Comparison of acidic strength of halophenols
Answers
I think that, this acidity order can be justified by considering a combined effect of +R effect, -I effect and intermolecular hydrogen bonding (where it is dominant).
For 4-fluorophenol, the +R effect of F is significant but weaker than -I effect. The electron pair on the 2p orbitals of F can easily delocalise with the benzene ring because of the stronger 2pπ-2pπ overlap, which makes +R effect significant . That's why it increases the electron density on the carbon near the −OH group to such an extent which decreases its ability to donate protons, greatly decreasing its acidity.
Considering intermolecular hydrogen bonding also, the acidity of the compound decreases further due to strongest intermolecular hydrogen bonding between the halogens.
For 4-chlorophenol, the +R effect is negligible due to much weaker 3pπ-2pπ overlap and thus the −I effect becomes more prominent. But Cl is electronegative enough to form strong intermolecular hydrogen bonding(HO−C6H4−Cl⋅⋅⋅H−O−C6H4−Cl⋅⋅⋅). That's why, it becomes difficult to remove hydrogen atoms from −OH group, decreasing its acidity.
For 4-bromophenol and 4-iodophenol, there is no strong hydrogen bonding due to much less electronegative Br and I atoms, and also +R effect is ineffective due to very weak 4pπ-2pπ and 5pπ-2pπ overlap. Only -I effect is acing for these two compounds in favour of acidity. As -I effect of Br is larger than I, the acidity of p-bromophenol is greater than the p-iodophenol.
Answer:
I think that, this acidity order can be justified by considering a combined effect of +R effect, -I effect and intermolecular hydrogen bonding