Requirements for Aromaticity


Uploaded by lamechivanes on 08.02.2010

Transcript:

From this slide, you can see that cyclical π systems,
which are aromatics,
or if they contain nitrogen, oxygen or sulfur
as in the case of tryptophan, they're called heteroaromatics,
aromatics and heteroaromatics are very common
in the building blocks of biological systems.
The amino acids, the nucleobases, which are
an important component in DNA and RNA building blocks,
are all heteroaromatics.
And enzymes recruit cofactors from vitamins,
such as nicotinamide or flavin, which are also heteroaromatics,
and so we need to learn a little bit more about the chemistry
of these cyclical π systems.
In this webcast,
we're going to introduce some of the key concepts,
and in the following webcast,
we're going to talk more
about the molecular orbitals
of the π systems of aromatic structures.
So here you see the π system of benzene.
Actually on the left,
you see the top view
where your eye is positioned above the ring
and so you can see the 6 atoms of the benzene ring,
and the 6 different π molecular orbitals of that.
We'll eventually understand these in great detail,
but I'm just showing you here
what the π system molecular orbitals look like.
From the side-on view where our eye is positioned
toward the edge of the plainer ring,
we can see the plane of the benzene ring
and those same 6 π molecular orbital systems
and we can see the nodal plane there very clearly.
One of the things we're going to do
and I just wanted to introduce it to you here
is to do molecular orbital calculations
based on what's known as
the simple Huckel molecular orbital approximation.
And we'll be able to look at and generate pictures
not too much different than the picture over here
and use that to figure out what's the highest occupied
and what's the lowest ah, unoccupied molecular orbitals,
the HOMO and LUMO of the π system.
That's going to be important,
but first, let me tell you what some of the key requirements
of an aromatic system are.
Aromatics are cyclical arrangements of atoms,
all of which will be sp2 hybridized.
For that reason, they're going to be conjugated systems.
They're going to be π conjugated systems.
In order to have maximum overlap
of those sp2 hybridized p-orbitals,
those p-orbitals remaining on sp2 hybridized atoms,
maximum overlap is going to require that those atoms
be a plainer set of atoms
so that those side-by-side p-orbitals
can participate in π-type bonding.
And finally, to maximize
the stability of the aromatic system,
there's a Huck- there's a rule that's known as Huckel's rule
in which the π electron count,
the number of π electrons in the cyclical system,
should be 4n+2 where n is an integer,
it could be 0, 1, 2, so if we could have
2, 6, or 10, for example, electrons would- would ah,
accommodate the Huckel's rule.

Let's just compare the difference between benzene,
our simplest of the aromatic systems,
to a cyclical arrangement of cyclohexatriene,
whose structure is shown here,
where we've pinned the double bond into those three locations
as opposed to the benzene system
in which those π bonds are in resonance
and delocalized around the ring.
It turns out that that special stability, the
presence of the cyclical arrangement of this π system
and the resonance that that brings, provides 36 kcal/mol
relative to this hypothetical 1,3,5-cyclohexatriene molecule.
And so that resonance stability
is an important reason which gives aromatic structures
stability and chemical resistance. 46.020 It turns out that that special stability, the
presence of the cyclical arrangement of this π system
and the resonance that that brings, provides 36 kcal/mol
relative to this hypothetical 1,3,5-cyclohexatriene molecule.
And so that resonance stability
is an important reason which gives aromatic structure