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 is the strongest acid among the following? 

F₃CCOOH

The correct answer is option 4. F₃CCOOH.

To determine the strongest acid among the given compounds, we need to consider the effect of electron-withdrawing groups on the acidity of carboxylic acids. The presence of electronegative substituents like fluorine can significantly increase the acidity of a carboxylic acid by stabilizing the conjugate base through inductive effects.

1. CH₃COOH (Acetic Acid):

Inductive Effect: The methyl group (CH₃) is slightly electron-donating due to hyperconjugation and inductive effects, which does not significantly increase acidity.

Acidity: This is the baseline for comparison and is the weakest acid among the given options due to the lack of strong electron-withdrawing groups.

2. FCH₂COOH (Fluoroacetic Acid):

Inductive Effect: The fluorine atom is highly electronegative and withdraws electron density through the sigma bond, stabilizing the conjugate base (fluoroacetate ion) and increasing the acidity of the acid.

Acidity: More acidic than acetic acid due to the strong electron-withdrawing effect of one fluorine atom.

3. F₂CHCOOH (Difluoroacetic Acid):

Inductive Effect: With two fluorine atoms attached, the electron-withdrawing effect is even stronger than in fluoroacetic acid, further stabilizing the conjugate base (difluoroacetate ion).

Acidity: More acidic than fluoroacetic acid due to the additive electron-withdrawing effect of the two fluorine atoms.

4. F₃CCOOH (Trifluoroacetic Acid):

Inductive Effect: With three fluorine atoms, the electron-withdrawing effect is the strongest among the given compounds, providing maximum stabilization to the conjugate base (trifluoroacetate ion).

Acidity: This is the strongest acid among the given options because the three fluorine atoms exert a very strong electron-withdrawing effect, significantly stabilizing the conjugate base.

The order of acidity based on the inductive effects of the electron-withdrawing fluorine atoms is:

\(\text{CH}_3\text{COOH} < \text{FCH}_2\text{COOH} < \text{F}_2\text{CHCOOH} < \text{F}_3\text{CCOOH}\)

Thus, the strongest acid among the given compounds is F₃CCOOH (trifluoroacetic acid).