Arrange the following compounds in order of their decreasing reactivity with an electrophile:
methoxy-benzene (I)
chloro-benzene (II)
nitro-benzene (III)

Question

Arrange the following compounds in order of their decreasing reactivity with an electrophile: 
methoxy-benzene (I) 
chloro-benzene (II) 
nitro-benzene (III)

Accepted Answer

To determine the order of reactivity of the given compounds with an electrophile, we need to consider the electronic effects of the substituents on the benzene ring. The reactivity of benzene derivatives towards electrophilic aromatic substitution is influenced by the nature of the substituents attached to the benzene ring.

1. **Methoxy-benzene (I):** The methoxy group (-OCH₃) is an electron-donating group due to its +R (resonance) effect. It donates electron density into the benzene ring, making it more electron-rich and thus more reactive towards electrophiles. The increased electron density enhances the ability of the benzene ring to stabilize the positive charge of the intermediate carbocation formed during the electrophilic attack.

2. **Chloro-benzene (II):** The chloro group (-Cl) is slightly electron-withdrawing due to its -I (inductive) effect, but it also has a +R effect due to the lone pairs on chlorine that can participate in resonance. However, the overall effect is less activating compared to the methoxy group. Therefore, chloro-benzene is less reactive than methoxy-benzene but more reactive than nitro-benzene.

3. **Nitro-benzene (III):** The nitro group (-NO₂) is a strong electron-withdrawing group due to both its -I and -R effects. It pulls electron density away from the benzene ring, making it electron-deficient and less reactive towards electrophiles. The electron-withdrawing nature of the nitro group significantly deactivates the benzene ring, making it the least reactive among the three compounds.

Based on the above analysis, the order of reactivity with an electrophile is: methoxy-benzene (I) > chloro-benzene (II) > nitro-benzene (III). Therefore, Option 1 ($I>II>III$) is correct.

**Explanation of why other options are incorrect:**

- **Option 2 ($III>II>I$):** This option suggests that nitro-benzene is more reactive than chloro-benzene and methoxy-benzene, which is incorrect. Nitro-benzene is the least reactive due to the strong electron-withdrawing nature of the nitro group.

- **Option 3 ($II>III>I$):** This option suggests that chloro-benzene is more reactive than methoxy-benzene, which is incorrect. The methoxy group is a stronger electron-donating group than the chloro group, making methoxy-benzene more reactive.

- **Option 4 ($I>III>II$):** This option suggests that nitro-benzene is more reactive than chloro-benzene, which is incorrect. Nitro-benzene is the least reactive due to the electron-withdrawing nature of the nitro group.

In conclusion, Option 1 ($I>II>III$) correctly represents the order of decreasing reactivity with an electrophile based on the electronic effects of the substituents.