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?

FCH₂COOH

The correct answer is option 1. \(FCH_2COOH\).

To determine the strongest acid among the given compounds, we need to consider the electron-withdrawing inductive effects of the halogen substituents. The stronger the electron-withdrawing effect, the more the conjugate base is stabilized, resulting in a stronger acid.

1. \(FCH_2COOH\) (Fluoroacetic Acid):

Inductive Effect: Fluorine is the most electronegative halogen, thus it has the strongest electron-withdrawing inductive effect. This significantly stabilizes the conjugate base (fluoroacetate ion).

Acidity: High, due to the strong electron-withdrawing effect of fluorine.

2. \(ClCH_2COOH\) (Chloroacetic Acid):

Inductive Effect: Chlorine is less electronegative than fluorine but still highly electronegative. It provides a strong electron-withdrawing inductive effect, though weaker than that of fluorine.

Acidity: High, but less than fluoroacetic acid.

3. \(BrCH_2COOH\) (Bromoacetic Acid):

Inductive Effect: Bromine is less electronegative than chlorine, resulting in a weaker electron-withdrawing inductive effect.

Acidity: Moderate, less than chloroacetic acid.

4. \(ICH_2COOH\) (Iodoacetic Acid):

Inductive Effect: Iodine is the least electronegative among the halogens listed, resulting in the weakest electron-withdrawing inductive effect.

Acidity: Lowest among the given compounds.

The order of acidity based on the inductive effects of the halogen substituents is:
\(\text{ICH}_2\text{COOH} < \text{BrCH}_2\text{COOH} < \text{ClCH}_2\text{COOH} < \text{FCH}_2\text{COOH}\)

Thus, the strongest acid among the given compounds is \(FCH_2COOH\) (fluoroacetic acid).