Re: ORGLIST: What are organic chemical reactions and how do they take place?

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From: Sengen Sun (sengensun$##$yahoo.com)
Date: Wed Oct 10 2007 - 20:59:04 EDT


Question:
> Dear Sengen,
>
> I have a question with your comment. It is
> common practice that organic chemists
> draw
the electron flow arrows from an
> electron-rich center to an electron-
> deficient center, which is essentially
> the same as that electrons flow toward > positive
potential.
> Is anything wrong with that?
> .........
......
---------------------------------
Answer:
At least a century ago (or two?), organic chemists
started to explain mechanisms based on charge
distribution in reactants, which is equivalent to the
electrostatic potential argument. Organic chemists
could correctly draw the arrows for the reactions such
as that between fulminic acid and ethyne a century ago
(!!!), based on the empirical knowledge that
electron-rich oxygen donates and electron-deficient
carbon of fulminic acid accepts electron. They could
until they could not.
.
At least, chemists became completely confused by the
regioselectivity of 1,3-dipolar cycloadditions, which
are a huge class of reactions in practical syntheses
of heterocycles (Christl & Huisgen: Tetrahedron Lett
1968, 5209). The regioselectivity made chemists
confused with some reactions (such as that between
fulminic acid and ethyne) that could have been well
understood a century ago. In my personal view, Huisgen
(maybe Christl also) should have been awarded a Nobel
prize for the pioneer experimental work, which not
only fascinated me for the last 27 years since I was
an under, but also has induced the unprecedented hot
debates in the literature for about 4 decades on
mechanisms. The regioselectivity demonstrates us how
the nature can trick us by hiding the dynamic events
from the state properties within the well-established
physical laws.
.
The regioselectivity is one of the few reasons that
the highest authority in theoretical chemistry
attempts to abolish the meaning of arrows for this
whole class of concerted cycloadditions, for which
they used a special word "non-classical".
.
A book of Modern Physical Organic Chemistry (by E.V.
Anslyn & D. A. Doughenty, Ed. 2006, page 894) says
"The mechanisms are best described by the orbital and
state correlation diagrams... The electron pushing
does not reflect how the reaction actually occurs".
This is just a recent example of many in the
literature that orbital control theories deny
electrostatic forces or the effect of electrostatic
potential. They don't mention the electrostatic forces
(or polar effect) until orbitals fail in prediction.

On the other hand, there have been strongly inquiring
minds in the literature for the recent decade who
attempt to re-establish the meaning of the arrows in
cycloadditions, but have led to the controversies what
is the correct direction for the arrows.
.
I have no problem for experimentalists to draw the
arrows based on their empirical knowledge. They do
until they cannot. Further, by the empirical
intuition, they often know when they should draw the
arrows backward toward less positive (or more
negative) potential such as in reverse reactions,
which are also normal reactions upon a switch.
.
A theory is totally different. That "electron flow is
always toward the positive potential" based on the
state property without a specified limit at the
theoretical level is strictly false. If it were true,
there would not be any polar molecules in this nature!
A referee of my JPC comment said this: "It is true
that in the absence of other (orbital-dependent)
factors...." Then this and the other referee wrote a
lot to explain how orbitals are useful and
complementary.... And one said "Frontier molecular
orbital theory works ...except when it doesn't"....
.
The history and the reality tell me a valid
theoretical task is to provide solid data to prove the
physical event - electrons shift towards positive
potential that does take place in a particular
reaction. It is valueless to just reiterate the
empirical hypothesis that is denied by the orbital
control theories deeply rooted in the mighty chemical
community.
.
Neither DFT nor AIM has provided a solution for the
unusual dynamic complexity of cycloadditions, although
I personally think that both DFT and AIM deserve the
best award in the world. The reality is that we often
cannot find tools to calculate what we need.
Therefore, computational and theoretical chemists are
needed as soon as we can recognize the challenges and
controversies. If some one claims an "attractive"
approach without a limit, another one must ask whether
more works are needed and why. That would be a
valuable and productive way to conduct theoretical
science.
.
Thanks to every one for attention.
.
Sengen

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