Question

What are the starting materials used in the industrial synthesis of the pesticide DDT?

• A. Chlorobenzene and Chloroform
• B. Chlorobenzene and Chloral
• C. Benzene and Chloral
• D. Chlorobenzene and Hexachloroethane

Hint:

Remember the formula: $2 \times \text{Chlorobenzene} + 1 \times \text{Chloral} \xrightarrow{\text{H}_2\text{SO}_4} \text{DDT} + \text{H}_2\text{O}$.

Answer 1

The Correct Option is B

Concept: DDT, or dichlorodiphenyltrichloroethane, is prepared through an acid-catalyzed condensation pathway that follows an electrophilic aromatic substitution mechanism ($\text{S}_\text{E}\text{Ar}$). The structural formula of DDT features two distinct halobenzene rings linked directly to a central chlorinated alkane backbone. To synthesize this framework, a highly reactive chlorinated carbonyl compound is required to serve as the structural core and source of the electrophile.

Step 1: Generating the electrophilic intermediate from the carbonyl precursor.
The synthesis utilizes trichloroacetaldehyde, commonly referred to as chloral ($\text{CCl}_3\text{CHO}$). In the presence of a strong dehydrating acid catalyst like concentrated sulfuric acid ($\text{H}_2\text{SO}_4$), the carbonyl oxygen of chloral becomes protonated. This protonation increases the electrophilic character of the carbonyl carbon, creating a highly reactive intermediate capable of attacking aromatic rings.

Step 2: Condensing the electrophile with the aromatic halide to finalize the DDT structure.
The activated intermediate undergoes successive electrophilic attacks by two separate molecules of chlorobenzene. The positions para to the chlorine atoms on the chlorobenzene rings act as the nucleophilic sites attacking the central carbon. As the condensation reaction proceeds, a molecule of water is eliminated ($\text{H}_2\text{O}$), locking the two chlorobenzene groups and the trichloroethane fragment into a single molecule: \[ 2\,\text{C}_6\text{H}_5\text{Cl} + \text{CCl}_3\text{CHO} \xrightarrow{\text{conc. H}_2\text{SO}_4} (\text{Cl-C}_6\text{H}_4)_2\text{CH-CCl}_3 + \text{H}_2\text{O} \] Thus, the industrial starting components are chlorobenzene and chloral.