Imperial College London

Henry Rzepa

Observation of the slow racemization of isobornyl chloride in a polar
solvent in 1923-24 by Meerwein led to the recognition that mechanistic
interpretation is the key to understanding chemical reactivity. The
hypothesis of ion pairs in which a **chloride anion** is partnered by a
**carbocation** long ago entered the standard textbooks (see
DOI [10.1021/ed800058c](http://dx.doi.org/10.1021/ed800058c) and
[10.1021/jo100920e](http://dx.doi.org/10.1021/jo100920e) for background
reading). But the intimate secrets of such ion-pairs are still perhaps
not fully recognised. Here, to tease some of them them out, I use the
NCI method, which has been the subject of several recent posts.

::: {#attachment_4199 .wp-caption .aligncenter style="width: 282px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/05/isobornyl.jpg "isobornyl"){.size-full
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width="272" height="324"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/05/isobornyl.jpg 544w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/05/isobornyl-251x300.jpg 251w"
sizes="(max-width: 272px) 100vw, 272px"}

NCI analysis of the iod-pair transition state for methyl migration in
isobornyl chloride. Click for 3D.
:::

To remind, the colour coding of the NCI surface is [blue=strongly
attractive]{style="color: #0000ff;"}, [red=strongly
repulsive]{style="color: #ff0000;"}, [green=weakly
attractive]{style="color: #00ff00;"}, [yellow=weakly
repulsive]{style="color: #ffcc00;"}. Shown above is the [ion-pair
transition state](http://hdl.handle.net/10042/to-8512){target="_blank"}
for \[1,2\]methyl migration. Note how the hydrogen bonds between the
chloride anion and the water molecules are clearly blue. Only slightly
weaker (with a turquoise tint) is a pair of hydrogen bonds between the
oxygen atoms and H-C bonds in the isobornyl cation. Such C-H...O bonding
in ion-pairs seems to be particularly important. There are other blue
regions, between an H...H pair, and a C-H bond and the carbon of the
migrating methyl group. Also noteworthy is that many atom pairs have
multi-coloured NCI regions, suggesting the interaction is not
homogenous, and can be both attractive AND repulsive between any pair of
atoms.

The NCI plot below shows the competing 1,6-hydride shift in isobornyl
chloride, again involving an ion-pair transition state.

::: {#attachment_4210 .wp-caption .aligncenter style="width: 239px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/05/isobornyl1.jpg "isobornyl1"){.size-full
.wp-image-4210 loading="lazy" decoding="async"
aria-describedby="caption-attachment-4210"
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width="229" height="188"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/05/isobornyl1.jpg 457w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2011/05/isobornyl1-300x246.jpg 300w"
sizes="(max-width: 229px) 100vw, 229px"}

NCI surfaces for the 1,6 hydride migration transition state in isobornyl
chloride. Click for 3D.
:::

Notice in this example how the migrating hydrogen supports an attractive
hydrogen bond to the chloride anion (ostensibly between a hydride atom
and an anionic chloride?), and again how there are a number of **blue**
regions elsewhere.

Modelling is increasingly focusing on these weaker interactions, that
probably mediate much (stereo)selectivity in organic reactions. How long
before such approaches themselves enter the text-books?

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a
<strong>
 chloride anion
</strong>
is partnered by a
<strong>
 carbocation
</strong>
long ago entered the standard textbooks (see DOI 10.1021/ed800058c and 10.1021/jo100920e for background reading).

Messaggi di Rogue Scholar

The inner secrets of an ion-pair: Isobornyl chloride rearrangements.