Question

Which of the following pairs of aryl halides can NOT be prepared directly by electrophilic substitution?

• A. Aryl bromide and aryl iodide
• B. Aryl chloride and aryl bromide
• C. Aryl fluoride and aryl chloride
• D. Aryl iodide and aryl fluoride

Hint:

Fluorine strongly deactivates the aromatic ring, making electrophilic substitution reactions difficult in aryl fluorides and aryl chlorides.

Answer 1

The Correct Option is C

Step 1: Electrophilic substitution in aryl halides.
Electrophilic substitution on aryl halides can be carried out directly, but the halide substituent must be able to stabilize the intermediate carbocation. Fluorine, being highly electronegative, strongly deactivates the aromatic ring, making electrophilic substitution difficult for aryl fluorides and also for aryl chlorides due to their bond strength.

Step 2: Analyzing the options.
(A) Aryl bromide and aryl iodide: These can undergo electrophilic substitution.
(B) Aryl chloride and aryl bromide: These can undergo electrophilic substitution.
(C) Aryl fluoride and aryl chloride: Correct — These cannot undergo electrophilic substitution directly because of the strong deactivating nature of fluorine and the strong C-Cl bond.
(D) Aryl iodide and aryl fluoride: Although aryl iodide is reactive, aryl fluoride is not, but they can be substituted under special conditions.

Step 3: Conclusion.
The correct answer is (C) Aryl fluoride and aryl chloride, as they cannot be prepared directly by electrophilic substitution.