Frederick Sanger’s early work on protein sequencing through the use of colorimetric labeling combined with liquid chromatography involves an important nucleophilic aromatic substitution (S_NAr) reaction in which the N-terminus of a protein is tagged with Sanger’s reagent. Understanding the inherent differences between this S_NAr reaction and other nucleophilic substitution reactions (S_N1 and S_N2) can be challenging for students learning organic chemistry. Here, both electrostatic potential (ESP) maps and natural bond orbital (NBO) analyses are employed to visualize and conceptualize Sanger’s key observation of the difference in reactivity between 2,4-dinitrochlorobenzene and 2,4-dinitrofluorobenzene. The utility of this method is extended to compare the reactivity of a series of halobenzenes for S_NAr fluorination, a widely used reaction in pharmaceutical and medicinal fields. In combination with experimental results from the literature, the ESP maps and NBO analyses are consistent with and provide excellent corroboration with the reactivity of different substrates toward S_NAr reactions.