Arrange the following set of compounds in the order of their decreasing relative reactivity with an electrophile`E^+`
I.Chlorobenzene
II.2,4 dinitrochlorobenzene
III.P-nitrochlorobenzene
IV.toluene V`.p-H_3C-C_6H_4-NO_2`

Options

A. `IgtIIgtIIIgtIVgtV`
B. `IVgtVgtIgtIIIgtII`
C. `VgtIVgtIIIgtIIgtI`
D. `IgtIIIgtIIgtVgtIV`

Answer

To arrange the given compounds in order of their decreasing relative reactivity with an electrophile (E^+), we need to consider the electron-donating or electron-withdrawing effects of the substituents on the benzene ring. Electrophiles are attracted to regions of high electron density, so compounds with electron-donating groups will be more reactive than those with electron-withdrawing groups.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Substituents:**
- I. Chlorobenzene: Contains a chlorine atom, which is a weakly electron-withdrawing group.
- II. 2,4-Dinitrochlorobenzene: Contains two nitro groups (strongly electron-withdrawing) and a chlorine atom.
- III. Para-Nitrochlorobenzene: Contains one nitro group (strongly electron-withdrawing) and a chlorine atom.
- IV. Toluene: Contains a methyl group (electron-donating) which increases electron density on the benzene ring.
- V. p-H3C-C6H4-NO2: Contains a methyl group (electron-donating) and a nitro group (electron-withdrawing) in the para position.

2. **Determine the Effects of Substituents:**
- Electron-donating groups (like -CH3) increase the reactivity towards electrophiles.
- Electron-withdrawing groups (like -NO2 and -Cl) decrease the reactivity towards electrophiles.
- The nitro group (-NO2) is a strong electron-withdrawing group, while the chlorine atom (-Cl) is a weak electron-withdrawing group.

3. **Rank the Compounds Based on Reactivity:**
- Toluene (IV) will be the most reactive due to the presence of the electron-donating methyl group.
- p-H3C-C6H4-NO2 (V) will be next, as the methyl group partially offsets the electron-withdrawing effect of the nitro group.
- Para-Nitrochlorobenzene (III) comes next, as it has one nitro group.
- 2,4-Dinitrochlorobenzene (II) is less reactive due to the presence of two nitro groups, which significantly withdraw electron density.
- Chlorobenzene (I) is the least reactive due to the weak electron-withdrawing effect of chlorine.

4. **Final Order of Reactivity:**
- IV (Toluene) > V (p-H3C-C6H4-NO2) > III (Para-Nitrochlorobenzene) > II (2,4-Dinitrochlorobenzene) > I (Chlorobenzene)

### Final Answer:
**The order of decreasing relative reactivity with an electrophile is:**
IV > V > III > II > I