Imperial College London

Henry Rzepa

Birch reduction of benzene itself results in 1,4-cyclohexadiene rather
than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?

![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/12/birch3.svg "birch3"){.aligncenter
.size-full .wp-image-8541 decoding="async" width="450"}

The mechanism, as elaborated in the previous [two
posts](http://www.ch.imperial.ac.uk/rzepa/blog/?p=8508){target="_blank"},
involves a one-electron transfer from a sodium atom to form the radical
anion, which is then protonated in a second step, and this is again
reduced to form a pentadienyl anion in the penultimate
step.\[cite\]10.1021/ja00159a078\[/cite\] The question now becomes why
does this anion protonate to give predominantly the less stable diene
product? The answer involves the actual structure of this anion. A
[calculation](http://hdl.handle.net/10.6084/m9.figshare.101004){target="_blank"}
at the ωB97XD/6-311+G(d,p)/SCRF=acetonitrile level for the ion pair
comprising the cyclohexadienyl anion and a Na(NH~3~)~3~^+^ counterion is
shown below.

::: {#attachment_8544 .wp-caption .aligncenter style="width: 289px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/12/birch-ip.jpg "birch-ip"){.wp-image-8544
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8544"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/12/benzene-ip.cml;measure 1 2;measure 4 5; ');"
width="279" height="295"}

Structure of the cyclohexadienyl ion pair. Click for 3D.
:::

From this, it appears that the sodium cation is η^2^ coordinated to each
of two relatively localised double bonds (1.37Å), resulting in the
negative charge accumulating on just the one carbon (red arrow), this
being the carbon that then exclusively receives a final proton. The
highest energy (-0.115 au) natural bond orbital (NBO) also emerges as
being located on this carbon (the next two highest energy NBOs only come
in at -0.303 au, and reside on each of the localised alkene bonds).

::: {#attachment_8547 .wp-caption .aligncenter style="width: 250px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/12/birch-ip-nbo.jpg "birch-ip-nbo"){.wp-image-8547
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8547"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/12/ip_mo42.cub.xyz;isosurface color orange blue wp-content/uploads/2012/12/ip_mo42.cub.jvxl translucent;zoom 80;');"
width="240" height="240"}

The highest energy NBO orbital. Click for 3D.
:::

The molecular electrostatic potential in effect integrates over all the
electrons (not just those in the highest orbital), resulting in a
function that measures the attractiveness of any point to a proton
(red). It too shows that the most attractive region (red) for a proton
is again on this carbon.

::: {#attachment_8551 .wp-caption .aligncenter style="width: 279px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/12/birch-ip-mep.jpg "birch-ip-mep"){.wp-image-8551
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8551"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/12/ip_mo42.cub.xyz;isosurface color red green wp-content/uploads/2012/12/ip_esp.cub.jvxl translucent;zoom 80;');"
width="269" height="274"}

Molecular electrostatic potential. Click for 3D.
:::

There is even evidence from crystal structures that this sort of motif
is possible. Thus the dianion of 1,4-diphenylbenzene (with
two Na(thf)~3~^+^ counter-ions)
reveals\[cite\]10.1107/S0567740881006833\[/cite\] this type of
coordination.  The buckling seen in the above mono-anion is inhibited by
the presence of cations on both sides of the di-anion, but the pattern
of short/long bonds seen above also manifests in the crystal structure.

::: {#attachment_8556 .wp-caption .aligncenter style="width: 311px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/12/BIRCH-XRAY1.jpg "BIRCH-XRAY"){.size-full
.wp-image-8556 loading="lazy" decoding="async"
aria-describedby="caption-attachment-8556"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/12/BABKOE1.cif;measure 2 3;measure 28 26;');"
width="301" height="361"}

Crystal model. Click for 3D.
:::

So the take home message is that the counter-ion (solvated sodium
cations) in the Birch reduction  of benzene itself may coordinate to the
anionic intermediates in the reductive process, and the resulting
geometry of this ion-pair determines the eventual product of
protonation.

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


Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?

Messaggi di Rogue Scholar

The mechanism of the Birch reduction. Part 3: reduction of benzene