Wednesday, April 3, 2013

Things I won’t include in my First Year Chemistry Textbook (except as cautionary historical episodes)

1. Negative electron affinities

How can you force something that doesn’t want an electron to accept an electron? How would you measure the energy in doing that? Okay, in an ionic lattice you can get the energy back somehow and do some algebra to get a number. Maybe this exercise gives you a number that is negative. Is that a number we should be confusing first year students with? I think, no.  Meaningfully, things that don’t like forming anions should be listed with a first electron affinity of ~0. Why? This would be consistent with what we tell students later about intermolecular forces, where electrons move to give transient dipoles giving attractive forces between anything and anything.

2. Hybridisation

Atoms have atomic orbitals. When you combine atoms in molecules, the resulting molecules have molecular orbitals. These things have physical significance and their properties can be calculated using quantum mechanics and group theory.  ‘Hybrid orbitals’ are a non-physical halfway house with no real explanatory power.  The labels sp, sp2, and sp3 are useful shorthand for motifs that recur in molecular orbitals. Nothing more.

3. Crystal Field Theory

We teach this theory and then immediately segue to the spectrochemical series, which is the opposite way around to what the theory predicts. This is okay if you are the kind of student who just memorises things for the exam without thinking about them, but if you are the kind of student who wants to make logical connections between the things you learn, you will just go: ‘WTF? Coordination chemistry is stupid.’ I suggest the handwaving explanation ‘d-orbital electrons go in MOs with antibonding character’ as a substitute introductory-level model.

4. Intermediate Stability as an Explanation for Selectivity

All first year organic chemistry textbooks will tell you that you get substitution of hydrogen in addition of HX to a double bond at the place where there are more hydrogens already because the intermediate is more stable. They will trot out this same kind of explanation for all sorts of other things. This is a bogus argument because it reverses causality. The reaction can’t go a particular way because it will form something that is more stable; how can it possibly know what it is going to form until it forms it? This ‘explanation’ is lazy shorthand for something that everyone ought to be taught in first year but isn’t.

Formation of an intermediate is endothermic by definition. The transition states of endothermic reactions usually resemble the products (Hammond’s Postulate). So the structure of the intermediate is important as a pointer towards the structure of the transition state. A stable intermediate implies a low energy transition state; that is, a lower activation energy.

Note that Hammond’s postulate is not called Hammond’s Law. And note that this is only going to be worth invoking for kinetically-controlled reactions; for thermodynamically-controlled reactions, we just have to tally up the bond enthalpies of the possible products... and that is a full and complete explanation for observed selectivity.