Yin and Yang: Electrophilic and Nucleophilic Reactions

In my opinion, the most awkwardly named reaction in all of chemistry is electrophilic aromatic substitution (and all of its three-worded cousins). This name suffers from the same problem as other named reactions: it is deceptively uninformative. I still recall raising an eyebrow in undergrad when I found out that the aromatic involved in this reaction is not the electrophile—the other reagents combine to generate the electrophile. The aromatic is the nucleophile. “Why the heck is the word ‘electrophilic’ stuffed before ‘aromatic’ in the name, then?!” When you really get down to it, the name doesn’t tell you much and has the potential to feed a novice a lot of incorrect information:

“So the reaction mixture is electrophilic, then?”
“Well no, the reaction involves a nucleophile and an electrophile, just like all polar organic reactions…”

“So the aromatic is electrophilic?”
“No, the aromatic is the nucleophile in these reactions.”
“But the name says electrophilic aromatic…!”
[Professor places face in palms]

Only once the student has seen copious examples of other electrophilic substitutions does s/he realize that the adjective refers to the conditions surrounding the substrate, not the substrate itself. The naming convention makes sense to a synthetic chemist interested in “decorating” a given substrate: the substrate is what it is, and we treat it with electrophilic or nucleophilic conditions to add groups to it. The names of substitution reactions clarify the reactivity of whatever’s coming into contact with the substrate (the reagents). To a student without a synthetic frame of mind though, without an inkling of the primacy of the substrate or even its identity, I don’t think this naming convention comes naturally. Continue reading →