Counterintuitive Chemistry

We get excited when our intuition tells us one thing but we observe something else.  Experimental observations and results that are counterintuitive allows us to understand the problems at a deeper level.  They also point to gaps in our understanding .   Anybody who has gone to a physics demonstration that includes a bed of nails or blowing air between two suspended balls realize the excitement among the audience when their prediction turns out to be wrong and the scientist provides an explanation for it.

We want to collect examples in chemistry are counterintuitive.  Therefore if you have one, send us your example.   Here some from our group:

1. Propane has the lowest melting point of all hydrocarbons. Most people would have expected methane to be the lowest.  Why?  Some say it is packing.  We think it is entropy.

2. Ultrasound increases the aggregation of conjugated polymers rather than break the aggregates. (see the paper by Reichmanis  (link))  This issue is related to the ‘straightening ‘ of chains upon sonication!

3. The entropy of a random packing of hard spheres is less than the entropy of an order array of hard spheres. (See  the paper by Frenkel (link)) Although this is more on the physics side, I see its application in the packing of atoms and morphologies.  We are using this in our research on OPVs.The key concept is the difference between configurational entropy and free-volume entropy.

4. HF is the weakest hydrohalo acid.  Most organic textbooks attribute this to a smaller size of F-.  This explanation is bogus.  The real answer lies in the ‘trapping’ of H+ between two F- complicating the equilibria.   Simply put, H+ in HF prefers to be with F- than with water.  (For equilibria, see ‘Chemistry of the Elements’ by Greenwood and Earnshaw)

5. SN2 at a tertiary carbon reported in 1,4,7-Trimethyloxatriquinane. See Link

6. The Optical Rotatory Power of Water!  Enough said… See paper by Kahr and co-workers (Link)

7. Electron transfer from F- to Naphthalenediimide.  In 2010, there was are report in JACS  that reports the formation of naphthalenediimide radical anion in the presence of F-. The authors report an electron transfer from F- to naphthalenediimide. [Link]  If this is true, what is the fate of the fluoride radical? Can fluoride act as an electron donor?

8. A Sigma Hole:  Why is there a short contact between halogen atoms in some crystal structures?  The halogen-halogen bonding has recently received attention because they show up in many crystal structures.  We have observed short contacts between F atoms in some of our crystal structures.  In the structure of Iodine (I2), we can observes specific bonding patterns that can be reasoned through HOMO-LUMO interactions.  The F-F interactions are still under debate. However, in the case of CF3X, where X is Cl, Br or I, computations indicate the presence of a hole along the C-X bond which inteacts with electron density on the halogen atoms.  Curious, check out this paper [Link]