The most reactive compound among the following toward electrophilic aromatic substitution reaction is:

The reactivity of benzene derivatives towards electrophilic substitution depends on the nature of the substituent group attached to the ring.

1. Activating Groups: Electron-donating groups increase the electron density in the benzene ring, making it more susceptible to electrophilic attack. The methyl group ($-CH_3$) in Toluene is an activating group due to the inductive effect ($+I$) and hyperconjugation. It activates the ring at ortho and para positions .
2. Deactivating Groups: Electron-withdrawing groups decrease electron density.

• Chlorine (in Chlorobenzene) is deactivating due to a strong $-I$ effect, although it is ortho-para directing .
• Nitro group ($-NO_2$ in Nitrobenzene) is a strong deactivating group due to $-I$ and $-R$ effects .

3. Conclusion: Toluene, having an activating group, is more reactive than Benzene (standard), Chlorobenzene (weakly deactivated), and Nitrobenzene (strongly deactivated).