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 →

Writing About Writing About the Second Law

I recently returned from vacationing in the UK, and just spent a couple of days in the West End of Glasgow, near Kelvingrove Park. Yes, the same Kelvin of scientific fame! Seeing his statue got me thinking about the second law of thermodynamics—enough that I was inspired to jot a few thoughts down about the second law.

The second law and entropy are two of the hardest topics to write about at a general chemistry level, in my opinion. Not only has there been fierce debate over the years as to the ideal intuitive notions and analogies for these topics, but related derivations with mathematical rigor can be painfully complicated. There’s a gulf here between the theory and practice of chemical thermodynamics that is difficult to navigate.

A while back I tried just to get down on paper a rigorous derivation of the definition of entropy in terms of heat and temperature, using the second law and a hypothetical thermodynamic cycle. While the work was mathematically correct, the writing made me—the author, mind you!—want to tear my eyeballs out recently. That text will never see the light of day in a general chemistry class. At that point, I wondered if I was even capable of dispensing with rigor to write a more intuitive piece. I’ve always found it difficult to write while sacrificing rigor because I still recall craving rigor and theory in the depths of my soul as a student.

The reality, of course, is that all chemists use heuristics, shortcuts, or metaphors when confronted with certain topics. The best chemist writers can navigate rigorous theory and metaphor with finesse, presenting derivations where the mind “wants” them and metaphors elsewhere. Tro is a good example—while he makes no effort to dumb down important equations, he also presents the practical metaphors that chemists most often use.

In the edition I have, he even manages to lay out all three general interpretations of entropy: entropy as disorder, entropy as energy dispersal, and the statistical interpretation. Color me jealous!