Both alcohols and phenols contain a hydroxyl group, but phenols are more acidic than alcohols. Whereas the acid strength of aliphatic alcohols mainly depends upon the inductive effects, and acid strength of phenols depends upon a combination of both inductive and resonance effects of the substituent and its position on the benzene ring.

Which of the following is the strongest acid? 

o-Chlorophenol

The correct answer is option 2. o-Chlorophenol.

Let us break down why o-Chlorophenol is the strongest acid among the options:

Acidity in Phenols:

Phenols are weak acids because they can release a proton \((H^+)\) from the hydroxyl group \((OH)\) attached to the benzene ring. The ease with which this proton is released determines the strength of the acid.

Impact of Halogen Substituents:

The halogen atoms \((F, Cl, Br, I)\) attached to the benzene ring in these molecules influence the acidity of the \(-OH\) group. These halogens tend to withdraw electron density from the ring due to their electronegativity. This electron withdrawal makes the oxygen atom in the -OH group more positive, weakening its hold on the proton \((H^+)\). As a result, the proton becomes easier to release, increasing the acidity. This effect is called the inductive effect (\(I\)-effect).

The Case of Fluorine (F):

Fluorine, despite being the most electronegative of the halogens, presents a unique situation. Due to its small size, the fluorine atom in o-fluorophenol can get very close to the hydrogen atom in the \(-OH\) group. This proximity allows them to form a special type of hydrogen bond called an intramolecular hydrogen bond.

Intramolecular Hydrogen Bond:

An intramolecular hydrogen bond is a hydrogen bond formed within the same molecule. In o-fluorophenol, the highly electronegative fluorine attracts the positive end \((δ^+)\) of the \(O-H\) bond, forming a strong intramolecular hydrogen bond. This bond ties up the proton \((H^+)\) more tightly, making it harder to release and reducing the acidity compared to the other halogenated phenols.

Order of Acidity:

Therefore, considering both the inductive effect (increasing acidity) and the intramolecular hydrogen bond (decreasing acidity) for each molecule, we get the following order of acidity:

o-Chlorophenol: Chlorine is larger than fluorine and doesn't form an intramolecular hydrogen bond, leading to the strongest inductive effect and highest acidity.

o-Bromophenol: Similar to chlorine, bromine also withdraws electrons effectively without a significant intramolecular hydrogen bond.

o-Iodophenol: Iodine is the largest and least electronegative halogen, resulting in a weaker inductive effect compared to chlorine and bromine.

o-Fluorophenol: Despite fluorine's high electronegativity, the intramolecular hydrogen bond weakens the acidity, making it the least acidic among the four.

In conclusion, o-chlorophenol exhibits the strongest acidity due to the combined effect of a strong inductive electron withdrawal by chlorine and the absence of a weakening intramolecular hydrogen bond.