`underset((I))("In Chlorobenzene") , underset((II))("2,4-dinitrochlorobenzene"), underset((III))("p- nitrochlorobenzene")`
The decreasing order of reactivity towards electrophliic substitution reaction is

Options

A. (I) gt (II) gt (III)
B. (I) gt (III) gt (II)
C. (II) gt (I) gt (III)
D. (III) gt (I) gt (II)

Answer

To determine the decreasing order of reactivity towards electrophilic substitution reactions for the compounds chlorobenzene, 2,4-dinitrochlorobenzene, and p-nitrochlorobenzene, we need to analyze the effects of substituents on the benzene ring.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (I) Chlorobenzene
- (II) 2,4-Dinitrochlorobenzene
- (III) p-Nitrochlorobenzene

2. **Understand Electrophilic Substitution**:
- Electrophilic substitution involves an electrophile attacking the benzene ring, resulting in the substitution of a hydrogen atom. The reactivity of the benzene ring towards electrophiles depends on the nature of the substituents already present on the ring.

3. **Determine the Nature of Substituents**:
- **Chlorobenzene (I)**: The chlorine atom is a weakly activating group due to its +R (resonance) effect, which increases electron density at the ortho and para positions. However, it also has a -I (inductive) effect, which is electron-withdrawing. The overall effect is that chlorobenzene is moderately reactive towards electrophilic substitution.

- **2,4-Dinitrochlorobenzene (II)**: The nitro groups (-NO2) are strong deactivating groups due to their -I and -R effects. They withdraw electron density from the ring, significantly decreasing the reactivity towards electrophilic substitution. Since there are two nitro groups at the 2 and 4 positions, the reactivity is very low.

- **p-Nitrochlorobenzene (III)**: This compound has one nitro group and one chlorine atom. The nitro group is deactivating, but the chlorine atom still provides some activating effect through resonance. However, the overall effect is still deactivating, but less so than in the case of 2,4-dinitrochlorobenzene.

4. **Rank the Reactivity**:
- The order of reactivity towards electrophilic substitution can be determined by considering the overall electron density on the benzene ring:
- **Chlorobenzene (I)**: Most reactive due to the weakly activating effect of chlorine.
- **p-Nitrochlorobenzene (III)**: Less reactive than chlorobenzene but more reactive than 2,4-dinitrochlorobenzene due to the presence of only one nitro group.
- **2,4-Dinitrochlorobenzene (II)**: Least reactive due to the strong deactivating effect of two nitro groups.

5. **Final Order**:
- The decreasing order of reactivity towards electrophilic substitution is:
- Chlorobenzene (I) > p-Nitrochlorobenzene (III) > 2,4-Dinitrochlorobenzene (II)

### Conclusion:
The correct sequence is:
**Chlorobenzene (I) > p-Nitrochlorobenzene (III) > 2,4-Dinitrochlorobenzene (II)**.