Our discussion of solvent and salt effects was quite interesting, especially if you think about today’s lecture on rate constants. An endo/exo ratio of 2 means a tiny free energy difference (~ 0.4 kcal/mol) between the endo and exo transition states. Even a solvent effect of 1000 on the overall rate means that the barrier has changed by only ~4 kcal/mol (assuming room temperature reactions).
As part of our discussion, Sarah drew 4 different transition states, 2 forming the exo isomer and 2 forming the endo isomer. Here’s what they look like as tube models. (IN/OUT refers to the position of the C=O; s-cis and s-trans provides the same information.)
If the hydrophobic packing argument is correct, the surface area of these transition states should be a good bit smaller than the surface area of the separated reactants. According to Spartan, the transition states have surface areas between 153-156 A2, while the reactants start with a total surface area of 194 A2 (90 acrolein + 104 cyclopentadiene).
Space-filling models of the transition state show just how tightly these molecules snuggle together. Although the distance between reactants is >2 A (measured nucleus-to-nucleus), this is far less than the sum of two carbon van der Waals radii (2.9-3 A).
Receive Chemistry 324 posts (RSS)
