Arrange the following anilines in decreasing order of basicity
1. `C_(6)H_(5)NH_(2)`
2. `o-CH_(3)C_(6)H_(4)NH_(2)`
3. `m-CH_(3)C_(6)H_(4)NH_(2)`
4. `p-CH_(3)C_(6)H_(4)NH_(2)`

Options

A. `4 gt 1 gt 2 gt 3`
B. `2 gt 4 gt 3 gt 1`
C. `1 gt 2 gt 3 gt 4`
D. `4 gt 3 gt 1 gt 2`

Answer

To determine the decreasing order of basicity of the given anilines, we need to analyze the effects of the substituents on the nitrogen atom in each compound. The compounds to be arranged are:

1. Aniline (`C6H5NH2`)
2. Ortho-toluidine (`o-CH3C6H4NH2`)
3. Meta-toluidine (`m-CH3C6H4NH2`)
4. Para-toluidine (`p-CH3C6H4NH2`)

### Step 1: Understand the Basicity of Anilines
Basicity in anilines is influenced by the availability of the lone pair of electrons on the nitrogen atom. Electron-donating groups (like methyl, `-CH3`) increase basicity by increasing electron density on the nitrogen, while electron-withdrawing groups decrease it.

### Step 2: Analyze the Substituent Effects
- **Methyl Group (`-CH3`)**: This group is an electron-donating group through both the inductive effect (+I) and hyperconjugation (+H).
- **Ortho Effect**: In ortho-toluidine, the methyl group is positioned adjacent to the amino group, which can create steric hindrance and reduce basicity despite the electron-donating effects.

### Step 3: Draw the Structures
1. Aniline: `C6H5-NH2`
2. Ortho-toluidine: `o-CH3C6H4-NH2`
3. Meta-toluidine: `m-CH3C6H4-NH2`
4. Para-toluidine: `p-CH3C6H4-NH2`

### Step 4: Compare Basicity
- **Ortho-toluidine**: The steric hindrance from the methyl group reduces the ability of the nitrogen to accept protons, making it the least basic.
- **Para-toluidine**: The methyl group is in the para position, which allows both +I and +H effects to operate without steric hindrance, making it the most basic.
- **Meta-toluidine**: The methyl group only exerts a +I effect here, which increases basicity but not as effectively as in the ortho or para positions.
- **Aniline**: Lacks any electron-donating substituents, making it less basic than para and meta-toluidine.

### Step 5: Final Order of Basicity
Based on the analysis:
1. Para-toluidine (most basic)
2. Meta-toluidine
3. Aniline
4. Ortho-toluidine (least basic)

Thus, the final order of basicity is:
**p-toluidine > m-toluidine > aniline > o-toluidine**