Problem 170 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound formed on heating chlorobenzene with chloral in the presence of concentrated sulphuric acid is [2004] (a) gammexane (b) DDT (c) freon (d) hexachloroethane

Step-by-Step Solution

Answer

The compound formed is DDT (option b).

Step 1: Understand the Reactants and Conditions

Identify the reactants: chlorobenzene and chloral, which are being heated together in the presence of concentrated sulphuric acid. Recognize these as key ingredients involved in organic synthesis.

Step 2: Recall the Relevant Reaction

Recall that when chlorobenzene reacts with chloral ( CCl_3CHO) in the presence of concentrated sulphuric acid, it leads to the formation of a specific compound through electrophilic aromatic substitution followed by further reactions.

Step 3: Identify the Compound Formed

The compound that forms from this reaction is DDT, or dichlorodiphenyltrichloroethane. This is a well-known pesticide, historically significant, and usually formed in the process described. Recognize the synthetic pathway to DDT from chlorobenzene and chloral using concentrated sulphuric acid.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry. It involves the replacement of a hydrogen atom in an aromatic ring, such as benzene, with an electrophile. The aromatic ring, with its pi electrons, acts as a nucleophile. This is because it can donate electrons to an electron-poor species, known as the electrophile. When these reactions occur, they generally follow a particular mechanism:

• Formation of a pi-complex: The electrophile first approaches the aromatic ring and forms a temporary interaction.
• Formation of a sigma complex (arenium ion): The electrophile binds to the aromatic ring, creating an unstable ion with a positive charge.
• Re-aromatization: To regain the stable aromaticity, the molecule loses a proton, reverting to the stable ring structure.

The reaction of chlorobenzene with an electrophilic species, like chloral in the presence of concentrated sulphuric acid, is a type of electrophilic aromatic substitution. The introduction of the trichloromethyl group (from chloral) points to the strong electrophilic nature as it replaces a hydrogen atom on the benzene ring.

DDT Synthesis

DDT, or dichlorodiphenyltrichloroethane, is synthesized through a classic organic reaction involving chlorobenzene and chloral. This reaction is significant due to its importance in the history of pesticide development. In the DDT synthesis:

• Chlorobenzene acts as an aromatic substrate that undergoes electrophilic substitution with chloral ( CCl_3CHO), facilitated by concentrated sulfuric acid.
• The reaction proceeds via the generation of an electrophile from chloral, which then attacks the aromatic ring of chlorobenzene, displacing a hydrogen atom.
• In the presence of an acidic environment provided by sulfuric acid, the process is expedited by stabilizing intermediates.

The formation of DDT was an important development, although its use is now limited due to environmental and health concerns. Nonetheless, understanding its synthesis helps in comprehending mechanisms typical to aromatic chemistry.

Chlorobenzene Reactions

Chlorobenzene is a simple aromatic compound with a chlorine atom bonded to a benzene ring. This halogenated compound is a pivotal starting material in many organic syntheses due to its reactivity. Important reactions involving chlorobenzene include:

• Electrophilic Aromatic Substitution: As discussed, chlorobenzene can participate in this type of reaction, forming products like DDT through interactions with electrophiles.
• Reduction Reactions: Catalytic hydrogenation can convert chlorobenzene to cyclohexane derivatives.
• Nucleophilic Substitution: Though more challenging due to the stability of the C-Cl bond, reactions can proceed under specific conditions using strong bases.

Each reaction pathway highlights the versatility of chlorobenzene as a synthetic building block. It demonstrates why understanding the chemical nature of chlorobenzene is vital for working with many aromatic compounds in industrial and research settings.