The precursor molecules used are chloral (CCl3CHO) and chlorobenzene (C6H5Cl).

Step-by-step solution

Identify the precursor molecules and the target molecule structure

Firstly, study the structure of DDT (dichlorodiphenyltrichloroethane), and observe that it contains two phenyl rings, a central ethane moiety, and five chlorine atoms. Notice that both phenyl rings are linked to the central ethane moiety, and the ethane moiety has three chlorine atoms attached to it. Based on the structure of DDT, we can identify two precursor molecules, chloral (CCl3CHO) and chlorobenzene (C6H5Cl).

Determine the reaction mechanism

Now that we have identified the precursor molecules, we can determine the sequence of steps that leads to the formation of DDT: 1. Nucleophilic addition reaction: The oxygen atom of chloral (CCl3CHO) acts as a nucleophile and attacks the carbon atom of chlorobenzene (C6H5Cl), resulting in the formation of a C-O bond and the release of a chloride ion (Cl⁻). - Equation: CCl3CHO + C6H5Cl -> \textrm{Intermediate 1} 2. Elimination reaction: The oxygen atom in Intermediate 1 is protonated, which results in the formation of an O-H bond, and the cleavage of the C=O bond. This proceeds via an E2 elimination mechanism, whereby the protonated oxygen atom acts as a leaving group, taking its bonding electrons with it and forming a water molecule. This step results in the formation of a C=C bond at the central ethane moiety. - Equation: \textrm{Intermediate 1} + H^+ -> \textrm{Intermediate 2} + H2O 3. Electrophilic aromatic substitution: Chlorobenzene (C6H5Cl) reacts with Intermediate 2, in which the carbon atom of the C=C bond acts as the electrophile, and the chlorobenzene acts as a nucleophile. The electrophilic carbon atom replaces one of the hydrogen atoms on the phenyl ring by a substitution reaction. This results in the formation of DDT. - Equation: \textrm{Intermediate 2} + C6H5Cl -> DDT + HCl In summary, the mechanism for the synthesis of DDT involves nucleophilic addition, elimination, and electrophilic aromatic substitution reactions. The precursor molecules chloral and chlorobenzene are key reactants in this process.