Henry Rzepa's Blog Chemistry with a twist

August 22, 2018

Early “curly” (reaction) arrows. Those of Ingold in 1926.

In 2012, I wrote a story of the first ever reaction curly arrows, attributed to Robert Robinson in 1924. At the time there was a great rivalry between him and another UK chemist, Christopher Ingold, with the latter also asserting his claim for their use. As part of the move to White City a lot of bookshelves were cleared out from the old buildings in South Kensington, with the result that yesterday a colleague brought me a slim volume they had found entitled The Journal of the Imperial College Chemical Society (Volume 6). 


May 28, 2014

Benzene: an oscillation or a vibration?

Filed under: Historical — Tags: , , — Henry Rzepa @ 9:28 am

In the preceding post, a nice discussion broke out about Kekulé’s 1872 model for benzene.[1] This model has become known as the oscillation hypothesis between two extreme forms of benzene (below). The discussion centered around the semantics of the term oscillation compared to vibration (a synonym or not?) and the timescale implied by each word. The original article is in german, but more significantly, obtainable only with difficulty. Thus I cannot access[1] the article directly since my university does not have the appropriate “back-number” subscription. So it was with delight that I tracked down an English translation in a journal that I could easily access.[2] Here I discuss what I found (on pages 614-615, the translation does not have its own DOI).



  1. A. Kekulé, "Ueber einige Condensationsproducte des Aldehyds", Annalen der Chemie und Pharmacie, vol. 162, pp. 77-124, 1872. http://dx.doi.org/10.1002/jlac.18721620110
  2. "Organic chemistry", Journal of the Chemical Society, vol. 25, pp. 605, 1872. http://dx.doi.org/10.1039/JS8722500605

November 7, 2011

Henry Armstrong: almost an electronic theory of chemistry!

Henry Armstrong studied at the Royal College of Chemistry from 1865-7 and spent his subsequent career as an organic chemist at the Central College of the Imperial college of Science and technology until he retired in 1912. He spent the rest of his long life railing against the state of modern chemistry, saving much of his vitriol against (inter alia) the absurdity of ions, electronic theory in chemistry, quantum mechanics and nuclear bombardment in physics. He snarled at Robinson’s and Ingold’s new invention (ca 1926-1930) of electronic arrow pushing with the put down “bent arrows never hit their marks“.  He was dismissed as an “old fogy, stuck in a time warp about 1894.” So why on earth would I want to write about him? Read on…


September 14, 2010

The oldest reaction mechanism: updated!

Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. Hence electronic arrow pushing as a term. But here I argue that the true origin of this immensely powerful technique in chemistry goes back to the 19th century. In 1890, Henry Armstrong proposed what amounts to close to the modern mechanism for the process we now know as aromatic electrophilic substitution [1]. Beyond doubt, he invented what is now known as the Wheland Intermediate (about 50 years before Wheland wrote about it, and hence I argue here it should really be called the Armstrong/Wheland intermediate). This is illustrated (in modern style) along the top row of the diagram.

The mechanism of aromatic electrophilic substitution



  1. "Proceedings of the Chemical Society, Vol. 6, No. 85", Proceedings of the Chemical Society (London), vol. 6, pp. 95, 1890. http://dx.doi.org/10.1039/PL8900600095

December 9, 2009

Clar islands in a π Cloud

Clar islands are found not so much in an ocean, but in a type of molecule known as polycyclic aromatic hydrocarbons (PAH). One member of this class, graphene, is attracting a lot of attention recently as a potential material for use in computer chips. Clar coined the term in 1972 to explain the properties of PAHs, and the background is covered in a recent article by Fowler and co-workers (DOI: 10.1039/b604769f). The concept is illustrated by the following hydrocarbon:


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