Imperial College London

Henry Rzepa

There is often a disconnect between how a text-book (schematically)
represents a reaction and a more quantitive "reality" revealed by
quantum mechanics. Is the bromination of ethene to give
1,2-dibromoethane one such example?

![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/ethene+br2.svg "ethene+br2"){.aligncenter
.size-full .wp-image-7965 decoding="async"}

Text-books will show how ethene interacts with bromine to form a cyclic
bromonium cation, which with the liberated bromide anion makes for an
ion-pair. Using quantum mechanics (DFT
ωB97XT/6-311G(d,p)/SCRF=dichloromethane), one soon realises that the
ion-pair will need to be stabilised, both by solvation and possibly by
interacting the bromide anion with a further bromine to form a
tribromide anion (as shown above). A transition state for this process
can indeed be located, and this is shown below, together with the [IRC
for the
process](http://hdl.handle.net/10.6084/m9.figshare.96457){target="_blank"}.

::: {#attachment_7970 .wp-caption .aligncenter style="width: 330px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/Br2b2.gif "Br2b"){.wp-image-7970
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7970"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/10/Br2.log;frame 61;connect (atomno=7) (atomno=8) PARTIAL;connect (atomno=10) (atomno=8) PARTIAL;measure 7 8;measure 8 10;vectors on;vectors 4;vectors scale 5.0; color vectors blue; vibration 20;animation mode loop;');"
width="320" height="213"}

Transition state for reaction between bromine and ethene. Click for 3D
:::

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| ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/Br2.svg "Br2"){.aligncenter .size-full .wp-image-7972 decoding="async" width="200"} | ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/Br2g.svg "Br2g"){.aligncenter .size-full .wp-image-7973 decoding="async" width="200"} |

The barrier (\~ 7 kcal/mol) is appropriately small for what is a rapid
reaction. The ion-pair formed is only stable by \~ 1 kcal/mol towards a
reverse reaction. Here however, the limitations of this relatively
simple solvation model (a continuum solvent and an extra bromine
molecule) may be under-estimating its stability. 

The interesting bit comes next. The ion-pair has got to relocate itself
to a position where the tribromide anion can back-side attack the
bromonium cation.^‡^ Ion-pairs are unlike normal molecules, where
covalent bonds determine fairly clearly what the geometry and its
conformations might be. There are no straightforward rules for
establishing what the geometry of an ion-pair might be, or indeed what
its energy relative to other poses might be, other than perhaps simply
minimising its dipole moment. In this example, quantum mechanics tells
us that in fact the ion-pair is about 2 kcal/mol MORE stable in its new
location, and so it can presumably diffuse over with a clear driving
force.^†^ Once it is back-side located, the reaction completes without
any enthalpic barrier; it is downhill all the way!

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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/br6.gif "br6"){.aligncenter .wp-image-7985 loading="lazy" decoding="async" width="300" height="301"} |                                                                                                                                                              |
| ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/br6.svg "br6"){.aligncenter .size-full .wp-image-7986 decoding="async" width="200"}                  | ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/br6g.svg "br6g"){.aligncenter .size-full .wp-image-7987 decoding="async" width="200"} |

This is a fairly simple model for the reaction between bromine and
ethene. Modelling it however is not that simple. Any reaction which
undergoes a transition from covalency to ionicity in one or more bond is
always a challenge. And ionic systems may not always be best represented
by (non-statistical) models which include only one molecule (as is the
case with ionic solids). But the picture I have shown here takes the
text-book schemes up one level, and we now have at least an estimate of
the energies involved, as well as geometries.

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

^†^ The geometry of TS2 is shown below. It has a dipole moment of 19.3D
and is some 9.6 kcal/mol in ΔG above TS1, which has a dipole moment of
18.7D. It may of course not be the lowest energy pathway for
reorganisation of the ion-pair.

::: {#attachment_7992 .wp-caption .aligncenter style="width: 173px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/ion-pair-migration.jpg "ion-pair-migration"){.wp-image-7992
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7992"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/10/IP.log;frame 3;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;');"
width="163" height="216"}

Click for animation.
:::

^‡^Where backside attack is prevented by steric bulk, this very ion-pair
can actually be isolated as a
crystal\[cite\]10.1021/ja00301a021\[/cite\]

::: {#attachment_8033 .wp-caption .aligncenter style="width: 288px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/BrBr31.jpg "BrBr3"){.wp-image-8033
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8033"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/10/DAKVUG.cif;');"
width="278" height="274"}

Crystal structure of a bromonium cation-tribromide anion pair. Click for
3D.
:::

It is also possible to isolate the bromonium cation using a different
counter-anion, by the same subterfuge of steric
hindrance\[cite\]10.1021/ja00085a027\[/cite\]. This structure also is
unusual in revealing an isolated hydronium ion cation, H~3~O^+^.

::: {#attachment_8036 .wp-caption .aligncenter style="width: 388px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/WEVPIW.jpg "WEVPIW"){.wp-image-8036
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8036"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/10/WEVPIW.mol2;');"
width="378" height="284"}

A cyclic bromonium ion using triflate counter-anions. Click for 3D
:::

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


There is often a disconnect between how a text-book (schematically) represents a reaction and a more quantitive “reality” revealed by quantum mechanics. Is the bromination of ethene to give 1,2-dibromoethane one such example? Text-books will show how ethene interacts with bromine to form a cyclic bromonium cation, which with the liberated bromide anion makes for an ion-pair.

Messaggi di Rogue Scholar

Text-books and the bromination of ethene.