Imperial College London

Henry Rzepa

Previously, I showed how conjugation in dienes and diaryls can be
visualised by inspecting bond lengths as a function of torsions. Here is
another illustration, this time of the mesomeric resonance on a benzene
ring induced by an electron donating substituent (an amino group) or an
electron withdrawing substituent (cyano).

![Scheme](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/08/benzenes.svg){decoding="async"
width="400"}

In both cases, you can see this resonance showing as a lengthening of
the C(ipso)-C(ortho) and C(meta)-C(para) bonds, and a contracting of the
C(ortho)-C(meta) bonds. Does this reflect in the measured structures?
The usual search is applied (R \< 5%, no disorder, no errors) and
qualified with the following:

1.  The amino has three bonds, and can bear either H, or 4-bonded carbon
    only.
2.  R on the ring can be either H or C.
3.  Three distances are defined.

![Scheme](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/08/search1.jpg){decoding="async"
width="440"}

The results of a search are shown below; the hotspot shows the
C-C(ortho) distance is close to 1.40Å, whilst the corresponding value
for C(ortho)-C(meta) is 1.38Å, a contraction of \~0.02Å. The contraction
is smaller for phenols (\~0.01Å).

![Scheme](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/08/aminophenyl.jpg){decoding="async"
width="440"}

The C(ortho)-C(meta) vs C(meta)-C(para) amino plot shows a cluster of
hotspots for which the former (1.38Å) is  shorter than the latter
(\~1.39Å) but the effect is less clear cut as the distance from the
substituent increases.

![Scheme](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/08/amino-m-p-phenyl.jpg){decoding="async"
width="440"}

For an electron withdrawing cyano substituent, C(ipso)-C(ortho) at
1.395Å is longer than C(ortho)-C(meta) at 1.385Å, although the
difference seems smaller than for the amino substituent. The
(ortho)-C(meta) to C(meta)-C(para) comparison is similar.

![Scheme](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/08/cyanophenyl.jpg){decoding="async"
width="440"}

![Scheme](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/08/cyano-m-p-phenyl.jpg){decoding="async"
width="440"}

These searches take but a few minutes to perform, and do serve as a
reality check on the oft-seen mesomeric π-resonance shown in all organic
text books.

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Previously, I showed how conjugation in dienes and diaryls can be visualised by inspecting bond lengths as a function of torsions.

Messaggi di Rogue Scholar

Mesomeric resonance in substituted benzenes: a crystallographic reality check.