Imperial College London

Henry Rzepa

As the title hints, I have [been here
before](http://www.ch.imperial.ac.uk/rzepa/blog/?p=1135){target="_blank"}.
The S~N~1 solvolysis mechanism of t-butyl chloride was central to the
flourishing of physical organic chemistry from the 1920s onwards, and it
appears early on in most introductory lecture courses and text books.
There we teach that it is a two-stage mechanism. Firstly the C-Cl bond
heterolyses to form a stable tertiary carbocation intermediate, which in
a second stage reacts with nucleophile (water) to form e.g. t-butanol.
This is contrasted with the [S~N~2
mechanism](http://www.ch.imperial.ac.uk/rzepa/blog/?p=4002){target="_blank"},
where these two stages are conflated into a single concerted process,
involving no intermediates. Here I explore an intrinsic reaction
coordinate for the hydrolysis of t-butyl chloride which attempts to
tease out whether this simple picture is realistic.

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/sn11.svg "sn1"){.aligncenter
.size-full .wp-image-5231 loading="lazy" decoding="async" width="331"
height="293"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/sn11.svg)

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/sn1-g1.svg "sn1-g"){.aligncenter
.size-full .wp-image-5232 loading="lazy" decoding="async" width="314"
height="252"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/sn1-g1.svg)

The basic model comprises t-butyl chloride and 16 water molecules. These
are subjected to a [wB97XD/6-311G(d,p)
calculation](http://hdl.handle.net/10042/to-9558){target="_blank"} with
a continuum water solvent field applied throughout. The functional is
different from the one I used last time, since  I wanted one that
included dispersion attractions. The basis set is also better.

1.  At IRC -5, we see the first stage of the mechanism, the cleavage of
    the C-Cl bond. Note how the [methyl
    flag](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5087){target="_blank"}
    waves at this point.
2.  At IRC 0.0 we have the transition state, at which point the
    gradients of the energy are precisely zero.
3.  At IRC +5, we have a very slight dip/inflexion point in the
    potential, but the gradients do not actually go to zero. This is the
    point that would correspond to the formation of a carbocation. The
    S~N~1 mechanism proper would require a formal intermediate here,
    with zero gradients.
4.  At IRC \~+15, we see a new phenomenon, the attack of a water
    molecule on the "almost" carbocation, reflecting in fact an S~N~2
    mechanism.
5.  At IRC \~+20, we see a slight blip, which in fact is reorganisation
    of the hydrogen bonds of the surrounding water molecules,
    accompanying the formation of an entirely ionic chloride.
6.  All these processes are animated in the diagram below, where you can
    see other features:
    1.  Note the methyl rotation just after the Cl has started leaving,
        and another when the C-O bond formation is completing.
    2.  Note the hydrogen bond reorganisation near the end.

::: {.wp-caption .aligncenter style="width: 355px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/sn1.gif "19"){.size-full
.wp-image-5186 loading="lazy" decoding="async"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('yellow');jmolApplet([450,450],'load wp-content/uploads/2011/10/sn1.log; frame 15; zoom 100; connect (atomno=14) (atomno=1) PARTIAL;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;');"
width="345" height="243"}

(anharmonic) IRC for hydrolysis of t-butyl chloride. Click to see the
(harmonic) transition normal mode.
:::

As I noted in my previous post on the topic, there are other
complexities, involving potential proton transfers amongst the water
molecules which is not reflected here. As is not unusual in science,
sometimes the most apparently simple processes turn out to have hidden
complexities.

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


As the title hints, I have been here before. The S
<sub>
 N
</sub>
1 solvolysis mechanism of t-butyl chloride was central to the flourishing of physical organic chemistry from the 1920s onwards, and it appears early on in most introductory lecture courses and text books. There we teach that it is a two-stage mechanism.

Messaggi di Rogue Scholar

The SN1 Mechanism for a third time. Exploration of the intrinsic reaction coordinate.