Back-to-School Trick QuestionsSeptember 14th, 2010
The kids are back in school, which means it’s time to mess with their heads. Since the first week of undergrad orgo is usually devoted to a review of electronic structure and bonding, why not try this on for size:
Fake Problem Set #1. (Insert fake instructions) NOTE: Draw all resonance structures where appropriate.
1) The protons shown in green for 2,4-pentanedione (I) are roughly 10^40 times more acidic that those on n-pentane (II). Provide an explanation.
2) The protons on benzene (III, C6H6) are roughly 10^7 times more acidic than those on n-pentane. Provide an explanation.
3) Boric acid (V) is a weak acid in water. It is roughly a million times more acidic than methanol (IV). Provide an explanation for the acidity of boric acid (by drawing chemical structures).
* All acidities based on pKa values in water (either measured or extrapolated) in Evans’ pKa table.
Answers after the jump…
1. The first part of this question is a fastball to set up the curves. (The major effect here is stabilization of the conjugate base of I by the electron delocalization described in the resonance structures shown below. The negative charge on the conjugate base of II is not similarly delocalized.)
2. Now that you’ve got the students thinking about resonance effects, you are bound to sucker more than a few into writing nonsensical resonance structures to explain the increased acidity of benzene relative to aliphatic hydrocarbons:
The moral of the story? Just because you can push electrons around Lewis structures with a pencil doesn’t mean that doing so makes any sense. The lone pair on the conjugate base of benzene is in an orbital orthogonal to the pi-system of the ring, so there’s no delocalization of the charge by a resonance effect. Also, early on in the class, more than a few kids forget about the implied hydrogens on organic structures. Drawing in the five implied hydrogens would make them realize the “alternate” resonance structures have Texas carbon issues.
The correct explanation for the increased acidity of benzene vs. alkanes is that the electron density is placed in an orbital with more s character (sp2 vs. sp3), and the electron-withdrawing (inductive) effect of the nearby sp2 (vs. sp3) carbons also helps stabilize the charge.
3. If the kids are still fixated on resonance—or if they’re thinking back to nitric acid and carbonic acid from gen chem—you are probably bound to encounter answers where boric acid is treated as a Bronsted acid (H+ donor, top reaction, in blue). The “real way” boric acid behaves as an acid in water is first by acting as a Lewis acid (bottom reaction, in black). In fairness to the undergrads, I’d bet >25% of graduate students around the country would not be able to correctly explain the acidity of boric acid.
Now…go terrorize your students and coworkers.