Imperial College London

Henry Rzepa

Text books will announce that during aromatic electrophilic
substitution, aromaticity is lost by the formation of a Wheland
intermediate (and regained by eliminating a proton). Is that entirely
true?[![wheland](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/wheland.svg){.aligncenter
.size-full .wp-image-11758
decoding="async"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/wheland.svg)

I will start by considering the simplest of all such
intermediates\[cite\]10.1021/ja00711a057\[/cite\],\[cite\]10.1021/ja00761a041\[/cite\]
the NMR of which was first reported by Olah and which is shown below
(red). The values in black are from a ωB97XD/6-311G(d,p) calculation
(chloroform)\[cite\]http://dx.doi.org/10.6084/m9.figshare.870473\[/cite\].
The discrepancy might be due to the difference in solvent
(HF-SbF~5~/SO~2~ClF-SO2F~2~) and my failure to include a counter-ion in
the calculation.^‡^ Actually, this result is only obtained at -134°C;
warming to -70°C increases the rate of [\[1,2\] ring
shifts](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10706 "Mechanistic arrow pushing. A proposed addition to its rules."){target="_blank"}
and only one "aromatic" peak at 8.09 ppm is observed.

::: {#attachment_11761 .wp-caption .aligncenter style="width: 408px"}
![Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/wheland-NNM.jpeg){.wp-image-11761
loading="lazy" decoding="async"
aria-describedby="caption-attachment-11761"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/12/wheland.log;frame 36;vectors on;vectors 4;vectors scale 8.0;color vectors green;vibration 6;animation mode loop;');"
width="398" height="310"}

Click for 3D to see vibrational modes
:::

 The computed MOs are shown below. MOs 21 and 15 are pretty much
identical with those for benzene itself (the former because of
symmetry),  so relatively little disruption of the conjugation there. An
NBO analysis shows that each C-H bond from the sp^3^ centre acts as a
donor to the π-system (E2 = 8.5 kcal/mol), which we know in another
guise as hyperconjugation or σ-conjugation. In this sense, the two C-H
bonds are acting as surrogates for a real p-AO on that carbon, and the
E2 value is certainly not insignificant. Certainly they are nowhere near
as good a donor as one real p-AO, but nonetheless, the two of them
together are good enough to result in retention of a significant measure
of cyclic π-conjugation, and hence probably aromaticity. The C-H bonds
are weakened as a result of their electron donation, which reduces their
normal mode wavenumber down by about 200 cm^-1^. The so-called
[Kekule-mode](http://www.ch.imperial.ac.uk/rzepa/blog/?p=485 "Longer is stronger."){target="_blank"},
which is depressed in benzene itself to about 1300 cm^-1^ because of the
effect termed π-distortivity\[cite\]10.1021/cr990363l\[/cite\], is
actually increased to 1442 cm ^-1^ in the Wheland
intermediate.\[cite\]10.6084/m9.figshare.870473\[/cite\] This mode is
normally elevated by any weakening of the distortive π-system (in [my
previous
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=485 "Longer is stronger."){target="_blank"},
I noted such an elevation induced by the quintet excited state of
benzene) and so we many presume the same effect operates in the Wheland
intermediate.

<table class="aligncenter" data-border="0" data-align="center">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td> <br />
&#10;<div id="attachment_11768" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/Wheland-20.jpeg"
class="wp-image-11768" decoding="async"
aria-describedby="caption-attachment-11768"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2013/12/Wheland_mo20.cub.xyz;isosurface color blue red wp-content/uploads/2013/12/Wheland_mo20.cub.jvxl;&#39;);"
width="200" alt="Click for 3D" />
<p>MO 20 Click for 3D</p>
</div></td>
<td> <br />
&#10;<div id="attachment_11769" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/Wheland-21.jpeg"
class="wp-image-11769" decoding="async"
aria-describedby="caption-attachment-11769"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2013/12/Wheland_mo21.cub.xyz;isosurface color blue red wp-content/uploads/2013/12/Wheland_mo21.cub.jvxl;&#39;);"
width="200" alt="Click for 3D" />
<p>MO 21 Click for 3D</p>
</div></td>
</tr>
<tr class="even">
<td colspan="2"><div id="attachment_11767"
class="wp-caption aligncenter" style="width: 310px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/Wheland-151.jpeg"
class="wp-image-11767" decoding="async"
aria-describedby="caption-attachment-11767"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2013/12/Wheland_mo15.cub.xyz;isosurface color blue red wp-content/uploads/2013/12/Wheland_mo15.cub.jvxl;&#39;);"
width="300" alt="Click for 3D" />
<p>MO 15 Click for 3D</p>
</div></td>
</tr>
</tbody>
</table>

To get a measure of any aromaticity deriving from a ring current, I
computed the NICS(1) value from the ring centroid. This latter was
itself obtained from the coordinates of the *ring-critical-point*
determined by a QTAIM analysis, and computed at that point and 1, 2 and
3Å above it. The NICS values are respectively -0.4, -6.1, -3.6 and -1.6
ppm. The maximum is indeed the NICS(1) point, and at that point, the
value indicates modest, but real π-aromaticity.

![wheland-NICS](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/12/wheland-NICS.jpeg){.aligncenter
.wp-image-11771 loading="lazy" decoding="async" width="269"
height="230"}

So does forming a Wheland intermediate disrupt all aromaticity? The
answer is a clear no! What might be interesting to compute (I have not
tried doing so here) is an actual energy for the stabilisation resulting
from the weak aromaticity present as a % of that present in benzene, so
that the text-books can be amended.

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

^‡^ It might also be that the exchange in the measured NMR spectrum was
not entirely suppressed, and so some residual averaging of the peak
positions remains.

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


Text books will announce that during aromatic electrophilic substitution, aromaticity is lost by the formation of a Wheland intermediate (and regained by eliminating a proton). Is that entirely true?

Postagens de Rogue Scholar

Does forming a Wheland intermediate disrupt all aromaticity?