Aims of Arrows

“In chemistry’s schemes mechanistic,
Find blend analytic-artistic…
See bonds’ breaking/forming;
Electrons’ unmooring:
All shown through line drawings, logistic.”

The 14 April 2023 limerick summarized an interesting overlap of chemistry and art, in terms of the drawings that chemists use to depict reactions.

“In chemistry’s schemes mechanistic, /
Find blend analytic-artistic… “

I’ve written often about my interest in the chemistry underlying artistic techniques, such as frescoes and cyanotypes.  Another way in which chemistry and art overlap is with respect to the symbolic representations used to describe the step-by-step progress of chemical reactions; these representations are called mechanisms (or “schemes mechanistic”).  

Organic chemists, especially, spend much time learning to draw these “blend[s] analytic-artistic,” in which the logical flow of a set of steps can be followed from reactants to products.  

“See bonds’ breaking/forming; /
Electrons’ unmooring: /
All shown through line drawings, logistic.”

Mechanisms are also called “electron-pushing diagrams” or “arrow-pushing diagrams.”  Pioneered by Robert Robinson (1886-1975), the drawings allow a chemist to easily show the breaking and forming of bonds between atoms, as well as the movement of electrons within and between molecules, using curved arrows.  The arrows begin where the electrons are and point to where they will go, as shown in the substitution- and elimination-themed poems from last autumn.  

I’ve written recently on the power of line drawings in organic chemistry, where even a simple hyphen can be reasonably read by a chemist as the molecule ethane.  To read through a multi-step mechanism is to see the immense utility of these “line drawings, logistic” when used in communication among chemists.  

Interestingly, throughout NaPoWriMo 2023, I found it a fun challenge to describe in verse the considerable variety of arrows in chemistry, so I plan to return to this topic, with some additional poem-expanding essays, in early 2024.  “Aims of arrows” can thus be read as both summarizing the goals of these mechanistic drawings and pointing towards themes in the spring term.