Consider the acidity of the carboxylic acids:
(a) `PhCOOH`
(b) `o-NO_(2) C_(6) H_(4) COOH`
(c) `m-NO_(2) C_(6) H_(4) COOH`
(d) `p-NO_(2) C_(6) H_(4) COOH`
Which of the following is the strongest acid and why?

Answer

To determine which of the given carboxylic acids is the strongest acid, we need to analyze the effects of substituents on the acidity of these compounds. The acids in question are:

1. **Benzoic acid (PhCOOH)**
2. **Ortho-nitrobenzoic acid (o-NO2C6H4COOH)**
3. **Meta-nitrobenzoic acid (m-NO2C6H4COOH)**
4. **Para-nitrobenzoic acid (p-NO2C6H4COOH)**

### Step 1: Understand the Role of the Nitro Group
The nitro group (NO2) is an electron-withdrawing group due to its -I (inductive) and -R (resonance) effects. This means that when a nitro group is attached to the aromatic ring of a carboxylic acid, it can stabilize the carboxylate ion (the conjugate base formed when the acid donates a proton) by pulling electron density away from the carboxylate ion.

**Hint:** Remember that electron-withdrawing groups increase acidity by stabilizing the conjugate base.

### Step 2: Analyze the Position of the Nitro Group
- **Ortho Position (o-NO2):** The nitro group at the ortho position has both -I and -R effects. Additionally, it experiences steric crowding (known as the ortho effect), which further enhances acidity.

- **Para Position (p-NO2):** The nitro group at the para position also exhibits -I and -R effects, but it does not have the additional steric crowding effect that is present in the ortho position.

- **Meta Position (m-NO2):** The nitro group at the meta position only exerts the -I effect and does not have a significant resonance effect on the carboxylate ion, making it less effective in stabilizing the conjugate base compared to the ortho and para positions.

**Hint:** Consider how the position of the nitro group affects both inductive and resonance stabilization of the carboxylate ion.

### Step 3: Compare Acidity
Based on the effects discussed:
- **Ortho-nitrobenzoic acid** will be the strongest acid due to the combined effects of -I, -R, and steric crowding.
- **Para-nitrobenzoic acid** will be the second strongest acid, as it has -I and -R effects but lacks the ortho effect.
- **Meta-nitrobenzoic acid** will be weaker than both ortho and para due to only having the -I effect.
- **Benzoic acid** will be the weakest since it has no electron-withdrawing groups.

### Conclusion
The strongest acid among the given options is **ortho-nitrobenzoic acid (o-NO2C6H4COOH)** due to the combined effects of resonance stabilization, inductive effects, and steric crowding.

**Final Answer:** The strongest acid is **o-NO2C6H4COOH (ortho-nitrobenzoic acid)** because of the enhanced stabilization of the carboxylate ion by the nitro group through both inductive and resonance effects, along with the ortho effect.