Imperial College London

Henry Rzepa

Do you fancy a story going from simplicity to complexity, if not
absurdity, in three easy steps? Read on! The following problem appears
in one of our (past) examination questions in introductory organic
chemistry. From relatively mundane beginnings, one can rapidly find
oneself in very unexpected territory.

::: {#attachment_3066 .wp-caption .aligncenter style="width: 281px"}
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+.jpg "CN+"){.size-full
.wp-image-3066 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3066" width="271" height="226"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+.jpg 1084w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+-300x250.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+-1024x854.jpg 1024w"
sizes="(max-width: 271px) 100vw, 271px"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+.jpg)

How would one make 3-nitrobenzonitrile?
:::

One teaches how to disconnect each group, identifying that both are
*meta*-directing towards electrophiles, and hence asking what an
appropriate electrophile might be? The "correct" answer is a nitronium
cation (nitric + sulfuric acids) acting upon *m*-directing benzonitrile.
But (sacrilege), why not a "cyonium" cation (CN^+^) on *m*-directing
nitrobenzene? Well, as a tutor one would normally swat it away on the
grounds it has never been previously observed (or that cyanide is always
seen as an anion, not a cation). But then one (or a student) asks, why
not? How about generating it from *e.g.* TfOCN. TfO^--^ is a jolly good
leaving group, one of the best. Well, this precursor truly appears never
to have been made (or even calculated!). By now (if encountered in a
tutorial), most chemistry students would be rather bemused. So the
process of following one's nose (more accurately, my nose) continues in
the peace and quiet of a blog, where a rather different readership might
be bemused (or inflamed).

::: {#attachment_3068 .wp-caption .aligncenter style="width: 108px"}
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN.jpg "CN"){.size-full
.wp-image-3068 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3068" width="98"
height="52"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN.jpg)

A Quadruple CN bond?
:::

One might start the same place a student would. How would one represent
this diatomic with bonds? How about the above? It has the merit that
both atoms are associated with a (shared) octet of electrons, in the
form of a quadruple bond. I did show this (briefly) to a colleague, but
they recoiled in horror, although it has to be said they were slightly
at a loss to actually explain their horror.

Well, time for calculations. How about CCSD/aug-cc-pVTZ (DOI:
[10042/to-6261](http://hdl.handle.net/10042/to-6261){target="_blank"})
or B3LYP/aug-cc-pVTZ (DOI:
[10042/to-6255](http://hdl.handle.net/10042/to-6255){target="_blank"}).
The latter allows a so-called *Wiberg bond index* to be computed (a
reasonably accepted index). This comes out at **3.55**, well on the way
to being quadruple. An NBO analysis (NBO 5.9) identifies **FOUR** NBO
orbitals with an occupancy of \~2.0, all designated BD (rather than
*e.g.* Lp). What are these NBOs like? (as it turns out, they are almost
identical to the MOs for this molecule).

<table data-border="o">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td colspan="2"><div id="attachment_3071" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_7.jpg"
title="CN+_7" class="size-full wp-image-3071" decoding="async"
aria-describedby="caption-attachment-3071"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2010/12/CN_mo7_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_7.jpg 394w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_7-300x205.jpg 300w"
sizes="(max-width: 394px) 100vw, 394px" width="200" />
<p>Orbital 7. Click for 3D</p>
</div></td>
</tr>
<tr class="even">
<td><div id="attachment_3072" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_6.jpg"
title="CN+_6" class="size-full wp-image-3072" loading="lazy"
decoding="async" aria-describedby="caption-attachment-3072"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2010/12/cn+_mo6_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_6.jpg 294w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_6-292x300.jpg 292w"
sizes="(max-width: 200px) 100vw, 200px" width="200" height="205" />
<p>Orbital 6 (π)</p>
</div></td>
<td><div id="attachment_3073" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_5.jpg"
title="CN+_5" class="size-full wp-image-3073" loading="lazy"
decoding="async" aria-describedby="caption-attachment-3073"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2010/12/cn+_mo5_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
width="200" height="170" />
<p>Orbital 5 (π)</p>
</div></td>
</tr>
<tr class="odd">
<td colspan="2"><div id="attachment_3074" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_4.jpg"
title="CN+_4" class="size-full wp-image-3074" loading="lazy"
decoding="async" aria-describedby="caption-attachment-3074"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2010/12/cn+_mo4_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_4.jpg 377w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_4-300x190.jpg 300w"
sizes="(max-width: 200px) 100vw, 200px" width="200" height="127" />
<p>Orbital 4. Click for 3D</p>
</div></td>
</tr>
<tr class="even">
<td colspan="2"><div id="attachment_3076" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN+_31.jpg"
title="CN+_3" class="size-full wp-image-3076" loading="lazy"
decoding="async" aria-describedby="caption-attachment-3076"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2010/12/cn+_mo3_0.02.jvxl;isosurface &quot;&quot; translucent;zoom 100;spin 3;&#39;);"
width="200" height="181" />
<p>Orbital 3. Click for 3D</p>
</div></td>
</tr>
</tbody>
</table>

