One of my favorite teaching moments throughout the entire year comes in Chapter 23, when, in the same lecture period, we examine aromatic substitution reactions proceeding through three different intermediates: arenium ion intermediates (first reaction below), Meisenheimer complexes (second reaction below), and benzyne intermediates (third reaction below).
I love this as a teaching moment because of how similar the reactants can appear to students, at least initially. But we work our way through the challenge by simply asking the questions: “What species will react with the aromatic ring,” “how will that species behave,” and “how will the aromatic ring behave?”
For the first reaction, students have come to appreciate in Chapter 22 that HNO3/H2SO4 produces NO2+, a powerful electrophile. Thus, the aromatic ring will react as the electron-rich species to produce the arenium ion intermediate and proceed through the electrophilic aromatic substitution mechanism.
For the second reaction, no such powerful electrophile is present, so electrophilic aromatic substitution doesn’t make sense. Instead, the amine—a moderate nucleophile—is present, and students know to expect a nucleophile to react with an electron-poor species. The strongly electron-withdrawing NO2 group attached to the aromatic ring helps ensure that the ring is electron-poor, so the amine nucleophile adds to produce the Meisenheimer complex, followed by elimination of Clˉ to complete the substitution.
Finally, for the third reaction, students recognize that the aromatic ring doesn’t have a strongly electron-withdrawing substituent attached, so nucleophilic addition to the ring doesn’t quite make sense; the negative charge that would develop in the ring wouldn’t be sufficiently stabilized. Meanwhile, NH2ˉ, a very strong base, reversibly deprotonates the ring, which is followed by elimination of Clˉ to produce the benzyne intermediate that reacts further to yield the substituted product.
To achieve a teaching moment like this one, I give credit to the mechanistic organization under which I teach. By organizing reactions according to mechanism from the outset, students see mechanistic patterns that convincingly simplify the chemistry. Students embrace mechanisms early and continually turn to mechanisms to understand reactions throughout the year. Seeing how species behave as nucleophiles, electrophiles, acids and bases becomes almost second nature for students, and it’s this mastery that students have gained that allows me to smoothly navigate them through what initially appear to be subtleties of the various aromatic substitution reactions.
Thinking back to my early years teaching under a functional group organization, where students did little more than memorize, students would be ill-equipped to deal with these apparent subtleties; navigating through the various aromatic substitution reactions would no doubt be much rockier.
I’d love to know…what has your experience been teaching these reactions under a functional group organization? How about a mechanistic organization?
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