Imperial College London

Henry Rzepa

Continuing my european visits, here are two photos from Bonn. First, a
word about how the representation of benzene evolved, attributed to
Kekulé.

::: {#attachment_12549 .wp-caption .aligncenter style="width: 210px"}
[![The sausage
formula](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule-sausage.gif){.size-full
.wp-image-12549 decoding="async"
aria-describedby="caption-attachment-12549"
width="200"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule-sausage.gif)

The sausage formula
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Above is his first effort, made in 1865.

::: {#attachment_12550 .wp-caption .aligncenter style="width: 360px"}
[![The bent bond
formula](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule-benzene.gif){.size-full
.wp-image-12550 decoding="async"
aria-describedby="caption-attachment-12550"
width="350"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule-benzene.gif)

The bent bond formula
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This one above is better, offered in 1866. But whilst what we now know
as the double bond (C=C) is perhaps understandably kinked (and we now
call these banana bonds, since nature tends to abhor kinks in electron
density), so too are the single bonds!

::: {#attachment_12548 .wp-caption .aligncenter style="width: 450px"}
[![The
plinth](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule1.jpg){.size-full
.wp-image-12548 decoding="async"
aria-describedby="caption-attachment-12548" width="440"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule1.jpg 880w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule1-300x223.jpg 300w"
sizes="(max-width: 880px) 100vw, 880px"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule1.jpg)

The plinth
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So when it came to erecting a statue in his honour around 1890, the
kinks were straightened out! The figure on the right (female) represents
science (and purity). The two chaps on the left are workers representing
industry. Note that they have still not quite gotten the lengths of the
putative single and double bonds in proportion. By 1872 of course,
Kekulé had
proposed\[cite\]10.1002/jlac.18721620110\[/cite\],\[cite\]10.1002/jlac.18721620211\[/cite\]
his oscillating model, the one that is taught to this day.

I feel I should add one modern interpretation to this concept. An
oscillation implies a frequency. Kekulé could only know that this
oscillation was fast on what might be called the "laboratory scale" (in
other words, no-one had been able to isolate the individual isomers of
substituted benzenes, which implies that the rate constant
inter-converting them was probably faster than k = 10^-3^ s^-1^ or a few
minutes half-life).^‡^ We now know that this oscillation is \~10^14^
s^-1^, the timescale of a molecular vibration! By the way, transition
state theory tells us that ***Ln*(k/T) = 23.76 -- ΔG^‡^/RT** where k is
the unimolecular rate constant, ΔG^‡^ the free energy barrier, T the
temperature and R the gas constant. Setting ΔG^‡^ to zero gives us a
rate constant of \~10^14^ s^-1^ (the barrier must be zero, or very close
to it).

::: {#attachment_12547 .wp-caption .aligncenter style="width: 450px"}
[![The
Statue](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule2.jpg){.size-full
.wp-image-12547 decoding="async"
aria-describedby="caption-attachment-12547" width="440"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule2.jpg 880w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule2-300x223.jpg 300w"
sizes="(max-width: 880px) 100vw, 880px"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/05/kekule2.jpg)

The Statue
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Here is the statue atop the plinth above. Apparently it is honoured by
the students with robes and other accoutrements on special occasions.

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^‡^Another famous timescale inference was by Beckmann in
1889\[cite\]10.1002/jlac.18892500306\[/cite\] when he deduced the
existence of a transient unseen intermediate in the (what he thought
was) racemisation of menthone. That intermediate of course was the enol.

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
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Continuing my european visits, here are two photos from Bonn. First, a word about how the representation of benzene evolved, attributed to Kekulé. The sausage formula  Above is his first effort, made in 1865. The bent bond formula  This one above is better, offered in 1866.

Messaggi di Rogue Scholar

Benzene. As you have never seen it represented before!