Physical Organic
Chemistry?!
Eh?
Kinetics through the looking glass.
Hello!
My name is Janette
Gunter. I am a Ph. D. graduate student in physical organic
chemistry, working for Prof. Claude F. Bernasconi.
Chemistry
is lots of fun........ especially when concerning (consuming)
ethanol.
But it can be frustrating
at times.......
Our group is primarily
interested in the investigation of kinetic and thermodynamic acidities
of various complexes. Kinetic acidities are observed by studying a set
of acids that belong to the same family. That means that we use
acids
of the same general structure except for one structural feature, the
probe
(in most cases this is a substituent on a phenyl ring). Take for
example the nitro alkane family, ArCH2NO2. Here the probe is the
substituent Z. The main objective is to
vary the acid without drastically changing its
chemical composition; this is done by changing Z. Notice that Z is
actually quite far removed from the acidic site. I will use the general
scheme shown below to express what is done in order to study the
acidities
of carbon acids.
When the substituent on the acid is varied and
the same base is used for each acid, a series of observed rate constants
(kobs) is obtained. The plot of these rate constants vs log
KaAH
yields a slope which is known as a.
When the Base is varied and the acid is kept
constant,
a series of rate constants for the base family for the deprotonation of
the acids is obtained. Plotting the observed rate constants vs the
pKaBH of the protonated base yields a slope
which is called the BrØnsted b
value.
When b
and a are
approximately the same, we would conclude that there is not a significant
imbalance in the Transition State, that is to say that charge
delocalization
does not lag behind proton transfer. When a
> b
we would say that charge delocalization lags behind proton
transfer.
In fact, for nitro alkanes an b
of 0.51 and an a of
1.29 are observed. This suggests that there is a large imbalance
between the proton transfer and the charge delocalization in the
Transition
State.
Since Fischer Carbenes,
like other carbon acids (specifically like nitro alkanes), contain an
activated
p system
it was believed that they would show a similar imbalance.
Interestingly enough, preliminary studies did not show an imbalance
(a
and b
values
were very similar). This information could be interpreted in 2
ways:
(1) the metal moiety in the Fischer carbenes does not behave like the
average
organic p-acceptor
or (2) the methoxy oxygen could be donating its p
electrons into the p
system.
In order to discern the reason for this lack of imbalance in the
transition
state, it was proposed that a compound which resembled the nitro alkane
system (possessing an activated p
system) yet also possessing a strong p
donor system should be studied. This is where my work
begins.
I study the kinetics of the deprotonation of
carbon
acids by secondary amines. Specifically, I study olefinic carbon acids
bearing both an electron withdrawing and an electron donating group (ewg-
C=C-edg). The "general" chemical structure for the compounds
I study is shown below (where Ar is a substituted phenyl ring):
The general scheme for the reaction studied is
given by scheme 1:
Scheme 1:
I also run ab
initio
calculations on proton transfers between various compounds to give us a
better idea what the transition state may look like. We use
identity reactions to help simplify these calculations. One of the
systems I am presently studying involves a transfer between nitro
acetaldehyde
and its anion (as shown in scheme 2).
Scheme 2:
We would expect the Transition State for the
proton
transfer to look something like this:
VIEW 1:
VIEW 2:
I will soon be starting on a new project. I will be studying
nucleophilic
vinylic substitutions in which the nucleophile is cyanamide. Our
goal is to observe the intermediate formed from the reaction of cyanamide
with a compound containing a carbon carbon double bond. In the
past,
this type of reaction with cyanamide has not generated an observable
intermediate;
therefore, we will be studying compounds that do not possess a leaving
group. This will stop the reaction at the point of the formation
of the intermediate (leaving group expulsion can not occur), thus making
the intermediate observable. The general reaction scheme is shown below
in scheme 3.
Scheme 3.
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