To determine the strongest acid among the given compounds, we need to understand the factors that affect acidity. For carboxylic acids, the key factor is the stability of the conjugate base. A more stable conjugate base means the acid is stronger. Stability is often influenced by electron-withdrawing groups (EWG) which stabilize the negative charge on the conjugate base through inductive effect or resonance.

The general formula for a carboxylic acid is R-COOH, and its acidity can be compared by looking at the group R. Electron-withdrawing groups (EWG) increase acidity by stabilizing the carboxylate anion, while electron-donating groups (EDG) decrease acidity by destabilizing the anion.

Let's analyze each option:

Option 1: CH₃CH₂CH(Cl)CO₂H (3-chlorobutanoic acid)
This has a chlorine atom attached to the carbon adjacent to the carboxylic acid group. Chlorine is an electron-withdrawing group due to its high electronegativity. It stabilizes the conjugate base through the -I effect, making this acid stronger than unsubstituted carboxylic acids.

Option 2: HCOOH (Formic acid)
Formic acid is stronger than acetic acid because it lacks an electron-donating alkyl group. The conjugate base (formate ion) is stabilized because there is no +I effect from alkyl groups.

Option 3: ClCH₂CH₂CH₂COOH (4-chlorobutanoic acid)
Here, the chlorine is attached to the terminal carbon, three atoms away from the carboxylic acid. The inductive effect of chlorine weakens with distance, so its electron-withdrawing effect is much weaker compared to when it is closer. This acid is only slightly stronger than butanoic acid.

Option 4: CH₃COOH (Acetic acid)
Acetic acid has a methyl group which is electron-donating (+I effect), destabilizing the conjugate base and making it weaker than formic acid.

Now, comparing the acids:

• The chlorine in option 1 is on the α-carbon (directly adjacent to COOH), providing strong -I effect.
• Option 2 (HCOOH) has no EDG, so it is fairly strong.
• Option 3 has chlorine on the γ-carbon, so its effect is weak.
• Option 4 has an EDG (methyl), making it the weakest.

The order of electron-withdrawing effect: α-Cl > no substitution > γ-Cl > EDG.
Thus, CH₃CH₂CH(Cl)CO₂H (3-chlorobutanoic acid) is the strongest acid because the chlorine is closest to the carboxylic group, providing the strongest inductive stabilization to the conjugate base.

Related Topics & Formulae

Acidity Constant (K_a) and pK_a:
The strength of an acid is quantified by its acid dissociation constant, K_a: $Ka=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[\mathrm{HA}\right]}$ A higher K_a means a stronger acid. Often, pK_a = -log₁₀K_a is used, and a lower pK_a indicates a stronger acid.

Inductive Effect:
Electron-withdrawing groups (e.g., -Cl, -NO₂) increase acidity by pulling electron density away from the acidic proton, stabilizing the conjugate base. Electron-donating groups (e.g., -CH₃) decrease acidity.

Position of Substituent:
The effect is strongest when the EWG is closer to the carboxylic group. For example, an α-chloro substituent has a much greater effect than a β or γ substituent.