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 given compounds in order of their decreasing relative reactivity with an electrophile (E+), we need to analyze the electron-donating and electron-withdrawing effects of the substituents present in each compound.

### Part (a): Compounds - Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene

1. **Identify the substituents**:
- Chlorobenzene (Cl)
- 2,4-Dinitrochlorobenzene (2,4-NO2)
- p-Nitrochlorobenzene (p-NO2)

2. **Determine the effects of substituents**:
- The nitro group (NO2) is a strong electron-withdrawing group due to its -I (inductive) and -R (resonance) effects. This decreases the electron density on the benzene ring, making it less reactive towards electrophiles.
- Chlorine (Cl) is also an electron-withdrawing group, but it has a +R (resonance) effect, which can donate electron density to the ring, making it less deactivating compared to NO2.

3. **Rank the compounds**:
- **Chlorobenzene**: Least electron-withdrawing effect, thus most reactive towards electrophiles.
- **p-Nitrochlorobenzene**: Contains one NO2 group, which withdraws electron density, making it less reactive than chlorobenzene but more reactive than 2,4-dinitrochlorobenzene.
- **2,4-Dinitrochlorobenzene**: Contains two NO2 groups, which significantly withdraw electron density, making it the least reactive towards electrophiles.

**Final Order for Part (a)**:
1. Chlorobenzene
2. p-Nitrochlorobenzene
3. 2,4-Dinitrochlorobenzene

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

1. **Identify the substituents**:
- Toluene (C6H5-CH3)
- p-H3C–C6H4–NO2 (p-Nitrotoluene)
- p-O2N–C6H4–NO2 (2,4-Dinitrotoluene)

2. **Determine the effects of substituents**:
- Toluene has a methyl group (CH3), which is an electron-donating group (+I effect), increasing electron density on the benzene ring and making it more reactive towards electrophiles.
- p-Nitrotoluene has one NO2 group that withdraws electron density but is still influenced by the methyl group, making it less reactive than toluene but more reactive than dinitrotoluene.
- p-O2N–C6H4–NO2 has two NO2 groups, which significantly withdraw electron density, making it the least reactive towards electrophiles.

3. **Rank the compounds**:
- **Toluene**: Most reactive due to the electron-donating effect of the methyl group.
- **p-Nitrotoluene**: Moderately reactive due to the balance between the methyl group and the nitro group.
- **p-Dinitrotoluene**: Least reactive due to the strong electron-withdrawing effect of two nitro groups.

**Final Order for Part (b)**:
1. Toluene
2. p-H3C–C6H4–NO2 (p-Nitrotoluene)
3. p-O2N–C6H4–NO2 (p-Dinitrotoluene)

### Summary of Final Orders:
- **Part (a)**: Chlorobenzene > p-Nitrochlorobenzene > 2,4-Dinitrochlorobenzene
- **Part (b)**: Toluene > p-Nitrotoluene > p-Dinitrotoluene