Which compound will readily undergo sulfonation?

Sulfonation is an electrophilic aromatic substitution (EAS) reaction in which a –SO₃H group (sulfonic acid group) is introduced into an aromatic ring. The reaction usually takes place when the compound is heated with concentrated sulfuric acid or fuming sulfuric acid (oleum), forming an aryl sulfonic acid. The rate of sulfonation depends on the nature of substituents already present on the aromatic ring.

Among the given compounds, toluene (C₆H₅CH₃) undergoes sulfonation most readily. The methyl group (–CH₃) attached to the benzene ring is an electron-donating group due to its +I (inductive) effect. It increases the electron density on the ring, especially at the ortho and para positions, making the ring more reactive toward electrophiles such as SO₃. As a result, toluene reacts faster with sulfuric acid than benzene.

In contrast, nitrobenzene (C₆H₅NO₂) contains a –NO₂ group, which is strongly electron-withdrawing through both –I and –M effects. This reduces the electron density on the benzene ring and makes it much less reactive toward electrophilic substitution. Hence, nitrobenzene undergoes sulfonation very slowly.

Chlorobenzene (C₆H₅Cl) also shows decreased reactivity compared to benzene because chlorine, although weakly donating by resonance, is strongly withdrawing inductively. This balance slightly deactivates the ring toward further substitution.

Benzene itself can be sulfonated, but the reaction rate is slower compared to toluene because benzene lacks an activating group.

Therefore, toluene sulfonates more readily than benzene, nitrobenzene, or chlorobenzene due to the activating effect of its methyl group. This concept is fundamental in understanding how substituents influence aromatic ring reactivity in electrophilic aromatic substitution reactions, an essential topic in organic chemistry