Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+
(a)   Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
(b) Toluene, `p-H_3C–C_6H_4–NO_(2), p-O_2N–C_6H_4–NO_2`.

Explanation

To solve the problem of arranging the compounds in order of their decreasing relative reactivity with an electrophile (E+), we need to analyze the electronic effects of the substituents present on the benzene rings of the given compounds.

### Step-by-Step Solution:

#### Part (a): Compounds - Chlorobenzene, 2,4-Dinitrochlorobenzene, p-Nitrochlorobenzene

1. **Identify the Compounds and Their Substituents**:
- Chlorobenzene (C6H5Cl)
- 2,4-Dinitrochlorobenzene (C6H3Cl(NO2)2)
- p-Nitrochlorobenzene (C6H4Cl(NO2))

2. **Determine the Nature of Substituents**:
- Chlorine (Cl) is an electron-withdrawing group (EWG) due to its electronegativity.
- Nitro groups (NO2) are strong electron-withdrawing groups.

3. **Analyze the Electron Density on the Benzene Ring**:
- Chlorobenzene has one EWG (Cl), which decreases the electron density but not significantly.
- p-Nitrochlorobenzene has two EWGs (Cl and NO2), which will further decrease the electron density.
- 2,4-Dinitrochlorobenzene has three EWGs (Cl and two NO2), leading to the lowest electron density.

4. **Order of Reactivity**:
- More electron density on the benzene ring leads to higher reactivity with electrophiles.
- Therefore, the order of decreasing reactivity is:
- **Chlorobenzene > p-Nitrochlorobenzene > 2,4-Dinitrochlorobenzene**

#### Part (b): Compounds - Toluene, p-H3C–C6H4–NO2, p-O2N–C6H4–NO2

1. **Identify the Compounds and Their Substituents**:
- Toluene (C6H5CH3)
- p-H3C–C6H4–NO2 (p-Nitrotoluene)
- p-O2N–C6H4–NO2 (p-Dinitrobenzene)

2. **Determine the Nature of Substituents**:
- Methyl (CH3) in toluene is an electron-donating group (EDG).
- Nitro groups (NO2) are strong electron-withdrawing groups.

3. **Analyze the Electron Density on the Benzene Ring**:
- Toluene has an EDG (CH3), increasing the electron density and making it very reactive.
- p-Nitrotoluene has one EDG (CH3) and one EWG (NO2), leading to moderate electron density.
- p-Dinitrobenzene has two EWGs (NO2), significantly decreasing the electron density.

4. **Order of Reactivity**:
- The order of decreasing reactivity with electrophiles is:
- **Toluene > p-Nitrotoluene > p-Dinitrobenzene**

### Final Order of Reactivity:

- **Part (a)**: Chlorobenzene > p-Nitrochlorobenzene > 2,4-Dinitrochlorobenzene
- **Part (b)**: Toluene > p-Nitrotoluene > p-Dinitrobenzene