When considering the acylation of the given compounds with acetyl chloride in the presence of aluminum chloride (AlCl3), we must focus on the ability of the substrates to stabilize the positive charge that develops during the reaction. In this case, we are comparing 4-dichlorobenzene, chlorobenzene, toluene, and benzene.
Out of these options, toluene will undergo the most rapid acylation. The reason for this is due to the presence of the methyl group attached to the benzene ring. The methyl group is an electron-donating group by resonance, which increases the electron density on the aromatic ring. This makes the ring more reactive towards electrophiles like the acylium ion generated from acetyl chloride and AlCl3.
In contrast, both chlorobenzene and 4-dichlorobenzene have chlorine atoms that are electron-withdrawing through resonance due to their electronegativity, which deactivates the ring towards electrophilic substitution. While 4-dichlorobenzene does have the dual deactivating effect of two chlorine atoms, toluene’s activating nature overshadows this.
Benzene, being a symmetrical aromatic compound without any substituents, has no activating or deactivating groups to influence the acylation process, making it slower than toluene as well.
Thus, in summary, the order of reactivity for acylation in the provided options, from fastest to slowest, is as follows: toluene > benzene > chlorobenzene > 4-dichlorobenzene. Therefore, toluene undergoes the most rapid acylation with acetyl chloride and AlCl3.