DDT synthesis step: In the classic preparation of DDT by condensing chloral with chlorobenzene, which catalyst is used to promote the reaction?

Introduction / Context:
DDT (dichloro-diphenyl-trichloroethane) is produced by electrophilic condensation of chloral (trichloroacetaldehyde) with chlorobenzene. The historical laboratory and industrial procedures employ strong acid catalysts to generate a reactive electrophile and drive condensation.

Given Data / Assumptions:

• Reactants: chloral and chlorobenzene.
• Reaction type: acid-catalyzed aromatic substitution/condensation.
• Strong acidic medium is required for effective conversion.

Concept / Approach:

Oleum (fuming sulfuric acid, containing dissolved SO3) provides a superacidic environment that protonates carbonyl and facilitates formation of the electrophilic species. This enhances rate and selectivity toward the DDT framework compared to weaker acids.

Step-by-Step Solution:

Verification / Alternative check:

Reference preparations cite concentrated H2SO4/oleum mixtures; oleum is specifically noted for improved activity due to SO3 content.

Why Other Options Are Wrong:

Dilute H2SO4: Insufficient acidity for efficient conversion. UV light: Not the standard catalytic route for this electrophilic condensation. None: Inaccurate because an acid catalyst is required.

Common Pitfalls:

Assuming any sulfuric acid strength suffices; underestimating the role of SO3 in creating a superacidic medium.

Final Answer:

Oleum (fuming sulfuric acid)