The correct order of acid strength of following aromatic acids is : Benzoic acid (I), 4-Nitrobenzoic acid (II), 4-Dinitrobenzoic acid (III), 4-Methoxybenzoic acid (IV)

Options

A. `I lt II lt III lt IV`
B. `IV lt III lt II lt I`
C. `IV lt I lt II lt III`
D. `I lt IV lt II lt III`

Explanation

To determine the correct order of acid strength for the given aromatic acids: Benzoic acid (I), 4-Nitrobenzoic acid (II), 4-Dinitrobenzoic acid (III), and 4-Methoxybenzoic acid (IV), we need to analyze the effects of the substituents on the benzene ring and their influence on the carboxylic acid group (-COOH).

### Step 1: Understand the structure of the acids
- **Benzoic acid (I)**: C6H5COOH
- **4-Nitrobenzoic acid (II)**: C6H4(NO2)COOH
- **4-Dinitrobenzoic acid (III)**: C6H3(NO2)2COOH
- **4-Methoxybenzoic acid (IV)**: C6H4(OCH3)COOH

### Step 2: Identify the effects of substituents
- **Nitro group (NO2)**: This is a strong electron-withdrawing group (EWG) due to its -I (inductive) and -M (mesomeric) effects. It increases the acidity of the carboxylic acid by stabilizing the negative charge on the conjugate base (carboxylate ion).
- **Methoxy group (OCH3)**: This is an electron-donating group (EDG) due to its +M effect, which decreases the acidity of the carboxylic acid by destabilizing the negative charge on the conjugate base.

### Step 3: Analyze the acids based on substituent effects
1. **4-Dinitrobenzoic acid (III)**: Contains two nitro groups, which strongly withdraw electrons. This will significantly increase the acidity.
2. **4-Nitrobenzoic acid (II)**: Contains one nitro group, which also increases acidity but to a lesser extent than the dinitro compound.
3. **Benzoic acid (I)**: Has no substituents that affect acidity, so it is less acidic than the nitro-substituted acids.
4. **4-Methoxybenzoic acid (IV)**: The methoxy group donates electrons, which decreases acidity, making it the least acidic of the four.

### Step 4: Order the acids based on their strength
From the analysis:
- **Strongest Acid**: 4-Dinitrobenzoic acid (III)
- **Second Strongest**: 4-Nitrobenzoic acid (II)
- **Third**: Benzoic acid (I)
- **Weakest Acid**: 4-Methoxybenzoic acid (IV)

### Final Order of Acid Strength
The correct order of acid strength is:
**4-Dinitrobenzoic acid (III) > 4-Nitrobenzoic acid (II) > Benzoic acid (I) > 4-Methoxybenzoic acid (IV)**