The decresing order of reactivity towards `SE` (substitution by electrophile) of the following is :
(I) Nitrobenzene
(II) Chlorobenzene
(III) Toluene
(IV) Benzene.

Options

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

Explanation

To determine the decreasing order of reactivity towards electrophilic substitution (SE) for the compounds given (Nitrobenzene, Chlorobenzene, Toluene, and Benzene), we need to analyze the effects of the substituents on the benzene ring. Here’s a step-by-step breakdown of the reasoning:

### Step 1: Identify the Compounds and Their Substituents
1. **Nitrobenzene (I)**: Contains a nitro group (-NO2), which is a strong electron-withdrawing group.
2. **Chlorobenzene (II)**: Contains a chloro group (-Cl), which is a weak electron-withdrawing group due to its -I effect.
3. **Toluene (III)**: Contains a methyl group (-CH3), which is an electron-donating group.
4. **Benzene (IV)**: The base structure without any substituents.

### Step 2: Analyze the Effects of Substituents
- **Toluene**: The methyl group is an electron-donating group (EDG) that increases the electron density on the benzene ring, making it more reactive towards electrophiles. Thus, it is the most reactive.

- **Benzene**: The benzene ring has a delocalized electron cloud, which allows it to react with electrophiles, but it does not have any substituents that enhance or diminish its reactivity significantly. It is less reactive than toluene but more reactive than the others with electron-withdrawing groups.

- **Chlorobenzene**: The chloro group has a -I effect (inductive effect) that withdraws electron density from the ring, making it less reactive towards electrophilic substitution than benzene.

- **Nitrobenzene**: The nitro group is a strong electron-withdrawing group (-NO2), which significantly decreases the electron density on the benzene ring, making it the least reactive towards electrophilic substitution.

### Step 3: Establish the Order of Reactivity
Based on the analysis:
1. **Toluene (III)** - Most reactive due to the electron-donating methyl group.
2. **Benzene (IV)** - Reactive but less than toluene.
3. **Chlorobenzene (II)** - Less reactive due to the weak electron-withdrawing effect of the chloro group.
4. **Nitrobenzene (I)** - Least reactive due to the strong electron-withdrawing effect of the nitro group.

### Final Order of Reactivity
Thus, the decreasing order of reactivity towards electrophilic substitution is:
**Toluene > Benzene > Chlorobenzene > Nitrobenzene**