Fluorine is more electronegative than chlorine but p-fluorobenzoic acid is a weaker acid than p-chlorobenzoic acid. Explain.

Explanation

Both fluorine and chlorine are involved in `-I` effect as well as `+M` effect (due to presence of lone electron paris) 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 `2p` 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.