Animated Solution for Chemistry - Organic Chemistry: The compound formed on heating chlorobenzene with chloral in the presence of concentrated sulphuric acid is

The synthesis of DDT is a classic example of an electrophilic aromatic substitution reaction that beautifully illustrates the principles of steric hindrance and dehydration in organic chemistry. Let's dive deep into the mechanics of this fascinating transformation.

Our journey begins with two primary reactants: chlorobenzene ($C_6{H}_{5}Cl$) and chloral ($CC{l}_{3}CHO$). Chloral is a highly reactive aldehyde. The presence of three highly electronegative chlorine atoms on the adjacent carbon exerts a massive inductive pull ($-I$ effect), making the carbonyl carbon exceptionally electrophilic. It is practically begging for an electron-rich species to attack it.

On the other side, we have chlorobenzene. The chlorine atom on the benzene ring is an ortho/para directing group due to its $+M$ (mesomeric) effect, which increases electron density at the ortho and para positions. However, because the attacking electrophile (derived from chloral) is incredibly bulky, the ortho positions are sterically hindered. Therefore, the reaction will almost exclusively take place at the para position.

To kickstart this reaction, we introduce concentrated sulfuric acid ($H_{2}S{O}_4$) and apply heat. Sulfuric acid is a legendary dehydrating agent in organic chemistry. Its primary mission here is to facilitate the removal of a water molecule.

When chloral is protonated by the acid, it becomes an even stronger electrophile. This highly reactive intermediate is now ready to attack the electron-rich para positions of the chlorobenzene rings.

Imagine the spatial arrangement: one molecule of chloral aligns itself between two molecules of chlorobenzene. The carbonyl oxygen of chloral and the hydrogen atoms located at the para positions of the two chlorobenzene rings come into close proximity.

Driven by the dehydrating power of concentrated sulfuric acid, these three atoms—one oxygen and two hydrogens—are stripped away to form a single molecule of water ($H_{2}O$). This elimination creates a structural vacuum that must be filled immediately.

With the water molecule gone, the electrophilic carbon of chloral forms direct covalent bonds with the para carbons of both chlorobenzene rings. The result is a massive, bridged structure.

This newly formed compound is p,p'-Dichlorodiphenyltrichloroethane, universally known as DDT. Historically, DDT was a revolutionary insecticide that played a massive role in eradicating malaria-carrying mosquitoes. However, due to its extreme chemical stability and tendency to bioaccumulate in the food chain, its use has been heavily restricted or banned globally.

Understanding this synthesis not only helps master electrophilic aromatic substitution but also connects textbook chemistry to real-world environmental science!