Question

Organic II lab: Relative Rates of Electrophilic Aromatic Substitution

In this experiment, acetic acid is used as a catalyst for the electrophilic substitution on aromatic rings. Would this catalyst work with nitrobenzene? Explain your answer. _______________________________________________

Answer 1

In any Electrophilic Aromatic Substitution Reaction (EASR) the electrophile is generated in situ using some reagents or by using the salt containing required electrophile with counter anion.

E.g. In case of bromination of aromatic ring acetic acid is used as catalyst. Acetic acid help in polarization of Br-Br bond and facilate supply of electrophile Br⁺ for substitution. Here H⁺ provided by acetic acid serve to hold Br^- ion so Br⁺ remains free enough to attack benzene ring electron density.

In case of Nitration of benzene ring the electrophile is NO₂⁺. The normal sources of NO₂⁺ are mixture of H2SO4+HNO3, Nitrofluoroborate etc. In each case the counter ion is stable enough. However if acetic acid used as catalyst it will provide H⁺ which could facilate generation of NO2 either by holding counter anion or by helping liberation of NO2 electrophile from reagent. In (H2SO4+HNO3) case H2SO4 is itself strong acid and acetic acid catalysis will be just wastage of reagent.