Imperial College London

Henry Rzepa

This story starts with an organic chemistry tutorial, when a student
asked for clarification of the  [Finkelstein
reaction](http://dx.doi.org/10.1002/cber.19100430257){target="_blank"
rel="noopener noreferrer"}. This is a simple S~N~2 type displacement of
an alkyl chloride or bromide, using sodium iodide in acetone solution,
and resulting in an alkyl iodide. What was the driving force for this
reaction he asked? It seemed as if the relatively strong carbon-chlorine
bond was being replaced with a rather weaker carbon-iodine bond. But its
difficult to compare bond strengths of discrete covalent molecules with
energies of ionic lattices. Was a simple explanation even possible?

All is not as it seems however. The traditional explanation, found by
the quick Google search linked above, is that the reaction illustrates
[Le Chatelier's
principle](http://en.wikipedia.org/wiki/Le_Chatelier's_principle){target="_blank"
rel="noopener noreferrer"}, whereby an equilibrium is driven over to
completion by removal of one of the products (in this case sodium
chloride or sodium bromide, which crystallize out of solution). Well, we
have replaced one possible (and probably complicated) explanation based
on bond strengths and ionic lattices by another based on the
solubilities of an ionic material in a moderately polar solvent. But all
we have done is ask a different question, which now becomes why is
sodium iodide highly soluble in acetone, whereas sodium chloride and
bromide are not? The answer to this is less easily found using Google!

A good start would be the crystal structure of any complex formed
between acetone and sodium iodide. Fortunately, one such does exist, and
it is shown below (sodium=yellow, iodine=purple).

::: {#attachment_446 .wp-caption .aligncenter style="width: 440px"}
![(Acetone)3.
NaI](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/05/acetonenai.jpg "acetonenai"){.size-full
.wp-image-446 loading="lazy" decoding="async"
aria-describedby="caption-attachment-446"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('yellow');jmolApplet([430,430],'load wp-content/uploads/2009/05/naiace.mol;zoom 120;cpk on;spin 3;','c1');"
width="335" height="252"}

(Acetone)3+NaI. NAIACE. Click for 3D.
:::

The formula shows three acetone molecules for each sodium iodide. The
carbonyl oxygen has two lone pairs of electrons, and each of these is
used to coordinate a (different) sodium cation. This allows each sodium
to be coordinated by a total of six lone pairs, giving it octahedral
coordination. This sets up what in fact is quite a rigid scaffold, with
the unusual feature of an approximately triangular shaped channel
running down the lattice (two such are shown above). The size of this
hole is determined by the methyl groups of the acetone, and it is into
this cavity that the halide ion must fit.

As it happens, the iodide anion is exactly the right size to produce a
perfectly snug fit up against those methyl groups (click on the image
above to view this). If a chloride or bromide anion were to be fitted
into the cavity, there would be empty space surrounding it. The cavity
itself is too rigid to collapse around the halide anion to absorb this
space. This means these halide anions are further away from the
positively charged sodium than they would like to be such that they
[minimize their ionic lattice
energies](http://en.wikipedia.org/wiki/Lattice_energy){target="_blank"
rel="noopener noreferrer"}. Instead they avoid fraternizing with the
acetone at all, and form a pure sodium chloride or sodium bromide
lattice (where the two oppositely charged ions CAN approach at optimal
distances). The result is that sodium chloride crystallizes out of
solution, and the Finkelstein reaction proceeds to completion!

::: {#attachment_445 .wp-caption .aligncenter style="width: 440px"}
![Acetone. NaI in spacefill
mode](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/05/acetonenai-1.jpg "acetonenai-1"){.size-full
.wp-image-445 loading="lazy" decoding="async"
aria-describedby="caption-attachment-445"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('yellow');jmolApplet([430,430],'load wp-content/uploads/2009/05/naiace.mol;zoom 120;spin 3;','c2');"
width="335" height="252"}

Acetone. NaI in spacefill mode. Click for 3D.
:::

But that is not quite the end of the story. If you view the acetone.NaI
lattice sideways (click on the diagram above to view this aspect), you
will find that in fact there is still space in the scaffold after all!
Each iodide anion has room above or below it, with space for exactly one
more iodine atom to fit without having to change the shape of the
scaffold. And indeed such a molecule has been
reported\[cite\]10.1107/S0108270103006395\[/cite\],\[cite\]10.5517/CC75TZ5\[/cite\]
but it is an odd one! The stoichiometry is now (acetone)~3~.NaI~2~,
which implies that the iodide anion has been joined by an iodine atom.
I~2~(-) is called a radical anion, and as such has an unpaired electron.
Just like two iodine atoms can couple their unpaired electrons to form a
covalent bond, so can two I~2~ radical anions, forming I~4~^2-^ \[or
I~3~^--^.I^--^\] or on to infinity as a linear iodine polymer, of
formula n\[I~4~^2-^\], with all the I...I distances equal at 3.224Å (a
system with no [Peierls
distortion](http://en.wikipedia.org/wiki/Peierls_transition)). Straight
rod-like polymeric chains of a single element might appear highly
unusual, but curiously, another class of elements that exhibits this
behaviour is Cu/Ag/Au and Ga,\[cite\]10.1002/anie.200601726\[/cite\] the
ultimate in thin wires!).

::: {#attachment_462 .wp-caption .aligncenter style="width: 440px"}
![Acetone.
NaI2](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/05/acetonenai2.jpg "acetonenai2"){.size-full
.wp-image-462 loading="lazy" decoding="async"
aria-describedby="caption-attachment-462"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('yellow');jmolApplet([430,430],'load wp-content/uploads/2009/05/gadmoo.mol;zoom 120;cpk on;spin 3;','c3');"
width="430" height="298"}

Acetone+NaI2. GADMOO. Click for 3D.
:::

Finally, it is worth noting that the same phenomenon occurs with the
[dimethylformamide.NaI](http://dx.doi.org/10.1107/S1600536807007052)
complex. In this example, only the NaI and not the NaI~2~complex has
been reported.

::: {#attachment_480 .wp-caption .aligncenter style="width: 440px"}
![DMF.
NaI](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/05/dmfnai.jpg "dmfnai"){.size-full
.wp-image-480 loading="lazy" decoding="async"
aria-describedby="caption-attachment-480"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('yellow');jmolApplet([430,430],'load wp-content/uploads/2009/05/naidmf02.pdb;zoom 120;cpk on;spin 3;','c4');"
width="249" height="246"}

DMF+NaI. Click for 3D.
:::

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

#### Acknowledgments

This post has been cross-posted in PDF format at
[Authorea](http://Authorea){rel="noopener" target="_blank"}.

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


This story starts with an organic chemistry tutorial, when a student asked for clarification of the  Finkelstein reaction. This is a simple S
<sub>
 N
</sub>
2 type displacement of an alkyl chloride or bromide, using sodium iodide in acetone solution, and resulting in an alkyl iodide. What was the driving force for this reaction he asked?

Postagens de Rogue Scholar

The mystery of the Finkelstein reaction