Problem 9 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

DDT is prepared by reacting chlorobenzene with: \(\quad\) (a) \(\mathrm{CHCl}_{3}\) (b) \(\mathrm{CCl}_{3} \mathrm{CHO}\) (c) \(\mathrm{CCl}_{4}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{6}\)

Step-by-Step Solution

Answer

The correct answer is (b) \( \mathrm{CCl}_{3} \mathrm{CHO} \).

Step 1: Recognize the Chemical Reaction

DDT is synthesized through a chemical reaction known as the Friedel-Crafts alkylation. This involves the reaction of chlorobenzene with a suitable reagent to produce DDT.

Step 2: Identify the Friedel-Crafts Alkylation Reagent

The key reagent required for the Friedel-Crafts alkylation reaction in the synthesis of DDT is chloral, which is chemically known as \( \mathrm{CCl}_{3} \mathrm{CHO} \).

Step 3: Apply the Knowledge

Identify the option from the given choices that represents chloral, \( \mathrm{CCl}_{3} \mathrm{CHO} \), the reagent that reacts with chlorobenzene to form DDT.

Step 4: Select the Correct Option

The correct reagent needed for the preparation of DDT is \( \mathrm{CCl}_{3} \mathrm{CHO} \), which corresponds to option (b) in the given list of choices.

Key Concepts

DDT synthesis

DDT, short for dichloro-diphenyl-trichloroethane, is synthesized through a process called Friedel-Crafts alkylation. This is an important type of reaction in organic chemistry that allows the introduction of alkyl groups into aromatic rings, such as benzene or its derivatives. In the synthesis of DDT, chlorobenzene acts as the aromatic compound.

• The key to this synthesis is the reaction between chlorobenzene and chloral.
• Chloral, a compound with the formula \( \mathrm{CCl}_{3} \mathrm{CHO} \), serves as the alkylating agent.

When performing this reaction, a catalyst such as aluminum chloride \( \mathrm{AlCl}_3 \) is often used to facilitate the formation of the C-C bond between the aromatic ring of chlorobenzene and the trichloroethyl group from chloral.

Through meticulous control of the reaction conditions and catalyst input, the target compound DDT can be effectively produced, highlighting the elegance of tailored organic synthesis strategies.

Chlorobenzene

Chlorobenzene serves as the base aromatic scaffold for the creation of DDT. It is a simple aromatic compound featuring a benzene ring with one hydrogen atom replaced by a chlorine atom.

• This substitution gives chlorobenzene a unique set of chemical properties.
• It makes it less reactive than benzene due to the electronegative chlorine, which offers some level of electron withdrawing.

Despite this decreased reactivity, chlorobenzene can still engage in electrophilic substitution reactions, which are essential for the Friedel-Crafts alkylation involved in DDT synthesis.

This characteristic, alongside its industrial availability and relatively low cost, makes chlorobenzene a practical choice for such syntheses.

Chloral

Chloral is the other key reactant in the synthesis of DDT. Known chemically as \( \mathrm{CCl}_{3} \mathrm{CHO} \), chloral is an aldehyde with three chlorine atoms attached to its central carbon.

• The presence of three chlorine atoms makes it highly reactive, particularly in reactions requiring a strong electrophile.
• In the context of DDT synthesis, chloral acts as the alkylating agent.

This means it provides the necessary carbon skeleton that will attach to the chlorobenzene, facilitating the formation of the DDT structure.

The role of chloral can be likened to that of a building block in the architecture of DDT, underscoring its importance in the production of this pesticide.

Chemical reactions

Chemical reactions are the cornerstone of chemistry, facilitating the transformation of substances through the rearrangement of atoms. In the case of synthesizing DDT, the type of reaction is a Friedel-Crafts alkylation.

• This involves the aromatic compound chlorobenzene reacting with chloral in the presence of a catalyst.
• The catalyst, often aluminum chloride, plays a crucial role in activating both reactants.

The catalyst helps by temporarily binding to the components involved, making the reactants more electrophilic and thus more likely to react.

Through understanding these reactions, chemists can manipulate molecular structures to create complex compounds like DDT, showcasing the precision and predictability of chemical transformations.