The order of decreasing reactivity towards an electrphilic reagent for the following,
(i). Benzene
(ii). Toluene.
(iii). Chlorobenzoic acid.
(iv). Phenol. Would.

Options

A. `d gt b gt a gt c`
B. `a gt b gt c gt d`
C. `b gt d gt a gt c `
D. `d gt c gt b gt a `

Answer

To determine the order of decreasing reactivity towards an electrophilic reagent for the compounds benzene, toluene, chlorobenzoic acid, and phenol, we need to analyze the electron density of each compound. The more electron-rich a compound is, the more reactive it will be towards electrophiles.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- (i) Benzene
- (ii) Toluene
- (iii) Chlorobenzoic acid
- (iv) Phenol

2. **Analyze Benzene:**
- Benzene has a stable aromatic structure with no electron-donating or withdrawing groups. Therefore, it has a moderate level of electron density.

**Hint:** Remember that benzene is a stable aromatic compound with no substituents affecting its electron density.

3. **Analyze Toluene:**
- Toluene has a methyl group (-CH3) attached to the benzene ring. The methyl group is an electron-donating group via the +I (inductive) effect, which increases the electron density of the ring compared to benzene.

**Hint:** Consider how alkyl groups like methyl can increase electron density through inductive effects.

4. **Analyze Chlorobenzoic Acid:**
- Chlorobenzoic acid contains a carboxylic acid group (-COOH) which is a strong electron-withdrawing group due to its -M (mesomeric) effect. This decreases the electron density of the benzene ring, making it less reactive towards electrophiles.

**Hint:** Recall that electron-withdrawing groups decrease electron density and thus reactivity towards electrophiles.

5. **Analyze Phenol:**
- Phenol has a hydroxyl group (-OH) which has a +M (mesomeric) effect, donating electron density to the benzene ring. This makes phenol more reactive than benzene and toluene towards electrophiles.

**Hint:** Remember that the -OH group can donate electron density through resonance, enhancing reactivity.

6. **Order the Reactivity:**
- Based on the analysis:
- Phenol (most reactive due to +M effect)
- Toluene (more reactive than benzene due to +I effect)
- Benzene (moderate reactivity)
- Chlorobenzoic acid (least reactive due to -M effect)

**Final Order:**
- **Phenol > Toluene > Benzene > Chlorobenzoic acid**

### Conclusion:
The order of decreasing reactivity towards an electrophilic reagent is:
**(iv) Phenol > (ii) Toluene > (i) Benzene > (iii) Chlorobenzoic acid.**