`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 the substituents (chlorine and nitro groups) on the electron density of the benzene ring.

### Step-by-Step Solution:

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

2. **Understand the Effects of Substituents**:
- The chlorine atom (Cl) is an electron-donating group through resonance, which increases the electron density of the benzene ring.
- The nitro group (NO2) is an electron-withdrawing group through resonance and inductive effects, which decreases the electron density of the benzene ring.

3. **Analyze Each Compound**:
- **Chlorobenzene (I)**: Contains only the chlorine substituent, which donates electron density to the ring, making it more reactive towards electrophilic substitution.
- **2,4-Dinitrochlorobenzene (II)**: Contains two nitro groups at the 2 and 4 positions, which significantly withdraw electron density from the ring. Although it also has a chlorine atom, the effect of the two nitro groups will dominate, leading to a lower electron density compared to chlorobenzene.
- **p-Nitrochlorobenzene (III)**: Contains one nitro group and one chlorine atom. The nitro group will withdraw some electron density, but not as much as in the case of 2,4-dinitrochlorobenzene. Thus, it has a higher electron density than 2,4-dinitrochlorobenzene but lower than chlorobenzene.

4. **Rank the Reactivity**:
- The compound with the highest electron density will be the most reactive towards electrophilic substitution.
- Therefore, the order of reactivity from highest to lowest is:
- (I) Chlorobenzene > (III) p-Nitrochlorobenzene > (II) 2,4-Dinitrochlorobenzene

### Final Answer:
The decreasing order of reactivity towards electrophilic substitution reaction is:
**Chlorobenzene > p-Nitrochlorobenzene > 2,4-Dinitrochlorobenzene**

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