I have several times used arrow pushing on these blogs. But since the rules for this convention appear to be largely informal, and there appears to be no definitive statement of them, I thought I would try to produce this for our students. This effort is here shared on my blog. It is what I refer to as the standard version; an advanced version is in preparation. Such formality might come as a surprise to some;

The concept of a “ hidden intermediate ” in a reaction pathway has been promoted by Dieter Cremer[cite]10.1021/ar900013p[/cite] and much invoked on this blog. When I used this term in a recent article of ours[cite]10.1021/jo401146k[/cite], a referee tried to object, saying it was not in common use in chemistry. The term clearly has an image problem.

A little while ago, I set out some interpretations of how to push curly arrows.

I occasionally delve into the past I try to understand how we got to our present understanding of chemistry.

Two years ago, I discussed how curly arrow pushing is taught, presenting four different ways of showing the arrows. One of the comments posted to that blog suggested that all of the schemes shown below were deficient in one aspect. Curly arrow pushing The issues were the stereo and regiochemistry. In particular, the diagram above carries no explicit information about the symmetry of the electrons from which the first arrow originates;

Recollect, Robinson was trying to explain why the nitroso group appears to be an o/p director of aromatic electrophilic substitution. Using σ/π orthogonality, I suggested that the (first ever) curly arrows as he drew them could not be the complete story, and that a transition state analysis would be needed. Here it is.  Let me set the scene on how this might be done.

The discussion appended to the post on curly arrows is continued here. Recollect the curly arrow diagram (in modern style) derived from Robinson’s original suggestion: The pertinent point is that the angle subtended at the nitrogen atom evolves from being bent (~115°) on the left, to linear (180°) on the right.

Little did I imagine, when I discovered the original example of using curly arrows to express mechanism, that the molecule described there might be rather too anarchic to use in my introductory tutorials on organic chemistry. Why? It simply breaks the (it has to be said to some extent informal) rules!

I was first taught curly arrow pushing in 1968, and have myself taught it to many a generation of student since. But the other day, I learnt something new.

Sometimes, connections between different areas of chemistry just pop out (without the help of semantic web tools, this is called serendipity). So here, I will try to join up some threads which emerge from previous posts. I had noted that antiaromaticity in cyclopropenium anion is lessened by the system adopting gross geometric distortions, which take the anionic lone pair out of conjugation from the ring.