Comments on: Curly arrow pushing: another reality check.

By: Henry Rzepa

Henry Rzepa — Fri, 21 Sep 2012 09:14:15 +0000

Ian Fleming is famous (amongst other things) for his book “Molecular orbitals and Organic Reactions”. A colleague of mine (thanks James!) suggested that another way of regarding curly arrow pushing is to think of the arrows as reminding (perhaps representing is too strong a way of putting it) us of the underlying orbital basis of reactions. In this scenario we think of a curly arrow as representing the interaction of a filled orbital with an empty one, very much in the manner of the Klopman-Salem perturbation expression. Rather loosely, the origin of the arrow would be the filled (donor) orbital and its destination the region of maximum overlap between it and the empty (acceptor, and probably antibonding) orbital. The arrow in this interpretation can represent the formation of a new bond by the overlap of these two orbitals, and also the cleavage of the antibonding region. I have tried to represent these concepts here for the example described above.

By: Henry Rzepa

Henry Rzepa — Sun, 05 Aug 2012 09:02:39 +0000

The trans addition almost certainly requires a different model; perhaps involving two HBr units, one on each side of the alkene, and possibly a proton transfer chain to avoid the dipole moment building up to too large a value if the solvent permits.

Many reactions indeed require a lot of movement with one component “getting to the other side” and I think we need to think about models that can achieve this.

I have studied such non-least motion mechanisms for quite different reactions. An early attempt to do this for the E2 elimination appeared in this post, where both syn and anti modes were studied, and this involved adding a second halide anion to the model. The same approach was used to try to find out the balance between a ring migration reaction and an E2 elimination. The relative energies of the two modes certainly favours the anti elimination. However, I am not satisfied that either of these models is yet complete, since the overall charge on the model is -1 (coming from the second halogen). I feel the model will only be complete when the system is overall neutral, and an appropriate charge-balancing cation is somehow added.

By: Paul Schleyer

Paul Schleyer — Sun, 05 Aug 2012 08:44:32 +0000

Very instructive animation and discussion, Henry! I note that a “least motion” cis addition pathway wins out, but without any indication of the “trans addition” we teach our students. How does this occur?
In that context I wonder about
(1) HBr addition to norbornene, where such “direct addition” completes with rearrangement-addition
(2) trans Br2 addition to ethylene in solvents like CCl4 is known. A bromonium ion may form, but how does the second Br get to the other side under such non-ionizing conditions?