(A) Although fluorine is more electronegative than chlorine, yet p-fluorobenzoic acid is a weaker acid than p-chlorobenzoic acid.
(R) Due to matching sizes of 2p orbitals of F and carbon, F has a stronger +R effect than Cl.

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(A) is true but (R) is false

Both (A) and (R) are true but (R) is not the correct explanation of (A)

Both (A) and (R) are true and (R) is the correct explanation of (A)

Both (A) and (R) are false

answer is A.

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Detailed Solution

Both fluorine and chlorine are involved in − Ieffect as well as + Meffect with the benzene ring. Whereas − I effect tends to increase the acidic strength, + M effect decreases the same. Fluorine has greater − I effect than chlorine since it is more electronegative in nature. But it has also greater magnitude of + M effect (opposing factor) compared to chlorine. This may be attributed to the reason that 2 p orbitals of carbon and fluorine have comparable sizes while this is not so in case of carbon and chlorine atoms. In the light of this, p- fluorobenzoic acid is a weaker acid than p-chlorobenzoic acid.