Orbitals 5 & 6 are standard π orbitals with no mystery (and 8 & 9, not
shown, are the matching π\* pair). Orbital 3 results from the overlap of
two 2s AOs (but note the curious little toroid at the carbon end).
Orbitals 4 and 7 (the LUMO) are the interesting ones. Nominally, the
result of overlapping two 2p~x~ AOs to give what should be a bonding and
antibonding pair, they both appear to be bonding in the C-N region!
Perhaps the quadruple bond is not looking quite so unlikely after all
(comprising \~double occupancy of orbitals  3-6)!

What about those stalwarts I often use in these blogs, QTAIM and ELF?
The former  (using the CCSD natural orbitals) has a ρ(r) of 0.346 and a
∇^2^ρ(r) of +2.01 at the bond-critical point (BCP). The former is
certainly a high value, although no calibration exists to compare it to
a quadruple bond. The Laplacian has a positive value at this point,
possibly an indication of a charge-shift bond (see
[this](http://www.ch.ic.ac.uk/rzepa/blog/?p=2251){target="_blank"} and
[this](http://www.ch.ic.ac.uk/rzepa/blog/?p=1722){target="_blank"} blog,
although more likely due to the adjacency of the bond critical point to
the core shell of the carbon atom). ELF (also using natural orbitals)
declares the presence of TWO disynaptic basins, with integrations of
5.39e and 2.44 (totalling 7.83e). The basins will each take the form of
a torus (see
DOI: [10.1021/ct100470g](http://dx.doi.org/10.1021/ct100470g){target="_blank"}).
Hm, perhaps, on reflection, this paragraph might not be entirely
suitable for an introductory tutorial to organic chemistry. The density
of mumbo-jumbo is rather high!

So starting from a simple retrosynthetic analysis of a simple aromatic
molecule, in which the less obvious route is at least considered, one
derives a "new" reagent, the cyonium cation CN^+^. In a effort to
analyse its bonding, one concludes that a quadruple bond needs to be
taken at least seriously. I would note as a warning that these diatomic
species can be really tricky to pin down, and the iso-electronc C~2~ is
a good example of that. But C~2~ has all sorts of issues, some of which
are avoided with CN^+^. So the last word is hardly written, but not a
bad outcome, I venture to suggest, of following one's nose in a
tutorial.

------------------------------------------------------------------------

I have appended to this post a 3D exploration of the ELF function,
showing the two torus basins referred to above.

 

::: {#attachment_3103 .wp-caption .aligncenter style="width: 234px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN-ELF.jpg "CN-ELF"){.size-full
.wp-image-3103 loading="lazy" decoding="async"
aria-describedby="caption-attachment-3103"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2010/12/CN+_0.795.jvxl;isosurface \"\" translucent;zoom 200;spin 3;');"
width="224" height="181"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN-ELF.jpg 448w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/12/CN-ELF-300x242.jpg 300w"
sizes="(max-width: 224px) 100vw, 224px"}

ELF function for CN+. Click for 3D
:::

------------------------------------------------------------------------

Henry Rzepa, URL:http://www.ch.imperial.ac.uk/rzepa/blog/?p=3065.
Accessed: 2011-06-04. (Archived by WebCite® at
[http://www.webcitation.org/5zBSjBjhM](http://www.webcitation.org/5zBSjBjhM){target="_blank"})

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


Do you fancy a story going from simplicity to complexity, if not absurdity, in three easy steps? Read on! The following problem appears in one of our (past) examination questions in introductory organic chemistry. From relatively mundane beginnings, one can rapidly find oneself in very unexpected territory. How would one make 3-nitrobenzonitrile?

Rogue Scholar Beiträge

Following one’s nose: a quadruple bond to carbon. Surely I must be joking!